709 copy The Meteoritical Society 2007 Printed in USA

Meteoritics amp Planetary Science 42 Nr 45 709ndash729 (2007)Abstract available online at httpmeteoriticsorg

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) Mineralogy geochemistry and comparison with Ivory Coast tektites

Christian KOEBERL1 Franz BRANDSTAumlTTER2 Billy P GLASS3Lutz HECHT4 Dieter MADER1 and Wolf Uwe REIMOLD4

1Department of Geological Sciences University of Vienna Althanstrasse 14 A-1090 Vienna Austria2Mineralogisch-Petrographische Abteilung Natural History Museum Burgring 7 A-1010 Vienna Austria

3Department of Geology University of Delaware Newark Delaware 19716 USA4Mineralogy Museum for Natural History Humboldt University in Berlin Invalidenstrasse 43 D-10115 Berlin Germany

Corresponding author E-mail christiankoeberlunivieacat

(Received 01 October 2006 revision accepted 01 January 2007)

AbstractndashIn 2004 an International Continental Scientific Drilling Program (ICDP) drilling project atthe Bosumtwi impact crater Ghana (105 km in diameter 107 Myr old) was performed to study thesediments that fill the lake as well as the underlying impactites In one (LB-05) of 16 cores drilled intothe lake sediments the zone between the impact breccias and the post-impact sediments waspenetrated preserving the final fine-grained impact fallback layer This ~30 cm thick layer containsin the top 10 cm ldquoaccretionaryrdquo lapilli microtektite-like glass spherules and shocked quartzgrains Glass particlesmdashmostly of splash form less than 1 mm sizemdashmake up the bulk of the grains(~70ndash78 by number) in the coarser size fraction (gt125 m) of the top of the fallback layer Aboutone-third of all quartz grains in the uppermost part of the layer are shocked with planar deformationfeatures (PDFs) almost half of these grains are highly shocked with 3 or more sets of PDFsK-feldspar grains also occur and some show shock deformation The abundance of shocked quartzgrains and the average shock level as indicated by the number of sets of PDFs for both quartz andK-feldspar decrease with depth into the layer The well-preserved glass spherules and fragments arechemically rather homogeneous within each particle and also show relatively small variationsbetween the various particles On average the composition of the fallback spherules from core LB-5Bis very similar to the composition of Ivory Coast tektites and microtektites with the exception of CaOcontents which are about 15 to 2 times higher in the fallback spherules This is a rare case in whichthe uppermost fallback layer and the transition to the post-impact sediments has been preserved in animpact structure its presence indicates that the impactite sequence at Bosumtwi is complete and thatBosumtwi is a very well-preserved impact crater

INTRODUCTION AND GEOLOGICAL SETTING

The Bosumtwi impact structure in south-centralGhana is associated with the Ivory Coast strewn field oneof the four known tektite strewn fields (eg Koeberl et al1997) Bosumtwi is a well-preserved complex impactstructure (centered at 06deg30 N 01deg25 W) and is situatedabout 32 km southeast of Kumasi the capital of theAshanti region of Ghana The crater has a distinct steeprim with elevations of up to 300 m above present-daylake level and is almost completely filled by LakeBosumtwi which is 8 km in diameter The crater issurrounded by a slight and irregular circular depression aswell as an outer ring of minor topographic highs with a

diameter of about 20 km (Jones et al 1981 Reimold et al1998 Wagner et al 2001)

The Bosumtwi impact crater was excavated in lowergreenschist facies metasediments (metagraywacke quartziticmetagraywacke metatuffs phyllites shales and schists) ofthe 21ndash22 Gyr old Birimian Supergroup Rocks to thesoutheast of the crater contain altered basic intrusives(Birimian metavolcanics) in addition to metasedimentsClastic Tarkwaian sediments occur further to the east andsoutheast and are thought to have been formed by the erosionof Birimian rocks A detailed review describing all aspects ofBosumtwi and a new geological map were recently publishedby Koeberl and Reimold (2005)

The Bosumtwi structure is one of the best-preserved

710 C Koeberl et al

terrestrial meteorite impact structures with proximal ejecta inthe form of suevite and other impact breccia deposited outsidethe crater rim to a distance of about 1 crater radius The crateris particularly important because of its association with theIvory Coast tektites which were first reported by Lacroix(1934) from an 80 km wide area in Ivory Coast (CocirctedrsquoIvoire) territory Microtektites were found in deep-sea coresoff the coast of West Africa (Glass 1968 1969) and related tothe tektites found on land These microtektites are up to onemillimeter in size and show a variety of shapes mostlyspherical shapes droplets tear-drops dumbbells andfragments of particles of these respective shapes Thegeographical distribution of microtektite-bearing deep-seacores has been used to determine the extent of the strewn field(eg Glass and Zwart 1979 Glass et al 1979 1991) and themicrotektite abundance (numbercm2) and size distributionwas used to accurately predict the size of the source craterlocated at Lake Bosumtwi (Glass and Pizzuto 1994 Glasset al 1991)

Tektites are now known to have formed duringhypervelocity impacts on Earth and to represent melts ofsurficial predominantly sedimentary precursor rocks ofupper crustal composition (see eg Koeberl 1994Montanari and Koeberl 2000 and references therein) Several

lines of arguments were used to conclude that Bosumtwi ismost likely the source crater of the Ivory Coast tektite strewnfield These include similar chemical compositions(Schnetzler et al 1967 Jones 1985) and similar isotopiccharacteristics of the tektites and rocks found at the crater(eg Schnetzler et al 1966 Lippolt and Wasserburg 1966Shaw and Wasserburg 1982 Koeberl et al 1998) as well asthe similar ages of tektites and Bosumtwi impact glasses (egGentner et al 1964 Storzer and Wagner 1977) Precisefission track and 40Ar-39Ar step-heating dating on both IvoryCoast tektites and Bosumtwi impact glass established areliable age of 107 plusmn 005 Myr for the Bosumtwi impactevent and the tektites (Koeberl et al 1997) and themagnetostratigraphically determined age of the Ivory Coastmicrotektite layer also agrees with the age of the Bosumtwicrater providing a firm basis for the link between theBosumtwi impact and the tektite-forming event

CORING AT BOSUMTWI

Bosumtwi is a hydrologically closed basin which has ledto the preservation of laminated varves in the lake sedimentsproviding a means for high-resolution (annual) paleoclimatereconstruction in a region for which so far little data exists

Fig 1 A location map showing the location of borehole LB-05B which was terminated in the uppermost fallback layer as well as the positionsof boreholes LB-07A and LB-08A which penetrated impact breccias and fractured bedrock in the deep crater moat and the flank of the centraluplift respectively Also shown are profiles along which seismic data were recovered (cf Scholz et al 2002 Karp et al 2002) and that wereused to position the drill sites

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 711

Together with the importance of Bosumtwi for understandingimpact processes this provided ample reason for aninternational and multidisciplinary drilling project primarilyfinanced by the International Continental Scientific DrillingProgram (ICDP) (cf Koeberl et al 2005 2006 2007) Theproject had two main scientific goals paleoenvironmentalstudies and impact studies From July to October 2004 16boreholes were drilled at 6 locations within Lake Bosumtwias part of the ICDP drilling project The GLAD lake drillingsystem (specifically constructed for drilling at lakes seewwwdoseccorg Koeberl et al 2007) was used to core anentire lacustrine sediment fill from lake floor to bedrock Atfive sites 14 separate holes were drilled into the lakesediments at two sites LB-07A and LB-08A thicksequences of impactites and fractured bedrock wererecovered In core LB-05A (see Fig 1 for location) thecomplete ~1 Myr lacustrine sediment fill was recovered fromthe crater interior ending in a layer about 30 cm thick withabundant spherules on top the upper part of which appearedto be an ldquoaccretionaryrdquo lapilli-bearing sediment (Figs 2aand 2b) This unit likely represents the final fallback ejecta atthe base of the first post-impact sediment and provides animportant age constraint for the overlying sedimentarysequence The present paper describes the components of thisuppermost fallback ejecta unit

SAMPLES AND EXPERIMENTAL METHODS

Borehole LB-05B was drilled at a depth of 74 m layerwater (650052degN 141595degW) (Fig 1) the total depthreached was 37056 m and coring was done from a depth of7590 m to 37056 m for a total core recovery of 29467 mThe core penetrated lake sediments but at the very bottom ofthe core section 117 (see Fig 2 for a view of a section of thecore) a different rock unit was reached This unit with somespherule-like objects was below the bottom of the lake

sediments and assumed to be the uppermost impactite layerNo sedimentary structures are recognizable in this unit afterdrying the friable material has the form of irregularfragments The millimeter-size spherules that are prominentin Fig 2 have an appearance similar to accretionary lapilli(see below)

The ~30 cm thick uppermost fallback ejecta unit wasdivided into three subunits (117A1 117A2 and 117A3)Hereafter these subunits will be referred to as A1 A2 andA3 respectively Sample A1 is from the uppermost part ofthe fallback unit (~5 cm thick) sample A2 is immediatelybelow A1 (~10 cm) and A3 (the lowermost ~15 cm) is justbelow A2 Parts of the cores from these subunits wereavailable for the present study From those segments thinsections were prepared and various aliquots of (mostly ofsubpart A1) were studied in Vienna Berlin and Delaware Atthe University of Delaware samples A1 and A2 were

Fig 2 a) Core segment 117A1 from the bottom of corehole LB-05B 3705 m below lake level immediately after recovery of the core Corediameter is ~5 cm Photo courtesy M Talbot b) A close-up of a 4 mm wide region of the fragment shown in (a) displaying the ldquoaccretionaryrdquospheruleslapilli Photo courtesy J Peck

Table 1 Relative abundances of the major and accessory mineral components of accretionary spherules Sp-1 to Sp-4 and a bulk rock sample of subsample 117-A1 from core LB-05B

Sp-1 Sp-2 Sp-3 Sp-4 Bulk A1

Abundant Qz Qz Qz Qz Qz Ab Ab Ab Ab AbChl Chl Chl Chl ChlIll Ill Ill Ill Ill

Accessory Ap Ap Ap Py ApEpi Epi Py Ru EpiPy Py Ru Sph PlRu Py

RuSph

Mineral identifications by electron microscopy with EDX analysisAbbreviations Qz = quartz Ab = albite Chl = chlorite Ill = ldquoilliterdquoillite-hydro-muscovite Ap = apatite Epi = epidote Pl = plagioclasePy = pyrite Ru = rutile Sph = sphene

712 C Koeberl et al

searched for glass particles and shock-metamorphosedgrains This was done by disaggregating subsamples in waterusing ultrasonics and sieved into five size fractions (gt500250ndash500 125ndash250 63ndash125 lt63 m) Dilute HCl was usedin an attempt to help disaggregate A1 but very littlecarbonate was present A2 was disaggregated without usingdilute HCl Sample A3 could not be disaggregated and wascrushed to less than 250 m and sieved into three sizefractions (125ndash250 63ndash125 and lt63 m) The 63ndash125 msize fraction of each of the three samples was separated into aheavy (specific gravity gt296) and a light (specific gravitylt296) fraction The heavy mineral fractions were searchedfor high-pressure polymorphs (eg coesite reidite) Aportion of each light fraction was mounted in Piccolyte onpetrographic slides and searched for shock-metamorphosedgrains containing planar deformation features using apetrographic microscope

In addition 4 millimeter-size ldquospherulesrdquo as well as a

section of the core fragment ~6 mm wide wereimpregnated with epoxy and prepared for polished thinsections These were studied by optical and electronmicroscopy and electron microprobe analysis Scanning andbackscattered electron images were examined and thecomposition and identity of minerals and glasses weredetermined by energy-dispersive spectrometry using aJEOL-6400 instrument in Vienna The composition of someglass fragments and spherules was studied by quantitativewavelength-dispersive microprobe analysis at the NaturalHistory Museum in Vienna using an ARL-SEMQinstrument (acceleration voltage 15 kV beam current20 nA) and international mineral standards Analyses weredone using a defocused beam to avoid loss of Na Datareduction was done with standard ZAF proceduresDetection limits are at 001 wt and the precision of thedata is better than 5 rel

A further polished thin section was studied for

Fig 3 a) and b) View of a 25 mm wide section of sample 117-A1 with numerous glassy spherules and fragments as well as a variety ofminerals in a fine-grained matrix a) Plane-polarized light b) Cross-polarized light c) Plane-polarized light microphotograph of twoaccretionary lapilli-like agglomerated particles (the spherule shapes that are prominent in Fig 2) which contain some glass spherules (up toa few hundred microm in size) and fragments as well as mineral and lithic fragments The diameter of each lapilli is about 1 mm

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 713

petrography and chemical composition of glass fragmentsusing a JEOL-8800 electron microprobe instrument (NaturalHistory Museum at Berlin) operating at 15 kV and 15 nAAnalyses were calibrated using Smithsonian internationalmineral standards Counting times were 20 s on peak and 10 son background The beam size was defocused to 3 microm toavoid loss of sodium

In addition several of the glass spherules and fragmentsthat were isolated at the University of Delaware fromsubsample A1 were analyzed for trace elements byinstrumental neutron activation analysis (see Koeberl 1993for details on method) and subsequently embedded in epoxypolished and analyzed by electron microprobe in Vienna asnoted above

RESULTS

Mineralogical and Petrographic Observations (BulkSample)

The following observations are based on thin-sectionstudies by optical and electron microscopy on bulk samplesof subsample A1 Sample A1 consists of fine- to medium-grained mineral fragments (quartz feldspars mica opaquesincluding Fe-sulfide hematite and Fe-Ti-oxides chloritemica) as well as lithic (metagraywacke mica schist andgranite-derived) microclasts in a phyllosilicate-rich matrixthat is mostly fine-grained (generally lt150 microm) andcomposed of clastic material (of the same minerals as in thelarger fragments) Table 1 gives the relative abundances ofmajor and accessory mineral phases as identified by electronmicroscopy (mineral identification aided by EDX analysis)Besides clastic components spherical and droplettear-shapedglass particles as well as irregularly shaped glass fragmentsoccur abundantly in this sample The size of these particlesranges from several hundred micrometers to about 1 mm

Table 2 Size data and percentage of glass in samples 117A1 and 117A2

SampleSample weight(g)

Size fraction(microm)

Weight(g) Weight percent

Percent glass(by number)

117A1a 13368gt500 00000 000 na250ndash500 00663 496 78125ndash250 00032 024 7063ndash125 00362 271 9

lt63 12311b 9209

117A2 25160gt500 08241 3275 0250ndash500 01321 525 0125ndash250 01748 695 063ndash125 01833 729 02

lt63 12017b 4776aCarbonate fraction removed with dilute HClbEstimated by subtracting weights of coarser fractions from sample weight

Fig 4 Photographs of spherules recovered from sample 117A1a) Spherical glass particles exhibiting a variety of surface texturesfrom shiny smooth to deeply corroded b) Examples of dumbbellsand teardrops The tails are broken off all of the teardrops some wereoriginally dumbbells that were broken into two teardrops c) Threeexamples of teardrop pairs fused together

714 C Koeberl et al

Some rounded glass bodies are aggregated with otherspherules or even irregularly shaped glass particles mineralmicro-clasts and secondary alteration products (Figs 3andashc)Where surrounded by a thin (up to several hundred micromwide) rim of finest-grained clastic material theseaccumulations resemble accretionary lapilli as shown by thefour spherules that were thin-sectioned They are about onemillimeter in diameter and contain a variety of mineral andlithic fragments as well as smaller glass spherules andfragments (Fig 3c) The glass particles are usuallysurrounded by a very thin rim of alteration phases whichmay locally thicken in the form of embayments into the glassphase (possibly where shallow cracks form on the surface ofthe glass particles)

Tiny quartz feldspar or mica inclusions have beennoted within such rims Some spherules and fragments arefully or partially altered Replacements includephyllosilicates and locally some barite hematite and otheroxide phases Alteration noticeably progressed from theoutside inward into glass particles most likely alongfractures With optical microscopy two distinct glass phasescould be distinguished on the basis of their respective lackof color or yellowish appearance in plane-polarized light

The colorless variety is generally strongly fractured andpitted and some of these pits seem to be aligned alongpossible flow structures In contrast the yellowish phasecould be polished much better and has a very homogeneousappearance with locally incipient crystallization beingevident in the form of tiny crystallites and crystallitestrings Some shocked quartz grains were seen in the thinsections of sample 117A1s planar fractures (PFs) areabundant and a few grains with PDFs have been seen aswell (see below)

Fig 5 Polished grain mounts of glass particles recovered from sample 117A1 a) A seemingly homogeneous sphere b) A heterogeneous oval-shaped glass particle containing spherical vesicles and mineral inclusions The surface is somewhat pitted c) A pair of teardrops fusedtogether The teardrops have different colors and exhibit different amounts of surface pitting d) An elongated irregular glass particle exhibitingflow structure and containing vesicles and mineral inclusions e) An angular frosted glass fragment with two large vesicular silica inclusionswhich protrude from the surface probably due to differential solution f) A quartz grain in a spherule with two sets of planar features

Table 3 The abundance of components in the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction of samples 117A1 and 117A2

117A1 117A2Number Percent Number Percent

Rock fragments 257 295 495Quartz 175 280 153 257Feldspar 93 149 56 94Opaque grains 15 24 33 55Other 29 46 58 97Glass particles 57 91 1 02

626 1000 596 1000

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

710 C Koeberl et al

terrestrial meteorite impact structures with proximal ejecta inthe form of suevite and other impact breccia deposited outsidethe crater rim to a distance of about 1 crater radius The crateris particularly important because of its association with theIvory Coast tektites which were first reported by Lacroix(1934) from an 80 km wide area in Ivory Coast (CocirctedrsquoIvoire) territory Microtektites were found in deep-sea coresoff the coast of West Africa (Glass 1968 1969) and related tothe tektites found on land These microtektites are up to onemillimeter in size and show a variety of shapes mostlyspherical shapes droplets tear-drops dumbbells andfragments of particles of these respective shapes Thegeographical distribution of microtektite-bearing deep-seacores has been used to determine the extent of the strewn field(eg Glass and Zwart 1979 Glass et al 1979 1991) and themicrotektite abundance (numbercm2) and size distributionwas used to accurately predict the size of the source craterlocated at Lake Bosumtwi (Glass and Pizzuto 1994 Glasset al 1991)

Tektites are now known to have formed duringhypervelocity impacts on Earth and to represent melts ofsurficial predominantly sedimentary precursor rocks ofupper crustal composition (see eg Koeberl 1994Montanari and Koeberl 2000 and references therein) Several

lines of arguments were used to conclude that Bosumtwi ismost likely the source crater of the Ivory Coast tektite strewnfield These include similar chemical compositions(Schnetzler et al 1967 Jones 1985) and similar isotopiccharacteristics of the tektites and rocks found at the crater(eg Schnetzler et al 1966 Lippolt and Wasserburg 1966Shaw and Wasserburg 1982 Koeberl et al 1998) as well asthe similar ages of tektites and Bosumtwi impact glasses (egGentner et al 1964 Storzer and Wagner 1977) Precisefission track and 40Ar-39Ar step-heating dating on both IvoryCoast tektites and Bosumtwi impact glass established areliable age of 107 plusmn 005 Myr for the Bosumtwi impactevent and the tektites (Koeberl et al 1997) and themagnetostratigraphically determined age of the Ivory Coastmicrotektite layer also agrees with the age of the Bosumtwicrater providing a firm basis for the link between theBosumtwi impact and the tektite-forming event

CORING AT BOSUMTWI

Bosumtwi is a hydrologically closed basin which has ledto the preservation of laminated varves in the lake sedimentsproviding a means for high-resolution (annual) paleoclimatereconstruction in a region for which so far little data exists

Fig 1 A location map showing the location of borehole LB-05B which was terminated in the uppermost fallback layer as well as the positionsof boreholes LB-07A and LB-08A which penetrated impact breccias and fractured bedrock in the deep crater moat and the flank of the centraluplift respectively Also shown are profiles along which seismic data were recovered (cf Scholz et al 2002 Karp et al 2002) and that wereused to position the drill sites

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 711

Together with the importance of Bosumtwi for understandingimpact processes this provided ample reason for aninternational and multidisciplinary drilling project primarilyfinanced by the International Continental Scientific DrillingProgram (ICDP) (cf Koeberl et al 2005 2006 2007) Theproject had two main scientific goals paleoenvironmentalstudies and impact studies From July to October 2004 16boreholes were drilled at 6 locations within Lake Bosumtwias part of the ICDP drilling project The GLAD lake drillingsystem (specifically constructed for drilling at lakes seewwwdoseccorg Koeberl et al 2007) was used to core anentire lacustrine sediment fill from lake floor to bedrock Atfive sites 14 separate holes were drilled into the lakesediments at two sites LB-07A and LB-08A thicksequences of impactites and fractured bedrock wererecovered In core LB-05A (see Fig 1 for location) thecomplete ~1 Myr lacustrine sediment fill was recovered fromthe crater interior ending in a layer about 30 cm thick withabundant spherules on top the upper part of which appearedto be an ldquoaccretionaryrdquo lapilli-bearing sediment (Figs 2aand 2b) This unit likely represents the final fallback ejecta atthe base of the first post-impact sediment and provides animportant age constraint for the overlying sedimentarysequence The present paper describes the components of thisuppermost fallback ejecta unit

SAMPLES AND EXPERIMENTAL METHODS

Borehole LB-05B was drilled at a depth of 74 m layerwater (650052degN 141595degW) (Fig 1) the total depthreached was 37056 m and coring was done from a depth of7590 m to 37056 m for a total core recovery of 29467 mThe core penetrated lake sediments but at the very bottom ofthe core section 117 (see Fig 2 for a view of a section of thecore) a different rock unit was reached This unit with somespherule-like objects was below the bottom of the lake

sediments and assumed to be the uppermost impactite layerNo sedimentary structures are recognizable in this unit afterdrying the friable material has the form of irregularfragments The millimeter-size spherules that are prominentin Fig 2 have an appearance similar to accretionary lapilli(see below)

The ~30 cm thick uppermost fallback ejecta unit wasdivided into three subunits (117A1 117A2 and 117A3)Hereafter these subunits will be referred to as A1 A2 andA3 respectively Sample A1 is from the uppermost part ofthe fallback unit (~5 cm thick) sample A2 is immediatelybelow A1 (~10 cm) and A3 (the lowermost ~15 cm) is justbelow A2 Parts of the cores from these subunits wereavailable for the present study From those segments thinsections were prepared and various aliquots of (mostly ofsubpart A1) were studied in Vienna Berlin and Delaware Atthe University of Delaware samples A1 and A2 were

Fig 2 a) Core segment 117A1 from the bottom of corehole LB-05B 3705 m below lake level immediately after recovery of the core Corediameter is ~5 cm Photo courtesy M Talbot b) A close-up of a 4 mm wide region of the fragment shown in (a) displaying the ldquoaccretionaryrdquospheruleslapilli Photo courtesy J Peck

Table 1 Relative abundances of the major and accessory mineral components of accretionary spherules Sp-1 to Sp-4 and a bulk rock sample of subsample 117-A1 from core LB-05B

Sp-1 Sp-2 Sp-3 Sp-4 Bulk A1

Abundant Qz Qz Qz Qz Qz Ab Ab Ab Ab AbChl Chl Chl Chl ChlIll Ill Ill Ill Ill

Accessory Ap Ap Ap Py ApEpi Epi Py Ru EpiPy Py Ru Sph PlRu Py

RuSph

Mineral identifications by electron microscopy with EDX analysisAbbreviations Qz = quartz Ab = albite Chl = chlorite Ill = ldquoilliterdquoillite-hydro-muscovite Ap = apatite Epi = epidote Pl = plagioclasePy = pyrite Ru = rutile Sph = sphene

712 C Koeberl et al

searched for glass particles and shock-metamorphosedgrains This was done by disaggregating subsamples in waterusing ultrasonics and sieved into five size fractions (gt500250ndash500 125ndash250 63ndash125 lt63 m) Dilute HCl was usedin an attempt to help disaggregate A1 but very littlecarbonate was present A2 was disaggregated without usingdilute HCl Sample A3 could not be disaggregated and wascrushed to less than 250 m and sieved into three sizefractions (125ndash250 63ndash125 and lt63 m) The 63ndash125 msize fraction of each of the three samples was separated into aheavy (specific gravity gt296) and a light (specific gravitylt296) fraction The heavy mineral fractions were searchedfor high-pressure polymorphs (eg coesite reidite) Aportion of each light fraction was mounted in Piccolyte onpetrographic slides and searched for shock-metamorphosedgrains containing planar deformation features using apetrographic microscope

In addition 4 millimeter-size ldquospherulesrdquo as well as a

section of the core fragment ~6 mm wide wereimpregnated with epoxy and prepared for polished thinsections These were studied by optical and electronmicroscopy and electron microprobe analysis Scanning andbackscattered electron images were examined and thecomposition and identity of minerals and glasses weredetermined by energy-dispersive spectrometry using aJEOL-6400 instrument in Vienna The composition of someglass fragments and spherules was studied by quantitativewavelength-dispersive microprobe analysis at the NaturalHistory Museum in Vienna using an ARL-SEMQinstrument (acceleration voltage 15 kV beam current20 nA) and international mineral standards Analyses weredone using a defocused beam to avoid loss of Na Datareduction was done with standard ZAF proceduresDetection limits are at 001 wt and the precision of thedata is better than 5 rel

A further polished thin section was studied for

Fig 3 a) and b) View of a 25 mm wide section of sample 117-A1 with numerous glassy spherules and fragments as well as a variety ofminerals in a fine-grained matrix a) Plane-polarized light b) Cross-polarized light c) Plane-polarized light microphotograph of twoaccretionary lapilli-like agglomerated particles (the spherule shapes that are prominent in Fig 2) which contain some glass spherules (up toa few hundred microm in size) and fragments as well as mineral and lithic fragments The diameter of each lapilli is about 1 mm

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 713

petrography and chemical composition of glass fragmentsusing a JEOL-8800 electron microprobe instrument (NaturalHistory Museum at Berlin) operating at 15 kV and 15 nAAnalyses were calibrated using Smithsonian internationalmineral standards Counting times were 20 s on peak and 10 son background The beam size was defocused to 3 microm toavoid loss of sodium

In addition several of the glass spherules and fragmentsthat were isolated at the University of Delaware fromsubsample A1 were analyzed for trace elements byinstrumental neutron activation analysis (see Koeberl 1993for details on method) and subsequently embedded in epoxypolished and analyzed by electron microprobe in Vienna asnoted above

RESULTS

Mineralogical and Petrographic Observations (BulkSample)

The following observations are based on thin-sectionstudies by optical and electron microscopy on bulk samplesof subsample A1 Sample A1 consists of fine- to medium-grained mineral fragments (quartz feldspars mica opaquesincluding Fe-sulfide hematite and Fe-Ti-oxides chloritemica) as well as lithic (metagraywacke mica schist andgranite-derived) microclasts in a phyllosilicate-rich matrixthat is mostly fine-grained (generally lt150 microm) andcomposed of clastic material (of the same minerals as in thelarger fragments) Table 1 gives the relative abundances ofmajor and accessory mineral phases as identified by electronmicroscopy (mineral identification aided by EDX analysis)Besides clastic components spherical and droplettear-shapedglass particles as well as irregularly shaped glass fragmentsoccur abundantly in this sample The size of these particlesranges from several hundred micrometers to about 1 mm

Table 2 Size data and percentage of glass in samples 117A1 and 117A2

SampleSample weight(g)

Size fraction(microm)

Weight(g) Weight percent

Percent glass(by number)

117A1a 13368gt500 00000 000 na250ndash500 00663 496 78125ndash250 00032 024 7063ndash125 00362 271 9

lt63 12311b 9209

117A2 25160gt500 08241 3275 0250ndash500 01321 525 0125ndash250 01748 695 063ndash125 01833 729 02

lt63 12017b 4776aCarbonate fraction removed with dilute HClbEstimated by subtracting weights of coarser fractions from sample weight

Fig 4 Photographs of spherules recovered from sample 117A1a) Spherical glass particles exhibiting a variety of surface texturesfrom shiny smooth to deeply corroded b) Examples of dumbbellsand teardrops The tails are broken off all of the teardrops some wereoriginally dumbbells that were broken into two teardrops c) Threeexamples of teardrop pairs fused together

714 C Koeberl et al

Some rounded glass bodies are aggregated with otherspherules or even irregularly shaped glass particles mineralmicro-clasts and secondary alteration products (Figs 3andashc)Where surrounded by a thin (up to several hundred micromwide) rim of finest-grained clastic material theseaccumulations resemble accretionary lapilli as shown by thefour spherules that were thin-sectioned They are about onemillimeter in diameter and contain a variety of mineral andlithic fragments as well as smaller glass spherules andfragments (Fig 3c) The glass particles are usuallysurrounded by a very thin rim of alteration phases whichmay locally thicken in the form of embayments into the glassphase (possibly where shallow cracks form on the surface ofthe glass particles)

Tiny quartz feldspar or mica inclusions have beennoted within such rims Some spherules and fragments arefully or partially altered Replacements includephyllosilicates and locally some barite hematite and otheroxide phases Alteration noticeably progressed from theoutside inward into glass particles most likely alongfractures With optical microscopy two distinct glass phasescould be distinguished on the basis of their respective lackof color or yellowish appearance in plane-polarized light

The colorless variety is generally strongly fractured andpitted and some of these pits seem to be aligned alongpossible flow structures In contrast the yellowish phasecould be polished much better and has a very homogeneousappearance with locally incipient crystallization beingevident in the form of tiny crystallites and crystallitestrings Some shocked quartz grains were seen in the thinsections of sample 117A1s planar fractures (PFs) areabundant and a few grains with PDFs have been seen aswell (see below)

Fig 5 Polished grain mounts of glass particles recovered from sample 117A1 a) A seemingly homogeneous sphere b) A heterogeneous oval-shaped glass particle containing spherical vesicles and mineral inclusions The surface is somewhat pitted c) A pair of teardrops fusedtogether The teardrops have different colors and exhibit different amounts of surface pitting d) An elongated irregular glass particle exhibitingflow structure and containing vesicles and mineral inclusions e) An angular frosted glass fragment with two large vesicular silica inclusionswhich protrude from the surface probably due to differential solution f) A quartz grain in a spherule with two sets of planar features

Table 3 The abundance of components in the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction of samples 117A1 and 117A2

117A1 117A2Number Percent Number Percent

Rock fragments 257 295 495Quartz 175 280 153 257Feldspar 93 149 56 94Opaque grains 15 24 33 55Other 29 46 58 97Glass particles 57 91 1 02

626 1000 596 1000

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 711

Together with the importance of Bosumtwi for understandingimpact processes this provided ample reason for aninternational and multidisciplinary drilling project primarilyfinanced by the International Continental Scientific DrillingProgram (ICDP) (cf Koeberl et al 2005 2006 2007) Theproject had two main scientific goals paleoenvironmentalstudies and impact studies From July to October 2004 16boreholes were drilled at 6 locations within Lake Bosumtwias part of the ICDP drilling project The GLAD lake drillingsystem (specifically constructed for drilling at lakes seewwwdoseccorg Koeberl et al 2007) was used to core anentire lacustrine sediment fill from lake floor to bedrock Atfive sites 14 separate holes were drilled into the lakesediments at two sites LB-07A and LB-08A thicksequences of impactites and fractured bedrock wererecovered In core LB-05A (see Fig 1 for location) thecomplete ~1 Myr lacustrine sediment fill was recovered fromthe crater interior ending in a layer about 30 cm thick withabundant spherules on top the upper part of which appearedto be an ldquoaccretionaryrdquo lapilli-bearing sediment (Figs 2aand 2b) This unit likely represents the final fallback ejecta atthe base of the first post-impact sediment and provides animportant age constraint for the overlying sedimentarysequence The present paper describes the components of thisuppermost fallback ejecta unit

SAMPLES AND EXPERIMENTAL METHODS

Borehole LB-05B was drilled at a depth of 74 m layerwater (650052degN 141595degW) (Fig 1) the total depthreached was 37056 m and coring was done from a depth of7590 m to 37056 m for a total core recovery of 29467 mThe core penetrated lake sediments but at the very bottom ofthe core section 117 (see Fig 2 for a view of a section of thecore) a different rock unit was reached This unit with somespherule-like objects was below the bottom of the lake

sediments and assumed to be the uppermost impactite layerNo sedimentary structures are recognizable in this unit afterdrying the friable material has the form of irregularfragments The millimeter-size spherules that are prominentin Fig 2 have an appearance similar to accretionary lapilli(see below)

The ~30 cm thick uppermost fallback ejecta unit wasdivided into three subunits (117A1 117A2 and 117A3)Hereafter these subunits will be referred to as A1 A2 andA3 respectively Sample A1 is from the uppermost part ofthe fallback unit (~5 cm thick) sample A2 is immediatelybelow A1 (~10 cm) and A3 (the lowermost ~15 cm) is justbelow A2 Parts of the cores from these subunits wereavailable for the present study From those segments thinsections were prepared and various aliquots of (mostly ofsubpart A1) were studied in Vienna Berlin and Delaware Atthe University of Delaware samples A1 and A2 were

Fig 2 a) Core segment 117A1 from the bottom of corehole LB-05B 3705 m below lake level immediately after recovery of the core Corediameter is ~5 cm Photo courtesy M Talbot b) A close-up of a 4 mm wide region of the fragment shown in (a) displaying the ldquoaccretionaryrdquospheruleslapilli Photo courtesy J Peck

Table 1 Relative abundances of the major and accessory mineral components of accretionary spherules Sp-1 to Sp-4 and a bulk rock sample of subsample 117-A1 from core LB-05B

Sp-1 Sp-2 Sp-3 Sp-4 Bulk A1

Abundant Qz Qz Qz Qz Qz Ab Ab Ab Ab AbChl Chl Chl Chl ChlIll Ill Ill Ill Ill

Accessory Ap Ap Ap Py ApEpi Epi Py Ru EpiPy Py Ru Sph PlRu Py

RuSph

Mineral identifications by electron microscopy with EDX analysisAbbreviations Qz = quartz Ab = albite Chl = chlorite Ill = ldquoilliterdquoillite-hydro-muscovite Ap = apatite Epi = epidote Pl = plagioclasePy = pyrite Ru = rutile Sph = sphene

712 C Koeberl et al

searched for glass particles and shock-metamorphosedgrains This was done by disaggregating subsamples in waterusing ultrasonics and sieved into five size fractions (gt500250ndash500 125ndash250 63ndash125 lt63 m) Dilute HCl was usedin an attempt to help disaggregate A1 but very littlecarbonate was present A2 was disaggregated without usingdilute HCl Sample A3 could not be disaggregated and wascrushed to less than 250 m and sieved into three sizefractions (125ndash250 63ndash125 and lt63 m) The 63ndash125 msize fraction of each of the three samples was separated into aheavy (specific gravity gt296) and a light (specific gravitylt296) fraction The heavy mineral fractions were searchedfor high-pressure polymorphs (eg coesite reidite) Aportion of each light fraction was mounted in Piccolyte onpetrographic slides and searched for shock-metamorphosedgrains containing planar deformation features using apetrographic microscope

In addition 4 millimeter-size ldquospherulesrdquo as well as a

section of the core fragment ~6 mm wide wereimpregnated with epoxy and prepared for polished thinsections These were studied by optical and electronmicroscopy and electron microprobe analysis Scanning andbackscattered electron images were examined and thecomposition and identity of minerals and glasses weredetermined by energy-dispersive spectrometry using aJEOL-6400 instrument in Vienna The composition of someglass fragments and spherules was studied by quantitativewavelength-dispersive microprobe analysis at the NaturalHistory Museum in Vienna using an ARL-SEMQinstrument (acceleration voltage 15 kV beam current20 nA) and international mineral standards Analyses weredone using a defocused beam to avoid loss of Na Datareduction was done with standard ZAF proceduresDetection limits are at 001 wt and the precision of thedata is better than 5 rel

A further polished thin section was studied for

Fig 3 a) and b) View of a 25 mm wide section of sample 117-A1 with numerous glassy spherules and fragments as well as a variety ofminerals in a fine-grained matrix a) Plane-polarized light b) Cross-polarized light c) Plane-polarized light microphotograph of twoaccretionary lapilli-like agglomerated particles (the spherule shapes that are prominent in Fig 2) which contain some glass spherules (up toa few hundred microm in size) and fragments as well as mineral and lithic fragments The diameter of each lapilli is about 1 mm

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 713

petrography and chemical composition of glass fragmentsusing a JEOL-8800 electron microprobe instrument (NaturalHistory Museum at Berlin) operating at 15 kV and 15 nAAnalyses were calibrated using Smithsonian internationalmineral standards Counting times were 20 s on peak and 10 son background The beam size was defocused to 3 microm toavoid loss of sodium

In addition several of the glass spherules and fragmentsthat were isolated at the University of Delaware fromsubsample A1 were analyzed for trace elements byinstrumental neutron activation analysis (see Koeberl 1993for details on method) and subsequently embedded in epoxypolished and analyzed by electron microprobe in Vienna asnoted above

RESULTS

Mineralogical and Petrographic Observations (BulkSample)

The following observations are based on thin-sectionstudies by optical and electron microscopy on bulk samplesof subsample A1 Sample A1 consists of fine- to medium-grained mineral fragments (quartz feldspars mica opaquesincluding Fe-sulfide hematite and Fe-Ti-oxides chloritemica) as well as lithic (metagraywacke mica schist andgranite-derived) microclasts in a phyllosilicate-rich matrixthat is mostly fine-grained (generally lt150 microm) andcomposed of clastic material (of the same minerals as in thelarger fragments) Table 1 gives the relative abundances ofmajor and accessory mineral phases as identified by electronmicroscopy (mineral identification aided by EDX analysis)Besides clastic components spherical and droplettear-shapedglass particles as well as irregularly shaped glass fragmentsoccur abundantly in this sample The size of these particlesranges from several hundred micrometers to about 1 mm

Table 2 Size data and percentage of glass in samples 117A1 and 117A2

SampleSample weight(g)

Size fraction(microm)

Weight(g) Weight percent

Percent glass(by number)

117A1a 13368gt500 00000 000 na250ndash500 00663 496 78125ndash250 00032 024 7063ndash125 00362 271 9

lt63 12311b 9209

117A2 25160gt500 08241 3275 0250ndash500 01321 525 0125ndash250 01748 695 063ndash125 01833 729 02

lt63 12017b 4776aCarbonate fraction removed with dilute HClbEstimated by subtracting weights of coarser fractions from sample weight

Fig 4 Photographs of spherules recovered from sample 117A1a) Spherical glass particles exhibiting a variety of surface texturesfrom shiny smooth to deeply corroded b) Examples of dumbbellsand teardrops The tails are broken off all of the teardrops some wereoriginally dumbbells that were broken into two teardrops c) Threeexamples of teardrop pairs fused together

714 C Koeberl et al

Some rounded glass bodies are aggregated with otherspherules or even irregularly shaped glass particles mineralmicro-clasts and secondary alteration products (Figs 3andashc)Where surrounded by a thin (up to several hundred micromwide) rim of finest-grained clastic material theseaccumulations resemble accretionary lapilli as shown by thefour spherules that were thin-sectioned They are about onemillimeter in diameter and contain a variety of mineral andlithic fragments as well as smaller glass spherules andfragments (Fig 3c) The glass particles are usuallysurrounded by a very thin rim of alteration phases whichmay locally thicken in the form of embayments into the glassphase (possibly where shallow cracks form on the surface ofthe glass particles)

Tiny quartz feldspar or mica inclusions have beennoted within such rims Some spherules and fragments arefully or partially altered Replacements includephyllosilicates and locally some barite hematite and otheroxide phases Alteration noticeably progressed from theoutside inward into glass particles most likely alongfractures With optical microscopy two distinct glass phasescould be distinguished on the basis of their respective lackof color or yellowish appearance in plane-polarized light

The colorless variety is generally strongly fractured andpitted and some of these pits seem to be aligned alongpossible flow structures In contrast the yellowish phasecould be polished much better and has a very homogeneousappearance with locally incipient crystallization beingevident in the form of tiny crystallites and crystallitestrings Some shocked quartz grains were seen in the thinsections of sample 117A1s planar fractures (PFs) areabundant and a few grains with PDFs have been seen aswell (see below)

Fig 5 Polished grain mounts of glass particles recovered from sample 117A1 a) A seemingly homogeneous sphere b) A heterogeneous oval-shaped glass particle containing spherical vesicles and mineral inclusions The surface is somewhat pitted c) A pair of teardrops fusedtogether The teardrops have different colors and exhibit different amounts of surface pitting d) An elongated irregular glass particle exhibitingflow structure and containing vesicles and mineral inclusions e) An angular frosted glass fragment with two large vesicular silica inclusionswhich protrude from the surface probably due to differential solution f) A quartz grain in a spherule with two sets of planar features

Table 3 The abundance of components in the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction of samples 117A1 and 117A2

117A1 117A2Number Percent Number Percent

Rock fragments 257 295 495Quartz 175 280 153 257Feldspar 93 149 56 94Opaque grains 15 24 33 55Other 29 46 58 97Glass particles 57 91 1 02

626 1000 596 1000

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

712 C Koeberl et al

searched for glass particles and shock-metamorphosedgrains This was done by disaggregating subsamples in waterusing ultrasonics and sieved into five size fractions (gt500250ndash500 125ndash250 63ndash125 lt63 m) Dilute HCl was usedin an attempt to help disaggregate A1 but very littlecarbonate was present A2 was disaggregated without usingdilute HCl Sample A3 could not be disaggregated and wascrushed to less than 250 m and sieved into three sizefractions (125ndash250 63ndash125 and lt63 m) The 63ndash125 msize fraction of each of the three samples was separated into aheavy (specific gravity gt296) and a light (specific gravitylt296) fraction The heavy mineral fractions were searchedfor high-pressure polymorphs (eg coesite reidite) Aportion of each light fraction was mounted in Piccolyte onpetrographic slides and searched for shock-metamorphosedgrains containing planar deformation features using apetrographic microscope

In addition 4 millimeter-size ldquospherulesrdquo as well as a

section of the core fragment ~6 mm wide wereimpregnated with epoxy and prepared for polished thinsections These were studied by optical and electronmicroscopy and electron microprobe analysis Scanning andbackscattered electron images were examined and thecomposition and identity of minerals and glasses weredetermined by energy-dispersive spectrometry using aJEOL-6400 instrument in Vienna The composition of someglass fragments and spherules was studied by quantitativewavelength-dispersive microprobe analysis at the NaturalHistory Museum in Vienna using an ARL-SEMQinstrument (acceleration voltage 15 kV beam current20 nA) and international mineral standards Analyses weredone using a defocused beam to avoid loss of Na Datareduction was done with standard ZAF proceduresDetection limits are at 001 wt and the precision of thedata is better than 5 rel

A further polished thin section was studied for

Fig 3 a) and b) View of a 25 mm wide section of sample 117-A1 with numerous glassy spherules and fragments as well as a variety ofminerals in a fine-grained matrix a) Plane-polarized light b) Cross-polarized light c) Plane-polarized light microphotograph of twoaccretionary lapilli-like agglomerated particles (the spherule shapes that are prominent in Fig 2) which contain some glass spherules (up toa few hundred microm in size) and fragments as well as mineral and lithic fragments The diameter of each lapilli is about 1 mm

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 713

petrography and chemical composition of glass fragmentsusing a JEOL-8800 electron microprobe instrument (NaturalHistory Museum at Berlin) operating at 15 kV and 15 nAAnalyses were calibrated using Smithsonian internationalmineral standards Counting times were 20 s on peak and 10 son background The beam size was defocused to 3 microm toavoid loss of sodium

In addition several of the glass spherules and fragmentsthat were isolated at the University of Delaware fromsubsample A1 were analyzed for trace elements byinstrumental neutron activation analysis (see Koeberl 1993for details on method) and subsequently embedded in epoxypolished and analyzed by electron microprobe in Vienna asnoted above

RESULTS

Mineralogical and Petrographic Observations (BulkSample)

The following observations are based on thin-sectionstudies by optical and electron microscopy on bulk samplesof subsample A1 Sample A1 consists of fine- to medium-grained mineral fragments (quartz feldspars mica opaquesincluding Fe-sulfide hematite and Fe-Ti-oxides chloritemica) as well as lithic (metagraywacke mica schist andgranite-derived) microclasts in a phyllosilicate-rich matrixthat is mostly fine-grained (generally lt150 microm) andcomposed of clastic material (of the same minerals as in thelarger fragments) Table 1 gives the relative abundances ofmajor and accessory mineral phases as identified by electronmicroscopy (mineral identification aided by EDX analysis)Besides clastic components spherical and droplettear-shapedglass particles as well as irregularly shaped glass fragmentsoccur abundantly in this sample The size of these particlesranges from several hundred micrometers to about 1 mm

Table 2 Size data and percentage of glass in samples 117A1 and 117A2

SampleSample weight(g)

Size fraction(microm)

Weight(g) Weight percent

Percent glass(by number)

117A1a 13368gt500 00000 000 na250ndash500 00663 496 78125ndash250 00032 024 7063ndash125 00362 271 9

lt63 12311b 9209

117A2 25160gt500 08241 3275 0250ndash500 01321 525 0125ndash250 01748 695 063ndash125 01833 729 02

lt63 12017b 4776aCarbonate fraction removed with dilute HClbEstimated by subtracting weights of coarser fractions from sample weight

Fig 4 Photographs of spherules recovered from sample 117A1a) Spherical glass particles exhibiting a variety of surface texturesfrom shiny smooth to deeply corroded b) Examples of dumbbellsand teardrops The tails are broken off all of the teardrops some wereoriginally dumbbells that were broken into two teardrops c) Threeexamples of teardrop pairs fused together

714 C Koeberl et al

Some rounded glass bodies are aggregated with otherspherules or even irregularly shaped glass particles mineralmicro-clasts and secondary alteration products (Figs 3andashc)Where surrounded by a thin (up to several hundred micromwide) rim of finest-grained clastic material theseaccumulations resemble accretionary lapilli as shown by thefour spherules that were thin-sectioned They are about onemillimeter in diameter and contain a variety of mineral andlithic fragments as well as smaller glass spherules andfragments (Fig 3c) The glass particles are usuallysurrounded by a very thin rim of alteration phases whichmay locally thicken in the form of embayments into the glassphase (possibly where shallow cracks form on the surface ofthe glass particles)

Tiny quartz feldspar or mica inclusions have beennoted within such rims Some spherules and fragments arefully or partially altered Replacements includephyllosilicates and locally some barite hematite and otheroxide phases Alteration noticeably progressed from theoutside inward into glass particles most likely alongfractures With optical microscopy two distinct glass phasescould be distinguished on the basis of their respective lackof color or yellowish appearance in plane-polarized light

The colorless variety is generally strongly fractured andpitted and some of these pits seem to be aligned alongpossible flow structures In contrast the yellowish phasecould be polished much better and has a very homogeneousappearance with locally incipient crystallization beingevident in the form of tiny crystallites and crystallitestrings Some shocked quartz grains were seen in the thinsections of sample 117A1s planar fractures (PFs) areabundant and a few grains with PDFs have been seen aswell (see below)

Fig 5 Polished grain mounts of glass particles recovered from sample 117A1 a) A seemingly homogeneous sphere b) A heterogeneous oval-shaped glass particle containing spherical vesicles and mineral inclusions The surface is somewhat pitted c) A pair of teardrops fusedtogether The teardrops have different colors and exhibit different amounts of surface pitting d) An elongated irregular glass particle exhibitingflow structure and containing vesicles and mineral inclusions e) An angular frosted glass fragment with two large vesicular silica inclusionswhich protrude from the surface probably due to differential solution f) A quartz grain in a spherule with two sets of planar features

Table 3 The abundance of components in the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction of samples 117A1 and 117A2

117A1 117A2Number Percent Number Percent

Rock fragments 257 295 495Quartz 175 280 153 257Feldspar 93 149 56 94Opaque grains 15 24 33 55Other 29 46 58 97Glass particles 57 91 1 02

626 1000 596 1000

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 713

petrography and chemical composition of glass fragmentsusing a JEOL-8800 electron microprobe instrument (NaturalHistory Museum at Berlin) operating at 15 kV and 15 nAAnalyses were calibrated using Smithsonian internationalmineral standards Counting times were 20 s on peak and 10 son background The beam size was defocused to 3 microm toavoid loss of sodium

In addition several of the glass spherules and fragmentsthat were isolated at the University of Delaware fromsubsample A1 were analyzed for trace elements byinstrumental neutron activation analysis (see Koeberl 1993for details on method) and subsequently embedded in epoxypolished and analyzed by electron microprobe in Vienna asnoted above

RESULTS

Mineralogical and Petrographic Observations (BulkSample)

The following observations are based on thin-sectionstudies by optical and electron microscopy on bulk samplesof subsample A1 Sample A1 consists of fine- to medium-grained mineral fragments (quartz feldspars mica opaquesincluding Fe-sulfide hematite and Fe-Ti-oxides chloritemica) as well as lithic (metagraywacke mica schist andgranite-derived) microclasts in a phyllosilicate-rich matrixthat is mostly fine-grained (generally lt150 microm) andcomposed of clastic material (of the same minerals as in thelarger fragments) Table 1 gives the relative abundances ofmajor and accessory mineral phases as identified by electronmicroscopy (mineral identification aided by EDX analysis)Besides clastic components spherical and droplettear-shapedglass particles as well as irregularly shaped glass fragmentsoccur abundantly in this sample The size of these particlesranges from several hundred micrometers to about 1 mm

Table 2 Size data and percentage of glass in samples 117A1 and 117A2

SampleSample weight(g)

Size fraction(microm)

Weight(g) Weight percent

Percent glass(by number)

117A1a 13368gt500 00000 000 na250ndash500 00663 496 78125ndash250 00032 024 7063ndash125 00362 271 9

lt63 12311b 9209

117A2 25160gt500 08241 3275 0250ndash500 01321 525 0125ndash250 01748 695 063ndash125 01833 729 02

lt63 12017b 4776aCarbonate fraction removed with dilute HClbEstimated by subtracting weights of coarser fractions from sample weight

Fig 4 Photographs of spherules recovered from sample 117A1a) Spherical glass particles exhibiting a variety of surface texturesfrom shiny smooth to deeply corroded b) Examples of dumbbellsand teardrops The tails are broken off all of the teardrops some wereoriginally dumbbells that were broken into two teardrops c) Threeexamples of teardrop pairs fused together

714 C Koeberl et al

Some rounded glass bodies are aggregated with otherspherules or even irregularly shaped glass particles mineralmicro-clasts and secondary alteration products (Figs 3andashc)Where surrounded by a thin (up to several hundred micromwide) rim of finest-grained clastic material theseaccumulations resemble accretionary lapilli as shown by thefour spherules that were thin-sectioned They are about onemillimeter in diameter and contain a variety of mineral andlithic fragments as well as smaller glass spherules andfragments (Fig 3c) The glass particles are usuallysurrounded by a very thin rim of alteration phases whichmay locally thicken in the form of embayments into the glassphase (possibly where shallow cracks form on the surface ofthe glass particles)

Tiny quartz feldspar or mica inclusions have beennoted within such rims Some spherules and fragments arefully or partially altered Replacements includephyllosilicates and locally some barite hematite and otheroxide phases Alteration noticeably progressed from theoutside inward into glass particles most likely alongfractures With optical microscopy two distinct glass phasescould be distinguished on the basis of their respective lackof color or yellowish appearance in plane-polarized light

The colorless variety is generally strongly fractured andpitted and some of these pits seem to be aligned alongpossible flow structures In contrast the yellowish phasecould be polished much better and has a very homogeneousappearance with locally incipient crystallization beingevident in the form of tiny crystallites and crystallitestrings Some shocked quartz grains were seen in the thinsections of sample 117A1s planar fractures (PFs) areabundant and a few grains with PDFs have been seen aswell (see below)

Fig 5 Polished grain mounts of glass particles recovered from sample 117A1 a) A seemingly homogeneous sphere b) A heterogeneous oval-shaped glass particle containing spherical vesicles and mineral inclusions The surface is somewhat pitted c) A pair of teardrops fusedtogether The teardrops have different colors and exhibit different amounts of surface pitting d) An elongated irregular glass particle exhibitingflow structure and containing vesicles and mineral inclusions e) An angular frosted glass fragment with two large vesicular silica inclusionswhich protrude from the surface probably due to differential solution f) A quartz grain in a spherule with two sets of planar features

Table 3 The abundance of components in the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction of samples 117A1 and 117A2

117A1 117A2Number Percent Number Percent

Rock fragments 257 295 495Quartz 175 280 153 257Feldspar 93 149 56 94Opaque grains 15 24 33 55Other 29 46 58 97Glass particles 57 91 1 02

626 1000 596 1000

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

714 C Koeberl et al

Some rounded glass bodies are aggregated with otherspherules or even irregularly shaped glass particles mineralmicro-clasts and secondary alteration products (Figs 3andashc)Where surrounded by a thin (up to several hundred micromwide) rim of finest-grained clastic material theseaccumulations resemble accretionary lapilli as shown by thefour spherules that were thin-sectioned They are about onemillimeter in diameter and contain a variety of mineral andlithic fragments as well as smaller glass spherules andfragments (Fig 3c) The glass particles are usuallysurrounded by a very thin rim of alteration phases whichmay locally thicken in the form of embayments into the glassphase (possibly where shallow cracks form on the surface ofthe glass particles)

Tiny quartz feldspar or mica inclusions have beennoted within such rims Some spherules and fragments arefully or partially altered Replacements includephyllosilicates and locally some barite hematite and otheroxide phases Alteration noticeably progressed from theoutside inward into glass particles most likely alongfractures With optical microscopy two distinct glass phasescould be distinguished on the basis of their respective lackof color or yellowish appearance in plane-polarized light

The colorless variety is generally strongly fractured andpitted and some of these pits seem to be aligned alongpossible flow structures In contrast the yellowish phasecould be polished much better and has a very homogeneousappearance with locally incipient crystallization beingevident in the form of tiny crystallites and crystallitestrings Some shocked quartz grains were seen in the thinsections of sample 117A1s planar fractures (PFs) areabundant and a few grains with PDFs have been seen aswell (see below)

Fig 5 Polished grain mounts of glass particles recovered from sample 117A1 a) A seemingly homogeneous sphere b) A heterogeneous oval-shaped glass particle containing spherical vesicles and mineral inclusions The surface is somewhat pitted c) A pair of teardrops fusedtogether The teardrops have different colors and exhibit different amounts of surface pitting d) An elongated irregular glass particle exhibitingflow structure and containing vesicles and mineral inclusions e) An angular frosted glass fragment with two large vesicular silica inclusionswhich protrude from the surface probably due to differential solution f) A quartz grain in a spherule with two sets of planar features

Table 3 The abundance of components in the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction of samples 117A1 and 117A2

117A1 117A2Number Percent Number Percent

Rock fragments 257 295 495Quartz 175 280 153 257Feldspar 93 149 56 94Opaque grains 15 24 33 55Other 29 46 58 97Glass particles 57 91 1 02

626 1000 596 1000

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 715

Grain Size and Composition

It is clear that sample A2 is much coarser-grained thanA1 (Table 2) No grains gt500 m in size were recovered fromA1 but grains gt500 m make up ~33 wt of A2 Converselythe lt63 size fraction makes up ~92 wt of A1 but only~48 wt of A2 No size data were obtained for A3 as thissample could not be disaggregated The gt500 microm size fractionof A2 consists primarily of rock fragments (generally schists)and some mineral grains (mostly quartz) Quartz grains areabundant in the 63ndash125 m size fraction of both A1 and A2but extremely rare in A3 Glass particles make up the bulk ofthe grains (~70 to 78 by number) in the coarser size

fractions (gt125 m) but only ~9 by number in the 63ndash125 m size fraction of A1 On the other hand no glassparticles were recovered from the gt125 m size fraction ofA2 and only a trace of glass was observed in the 63ndash125 msize fraction of A2 No glass particles were observed in A3only mineral grains and fragments

Glass Particles

No glass particles were found in the gt125 m sizefraction of A2 or A3 thus the following discussion appliesonly to sample A1 The glass particles consist of splash forms(spheres ovals teardrops dumbbells and discs) irregular

Fig 6 Examples of grains from sample 117A1 which exhibit planar deformation features (PDFs) a) A quartz grain containing two well-defined sets of PDFs b) A quartz grain containing two sets of PDFs c) A quartz grain containing 3 sets of PDFs d) A polycrystalline grainconsisting of quartz One subgrain contains three sets of PDFs the other grain two sets of PDFs e) A translucent pale yellowish brown(toasted) quartz grain containing at least 4 sets of PDFs f) A rock fragment containing a quartz grain with two sets of PDFs

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

716 C Koeberl et al

shapes and fragments (Figs 4 and 5) Most glass particles arepale brown in color but a few are dark brown They arevariably transparent to translucent Some have some shinysmooth surfaces but most are pitted or deeply corroded Thetranslucent ones generally have a finely pitted surface givingthem a ldquofrostedrdquo appearance Some have really exotic shapesthat appear to have formed by solution of splash formsleaving silica-rich regions which based on their opticalappearance may be lechatelierite

Whole splash forms make up ~38 by number of theglass particles in the 250ndash500 m size fraction but only ~9of the glass particles in the 125ndash250 m size fractionConversely glass fragments make up nearly 40 by numberof the 250ndash500 m size fraction and 70 by number of the125ndash250 m size fraction The remainder of the glassparticles are obvious fragments of splash forms badly etchedparticles that could have originally been splash forms andirregular forms Approximately 80 by number of the splashforms are spherical to oval in shape The remainder are

Fig 7 Color images of single quartz grains from fallback layer 111A1 showing multiple sets of planar deformation features (PDFs)

Table 4 Percent grains containing planar deformation featuresa

117A1 117A2Number Percent Number Percent

Rock fragmentsWithout PDFs 229 891 281 953With PDFs 28 109 14 47Total 257 295

QuartzWithout PDFs 124 709 143 935With 1 set 14 8 5 33With 2 sets 14 8 4 26With 3 or more sets 23 131 1 07Total 175 153

FeldsparWithout PDFs 72 774 48 857With PDFs 21 226 8 143Total 93 56

aIn the light fraction (specific gravity lt296) of the 63ndash125 microm size fraction

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 717

Table 5 Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

A1-1 6460 075 1623 lt001 604 326 330 315 208 9941A1-2 6408 069 1656 002 590 310 321 306 199 9861A1-3 6530 070 1684 lt001 617 319 323 302 201 10046A1-4 6631 071 1669 lt001 610 310 332 306 202 10131A1-5 6638 064 1632 006 586 307 327 318 203 10081A1-6 6664 061 1604 005 553 269 303 303 210 9972A1-7 6981 057 1387 lt001 467 238 253 280 198 9861A1-8 7260 045 1207 lt001 403 188 211 258 204 9776A1-9 7456 040 1074 lt001 357 161 174 226 203 9691A1-10 7527 037 978 lt001 346 149 157 189 188 9571Avg(1-7) 6616 067 1608 004 575 297 313 304 203

A2-1 6666 067 1648 003 588 283 328 310 210 10103A2-2 6678 066 1634 002 606 291 330 308 213 10128A2-3 6627 064 1631 003 568 286 316 309 211 10015A2-4 6565 068 1622 002 564 294 329 319 210 9973A2-5 6562 067 1617 005 573 288 320 303 209 9944A2-6 6818 063 1585 003 562 275 302 288 208 10104A2-7 6770 058 1517 lt001 538 256 301 290 198 9928A2-8 6624 059 1547 003 555 275 311 297 207 9878A2-9 6515 065 1624 lt001 595 283 325 296 213 9916A2-10 6614 069 1628 lt001 586 292 327 299 202 10017Avg(1-10) 6644 065 1605 003 574 282 319 302 208

B1-1 6835 046 1544 003 509 243 265 326 225 9996B1-2 6830 048 1534 002 472 243 269 308 217 9923B1-3 6923 049 1528 lt001 471 227 268 308 216 9990B1-4 6975 050 1559 lt001 489 231 271 294 208 10077B1-5 7006 051 1504 lt001 467 211 266 308 207 10020B1-6 7024 052 1526 lt001 458 217 267 304 206 10054B1-7 7000 055 1542 lt001 472 226 266 324 206 10091B1-8 7014 050 1511 lt001 471 222 254 328 199 10049B1-9 7028 050 1511 lt001 469 222 255 322 202 10059B1-10 6974 051 1545 lt001 456 224 258 316 190 10014Avg(1-10) 6961 050 1530 003 473 227 264 314 208

B2-1 6979 065 1839 lt001 476 128 104 225 204 10020B2-2 7138 059 1778 lt001 465 122 084 204 210 10060B2-3 7313 056 1647 lt001 436 109 069 175 214 10019B2-4 7486 055 1555 lt001 423 098 060 161 203 10041B2-5 7551 048 1539 lt001 432 101 062 138 196 10067B2-6 7320 054 1688 lt001 490 109 070 168 194 10093B2-7 7014 059 1877 lt001 531 131 082 181 189 10064B2-8 6948 059 1913 lt001 577 144 086 190 183 10100B2-9 6904 059 1933 lt001 605 138 092 182 186 10099B2-10 6859 063 1922 lt001 600 145 093 203 180 10065Avg(1-10) 7151 058 1769 lt001 504 123 080 183 196

C1-1 6518 064 1854 lt001 608 352 244 255 188 10083C1-2 6554 074 1820 lt001 599 356 248 267 180 10098C1-3 6537 073 1827 lt001 606 352 249 270 185 10099C1-4 6535 079 1836 lt001 582 355 248 263 180 10078C1-5 6575 069 1807 003 619 343 244 258 180 10098C1-6 6618 070 1771 lt001 608 340 240 261 184 10092C1-7 6648 074 1748 lt001 571 330 239 277 184 10071C1-8 6638 069 1773 lt001 582 330 230 258 180 10060C1-9 6614 068 1778 lt001 575 332 238 254 181 10040

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

718 C Koeberl et al

C1-10 6625 077 1758 lt001 604 345 238 261 175 10083Avg(1-10) 6586 072 1797 003 595 344 242 262 182

C2-1 5552 052 1539 lt001 549 254 155 191 155 8447C2-2 6455 060 1729 003 573 251 193 201 183 9648C2-3 6691 064 1743 lt001 558 252 193 211 184 9896C2-4 6851 065 1696 lt001 573 259 196 198 188 10026C2-5 6988 064 1724 lt001 567 245 188 198 187 10161C2-6 6958 069 1722 lt001 573 249 190 186 184 10131C2-7 6931 065 1645 lt001 573 246 194 180 191 10025C2-8 7025 063 1618 lt001 549 229 183 186 192 10045C2-9 6889 062 1650 lt001 558 241 182 194 186 9962C2-10 6781 067 1726 002 597 256 200 186 181 9996Avg(3-10) 6918 065 1683 002 570 246 190 190 187

D1-1 6727 061 1573 lt001 611 292 240 248 170 9922D1-2 6764 065 1575 lt001 600 303 229 237 179 9952D1-3 6761 064 1588 lt001 608 280 239 227 173 9940D1-4 6765 060 1626 lt001 615 301 240 268 166 10041D1-5 6654 063 1589 003 606 284 246 248 174 9867D1-6 6724 060 1576 lt001 586 296 238 232 174 9886D1-7 6652 064 1596 003 600 298 234 237 171 9855D1-8 6717 062 1570 002 593 294 244 247 167 9896D1-9 6653 061 1580 003 600 289 246 241 167 9840D1-10 6599 062 1605 lt001 602 308 247 245 173 9841Avg(1-10) 6702 062 1588 003 602 295 240 243 171

D2-1 6605 074 1731 003 602 326 275 235 176 10027D2-2 6661 070 1717 002 601 323 276 241 175 10066D2-3 6673 074 1713 lt001 624 323 282 214 172 10075D2-4 6653 066 1742 lt001 624 309 283 239 170 10086D2-5 6676 076 1727 lt001 599 318 283 230 160 10069D2-6 6655 072 1762 003 571 319 284 224 171 10061D2-7 6648 071 1731 005 593 311 285 220 174 10038D2-8 6611 073 1716 003 615 320 285 229 164 10016D2-9 6685 066 1729 lt001 593 322 276 216 170 10057D2-10 6588 065 1661 lt001 615 309 275 220 165 9898Avg(1-10) 6646 071 1723 003 604 318 280 227 170

E1-1 6669 037 909 lt001 835 185 181 363 281 9460E1-2 7016 036 911 lt001 677 131 107 300 315 9493E1-3 6865 037 990 lt001 637 109 085 292 329 9344E2-1 5968 044 1103 008 1178 370 373 408 229 9681E2-2 6030 050 1135 009 1168 355 369 396 229 9741E2-3 6045 048 1204 005 1028 320 330 397 247 9624

F1-1 6668 075 1648 009 580 336 273 212 174 9975F1-2 6638 069 1627 008 600 335 277 211 169 9934F1-3 6574 068 1715 012 576 325 286 212 159 9927F1-4 6588 066 1748 012 611 325 291 211 160 10012F1-5 6675 072 1679 012 613 335 296 227 170 10079F1-6 6734 075 1677 012 605 322 285 211 169 10090F1-7 6712 070 1674 014 606 313 290 229 169 10077F1-8 6703 070 1704 014 572 302 287 222 176 10050F1-9 6722 069 1714 012 598 320 304 221 166 10126F1-10 6701 067 1733 017 628 322 310 230 170 10178Avg(1-10) 6672 070 1692 012 599 324 290 219 168

F2-1 6514 068 1654 018 617 322 326 320 198 10037F2-2 6546 067 1661 020 602 340 337 302 189 10064

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 719

teardrop dumbbell and disc shapes in decreasing order ofabundance Most of the teardrops have broken tails (Fig 4)some originally could have been dumbbells that broke in themiddle to form two teardrops Several examples of fused pairsof side-by-side teardrops were recovered (Fig 4) The largestsphere was an oblate sphere with a diameter of ~480 m Thelargest teardrop was 770 m long

The splash forms and angular fragments of glass rangefrom homogeneous without any inclusions or vesicles tohighly vesicular and containing numerous mineral inclusionsand lechatelierite particles (Fig 5) The mineral inclusionsappear to be quartz and one quartz grain appears to have twosets of planar deformation features (PDFs) (Fig 5)

Shocked Minerals

The 63ndash125 m size fractions of A1 and A2 weresearched for quartz grains exhibiting PDFs Approximately600 grains from each sample were counted Both samplescontain a high percentage of rock and mineral fragments (orfine-grained ejecta that were not entirely disaggregated)(Table 3) Quartz and K-feldspar make up the bulk of theremainder of each sample Both A1 and A2 contain rock andmineral grains exhibiting PDFs but grains exhibiting PDFsare more abundant in A1 (Table 3 Figs 6 and 7)Approximately 29 and 7 by number of the quartz grains inA1 and A2 respectively contain PDFs In A1 45 by

F2-3 6574 070 1625 014 593 346 332 307 192 10053F2-4 6546 064 1681 011 593 339 342 317 199 10092F2-5 6527 068 1659 020 595 325 331 316 202 10043F2-6 6611 068 1613 015 589 310 326 315 196 10043F2-7 6610 062 1645 015 591 319 314 309 196 10061F2-8 6617 069 1630 014 610 321 323 286 203 10073F2-9 6614 070 1645 009 572 321 319 289 203 10042F2-10 6586 066 1646 018 584 317 311 293 201 10022Avg(1-10) 6575 067 1646 015 595 326 326 305 198

F3-1 6599 059 1657 012 574 292 232 268 205 9898F3-2 6581 068 1675 008 591 308 238 247 208 9924F3-3 6713 061 1644 011 587 292 232 245 205 9990F3-4 6635 058 1583 008 578 295 225 233 196 9811F3-5 6605 063 1614 015 595 303 236 243 194 9868Avg(1-5) 6627 062 1635 011 585 298 233 247 202

G-1 6804 074 1763 010 582 286 273 241 190 10222G-2 6720 066 1702 005 559 304 257 262 185 10061G-3 6783 071 1725 003 556 281 279 224 185 10106

H-1 6806 068 1738 009 542 261 276 239 183 10122H-2 6782 069 1731 003 541 214 274 218 186 10019H-3 6711 062 1696 005 611 290 272 231 178 10056

I1-1 6955 065 1582 000 544 210 235 218 213 10023I1-2 7004 066 1597 006 523 250 230 228 205 10110

I2-1 6716 053 1839 015 570 201 228 278 235 10135I2-2 6765 058 1804 008 579 203 223 268 235 10142I2-3 6959 044 1474 006 431 178 153 196 240 9681

I3-1 6615 071 1836 007 568 300 283 199 152 10032I3-2 6728 067 1892 011 576 276 280 213 161 10205I3-3 6688 065 1855 004 566 295 268 193 161 10095

I4-1 6641 079 1947 003 442 310 256 225 168 10071I4-2 6662 071 1961 007 429 265 265 239 174 10073I4-3 6650 072 2004 000 459 319 273 231 162 10171I4-4 6638 072 1966 006 439 314 292 245 173 10145All data by electron microprobe analysis (Vienna) See Figs 9andashf for profile locationsldquoAvgrdquo followed by numbers indicates the average compositions calculated for the respective glass spherules or fragments from data for unaltered parts of

the various profiles (the numbers indicate which of the data points were used for the averaging) All data (G to I) by electron microprobe analysis (Berlin) See Fig 10 for analyses locations

Table 5 Continued Major-element variation (data in wt) along profiles across glass spherules and fragments in fallback layer subsample 117A1

SiO2 TiO2 Al2O3 MnO FeO MgO CaO Na2O K2O Total

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

720 C Koeberl et al

number of the quartz grains with PDFs have 3 or more setswhereas in A2 only 10 by number of the quartz grains withPDFs have 3 or more sets (Table 4) indicating average shockpressures for these grains in excess of ~20 GPa (eg French1998 Huffman and Reimold 1996) A1 also has a higherpercentage of K-feldspar grains with PDFs than A2 (~23 and14 by number respectively) Some of the rock fragments(or polymineralic grains) in A1 and A2 contain quartz grainswith PDFs (~11 and 5 by number respectively) Apreliminary search of the 63ndash125 m size fraction of A3indicates that it contains little if any quartz grains no grainscontaining PDFs were observed

X-ray diffraction (XRD) patterns were obtained forseveral white opaque grains from A1 which were thought to

be shocked quartz grains that might contain coesite andorstishovite No coesite or stishovite lines were observed in anyof the XRD patterns (in contrast to findings of coesite infallout ejecta deposits around Bosumtwi) (Littler et al 1961)A search was made for shocked zircons in the heavy mineralfractions of samples A1 A2 and A3 Only one zircon grainwas recovered and it exhibited no signs of shockmetamorphism

Chemical Composition of Glass

Electron microprobe analyses were made of several ofthe glass fragments and glass spherules In some of thespherules and fragments compositions are somewhat

Fig 8 An overview image of large polished thin section of a bulk fragment from fallback layer sample 117A1 The image is a mosaic ofbackscattered electron images and indicates the locations of the areas shown in Fig 9 where glass fragments and spherules were analyzed byelectron microprobe

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 721

Fig 9 andashf) Backscattered electron images of areas within a larger thin section (see Fig 8) The positions of electron microprobe profiles areindicated by white dashed lines The abbreviations (A1 A2 etc) refer to the data given in Table 5 Within each profile the analyses startedat the bottom of the line and ended at the top of the line analyses (for number of analyses in each profile see Table 4) were regularly spacedalong the length of the profile

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

722 C Koeberl et al

variable This appears to be due to incompletely digestedminerals grains as some zones are almost pure silica Mostother glass fragments and spherules (and other splash forms)are of rather uniform composition Table 5 gives detailed dataon the variation of compositions within 13 different glassfragments and spherules as well as the average compositionsfor each of the particles The locations of the areas in whichthe glasses were analyzed in the thin sections are shown in amosaic of backscattered electron images in Fig 8 Majorelement compositions were also measured by electron

microprobe along profiles across the glasses in most cases 10points were measured per profile Figure 9 shows the exactlocations of the profiles measured in Vienna on the variousglass fragments and spherules and Fig 10 shows thelocations on another thin section analyzed in Berlin In mostcases the data for the individual analysis points showed verylittle radial change in composition over tens to hundreds ofmicrometers Figure 11 gives two examples of the variationsof selected major element contents along two of the profilesconfirming the absence of any significant heterogeneities

Fig 10 Backscattered electron images of areas within a thin section of sample 117A1 that was studied by electron microprobe in Berlin Thepositions of electron microprobe analyses (G to I) refer to data listed in Table 5

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 723

In most cases totals for these analyses are close to100 wt indicating that the glasses are not significantlyhydrated (altered) Variations of the average compositions of11 of the 13 glass particles (excluding the altered ones ofprofiles E1 and E2) are limited and range from about30 relative (rel) for Ca and Mg to 5 rel and less for SiAl and Fe In a few cases the backscattered electron imagesshow slight variations in contrast indicating alteration ororiginal heterogeneities One such case is shown in Fig 9e(data in Table 5) where low totals in parts of the profileindicate incipient alteration of the glass to phyllosilicateminerals Where such alteration occurs the spherical outlineof the glasses are retained (eg Fig 10a) and the bulk of theglass remaining inside such a spherule has a similarcomposition to that of unaltered glass spherules For exampleelectron microprobe measurements made in Berlin of theglass within the spherule shown in Fig 10a give values(Table 5 points G1ndashG3) that are very similar to those forother unaltered glass given in Table 5 Data obtained in Berlinand in Vienna agree within the errors of analysis

The data for the glass spherules and fragments analyzedfor both major- and trace-element contents are given inTable 6 together with comparison values for Ivory Coasttektites and microtektites Despite the more limited precisionof the current trace-element data compared to the earliermicrotektite data a general similarity is evident The contentsof refractory elements such as Sc Rb the rare earth elements(REEs) Hf and Th are very similar The siderophile elementsshow more variation in abundance the Co contents of all ofthe 117A1 glass spherules is somewhat higher than those ofthe microtektites and a few of the samples also haverelatively high Cr and Ni contents

DISCUSSION

The uppermost fallback ejecta unit is normally sizegraded with the proportion of material with larger grain sizeincreasing with depth The highest glass content (includingspherules) and highest degree of shock metamorphism (asindicated by the proportion of grains with multiple sets ofPDFs) occurs at the very top Both glass content and degree ofshock metamorphism decrease abruptly with depth

The Ivory Coast microtektites which are believed to bederived from the Bosumtwi crater are generally darker andmore greenish in color than the spherules found in theuppermost fallback ejecta layer at Bosumtwi crater Theshapes are similar but the Ivory Coast microtektites have agreater range in size and a different size distribution than theBosumtwi crater spherules The largest spherical Ivory Coastmicrotektite is ~500 m in diameter and the abundanceincreases with decreasing size down to at least 60 m Thelargest Bosumtwi spherule is about ~480 m but most of thespherical Bosumtwi spherules are about 200 m in diameterand none are smaller that 160 m in diameter In contrast to

the Bosumtwi spherules the Ivory Coast microtektites aredevoid of mineral inclusions and are generally less vesicular(eg Glass 1968 1969 Glass and Zwart 1979)

The greener color of the Ivory Coast microtektites mightbe the result of the higher temperature of formation of themicrotektites compared with the Bosumtwi spherules whichcaused reduction of the iron resulting in the greenish color ofthe glass A higher temperature of formation of themicrotektites is supported also by the lack of mineralinclusions and generally fewer vesicles in the microtektitescompared with the Bosumtwi spherules

A comparison of the compositions of the glass fragmentsand spherules in sample LB-05B-117-A1 with those of IvoryCoast tektites and microtektites shows relative similarity(Figs 12 and 13) Detailed data are given in Tables 6 and 7The contents of SiO2 Al2O3 FeO and K2O in the fallbackglasses agree very well with both tektite and microtektite data

Fig 11 Profiles of elemental compositions across (a) glass fragmentB1 and (b) glass spherule D1 showing a fairly homogenouscomposition in both cases

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

724 C Koeberl et alTa

ble

6 T

he c

ompo

sitio

ns o

f 15

glas

s sph

erul

es a

nd fr

agm

ents

from

sam

ple

117A

1 M

ajor

ele

men

ts (i

n w

t) b

y el

ectro

n m

icro

prob

e tr

ace

elem

ents

(in

ppm

) by

neu

tron

activ

atio

n an

alys

is

Sam

ple

1

34

67

89

10

12

15

16

17

18

19

20

Tekt

ites

avg

Mic

rote

ktite

sW

eigh

t (micro

g)29

667

642

022

133

033

856

935

552

451

034

435

145

043

654

4

SiO

260

00

701

367

50

674

466

79

681

168

86

663

267

84

674

567

78

677

067

89

675

965

09

675

867

37

TiO

20

980

530

690

720

680

610

530

630

650

590

630

630

520

550

680

560

59A

l 2O3

121

915

41

156

716

38

168

913

28

138

614

94

156

015

81

153

316

58

155

813

98

179

816

74

170

7M

nO0

090

020

050

050

070

090

080

070

050

080

080

080

070

070

060

060

07Fe

O8

264

596

245

925

995

975

886

286

156

175

645

495

325

875

876

166

40M

gO8

192

113

543

153

003

192

763

203

023

133

273

232

342

932

833

463

70C

aO5

412

722

902

682

562

592

092

992

512

593

443

252

212

862

341

381

22N

a 2O

2

673

492

321

592

592

132

762

882

372

522

142

072

882

992

871

901

63K

2O1

351

821

641

681

991

472

262

042

001

851

541

662

232

062

091

951

86To

tal

991

410

083

100

5499

60

100

5697

43

990

799

34

100

1910

018

998

410

069

990

598

90

998

099

79

999

1

Sc25

918

717

614

815

218

916

116

617

118

319

816

615

717

117

714

717

9C

r28

583

212

9913

515

613

615

915

812

514

016

010

716

314

724

429

3C

o71

430

837

547

540

544

132

641

946

552

158

956

753

356

353

526

732

7N

i15

0lt1

000

lt700

650

lt100

0lt1

000

510

520

lt100

017

043

0lt1

000

180

lt700

lt600

157

224

Rb

4796

lt150

lt100

lt100

3020

140

160

lt100

110

5858

lt100

lt100

6667

Zr18

0lt6

00lt1

000

lt200

lt500

lt500

160

lt600

lt100

020

058

0lt4

0047

028

0lt1

000

134

215

Sblt1

016

lt1lt1

lt1lt1

lt1lt1

035

056

041

lt10

60

74lt1

023

02

Ba

740

780

640

100

480

700

640

700

620

124

380

lt500

510

670

lt600

327

620

La18

617

720

715

520

418

819

715

219

518

820

317

719

519

319

820

725

9C

e44

397

389

3537

4042

834

344

242

4735

4244

542

419

551

Nd

lt50

205

lt50

lt30

lt30

lt35

245

2228

2320

2022

2325

218

273

Sm3

243

614

153

324

383

83

933

773

953

124

133

653

513

84

13

955

1Eu

164

116

175

14

17

15

125

178

138

097

154

125

131

153

12

12

143

Gd

32

lt44

12

7lt5

lt54

13

84

24

14

84

23

83

53

53

344

4Tb

071

06

058

03

056

059

07

07

05

06

06

06

lt1lt1

035

056

074

Yb

22

236

254

18

219

215

241

208

217

272

17

25

161

22

131

179

207

Lu0

30

50

5lt1

05

061

057

045

067

063

04

06

04

05

027

024

031

Hf

345

402

lt56

74

274

53

893

134

133

565

256

154

1lt5

288

338

428

Talt2

07

lt2lt1

lt1lt1

lt10

4lt1

lt10

44lt1

04

lt10

530

340

42Th

115

324

322

lt54

383

373

853

963

682

623

772

22

684

514

273

543

99U

15

064

lt2lt2

131

lt1lt1

lt1lt2

lt1lt1

lt1lt1

lt10

870

940

64

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 725

(Koeberl et al 1997 Glass et al 2004) Contents of MgO areslightly lower in the fallback glass and of Na2O slightlyhigher than in Ivory Coast tektites but the variations arewithin factors of 08 and 13 respectively The abundance ofCaO is markedly higher in the fallback glass than in the IvoryCoast tektites the average CaO content in the glass from117A1 is 254 wt as compared to 138 wt in the tektites(Koeberl et al 1997) and 122 wt (4 microtektites Koeberlet al 1997) and 178 wt (16 microtektites Glass et al2004) This results in enrichments by a factor of about 15ndash2compared to Ivory Coast tektites

The differences are also interesting in view of chemicalcompositions of target rocks and suevitic breccias atBosumtwi (cf Koeberl et al 1998 Ferriegravere et al 2007 Coneyet al 2007) Of the metasedimentary target rocks atBosumtwi only phyllitesshalesslates have CaO contentssimilar to those of the tektites the overall average is moresimilar to the composition of the fallback glasses Curiouslythe average CaO content of fallback suevite (Ferriegravere et al2007) is higher (similar to those of the glasses) than that offallout suevite (Boamah and Koeberl 2003) which is moresimilar to that of the distal ejecta (tektites and microtektites)(Table 7) It might be possible to extend the speculationmentioned abovemdashthat the microtektites were subjected tohigher temperatures than the fallback glassesmdashto thechemical composition where removal of volatile phases wasmore efficient for the distal ejecta at higher temperaturesNevertheless the average composition of the fallback glasses(proximal ejecta) agrees fairly well with that of the distalejecta as is shown by the Harker diagrams in Fig 13 whereit is evident that only the CaO content is significantlydifferent Good agreement exists also with the average targetrock and suevitic breccia compositions at Bosumtwi(Table 7)

It is interesting to note that the chondrite-normalizedplatinum group element (PGE) abundance pattern of a (bulk)subsample of 117A1 is different from the patterns of targetrocks and suevites outside the crater (Dai et al 2005) andwithin the LB-07A and LB-08A drill cores (Goderis et al2007) This observation could indicate the presence of a smallmeteoritic component in the fallback layer (as noted byGoderis et al 2007) However such a component cannot bevery large because the total PGE content of the 117 samplesas especially noted for Ir is not significantly higher than thatof other Bosumtwi impactite samples

SUMMARY AND CONCLUSIONS

A microbreccia containing accretionary lapilli mineraland lithic clasts including shocked quartz as well asmicrotektite-like glass spherules and glass fragments ofcompositions similar to Ivory Coast tektites was found in acore from borehole LB-05B in Lake Bosumtwi at theinterface between impact breccias and post-impact lake

sediment at Bosumtwi This material represents a layer of thelast fine-grained fallback deposit Glasses as fragments andsplash forms (eg spherules teardrops and dumbbells)make up the majority of the gt125 microm size fraction in the topof the fallback layer The fallback layer is graded in terms ofgrain size over the about 30 cm length of core studied herefrom fine-grained in the top part (which is rich in shockedquartz and glass) and relatively coarser-grained towards thebottom of the section

Shocked minerals particularly quartz and K-feldsparare also common at or near the top of the layer but both theirabundance and the shock degree recorded by them decreasewith depth within a few tens of centimeters The chemicalcomposition of these glasses is very similar (with factors ofless than 15 for most elements) to the compositions of IvoryCoast tektites and microtektites with exception of the CaOcontent which is higher by a factor of 2 However thedifference is less compared to fallback suevites (within thecrater) which also have higher Ca contents that the falloutsuevites (outside the crater) It appears that the distal ejecta(tektites and microtektites) experienced somewhat highertemperatures of formation than the proximal ejecta

The whole fallback layer was deposited after theformation of the crater including the immediate post-impactmodification phase was completed and all breccia hadalready filled the crater Although numerical simulation of theBosumtwi crater formation (Artemieva et al 2004) did notinvolve the entire duration of cratering to the end of themodification stage it can be assumed that the fallback layerwas deposited within hours after the impact This agrees withconsiderations of atmospheric settling of particles larger than

Fig 12 The ratio of overall average (95 analyses) of glass fragmentsand glass spherules in fallback layer sample 117A1 versus averagecompositions of Ivory Coast tektites and microtektites (data fromKoeberl et al 1997) (Table 5) The glasses in the fallback layer havemarkedly higher contents of Ca somewhat higher contents for Naslightly higher contents of Ti and slightly lower contents of Fe Mgand Mn compared with both tektites and microtektites

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

726 C Koeberl et al

Fig 13 Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared with data for IvoryCoast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate the similarity incomposition for most major elements except for CaO

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 727

50 μm from heights of about 10 km (Bluth and Rose 2004)The small (or absent) meteoritic component in the fallbacklayer deposited directly into the crater agrees with thesuggestion of an oblique impact (angle between about 30 to45deg from the horizontal) which has also been invoked toexplain the asymmetric distribution of the tektites relative tothe crater location (Artemieva et al 2004)

Fine-grained fallback layers in impact structures are veryrare and restricted to well-preserved impact structures thatwere buried after their formation (eg at Chesapeake Bay)(Poag 2002) or where the impactites within the crater

depression are covered with post-impact sediments (as in thiscase) The presence of the uppermost fallback layer atBosumtwi indicates that the fallback and breccia units withinthe crater are complete which confirms that Bosumtwi isprobably the youngest and best-preserved large compleximpact structure on Earth

AcknowledgmentsndashDrilling at Bosumtwi was supported bythe International Continental Scientific Drilling Program(ICDP) the US NSF-Earth System History Program undergrant no ATM-0402010 Austrian FWF (project P17194-

Fig 13 Continued Harker diagrams of major oxide contents in the glasses from the fallback layer in core BL-5B (this work) compared withdata for Ivory Coast tektites (Koeberl et al 1997) and Ivory Coast microtektites (Koeberl et al 1997 Glass et al 2004) The plots indicate thesimilarity in composition for most major elements except for CaO

Table 7 Major-element composition of Ivory Coast tektites and microtektites (from Koeberl et al 1997) compared with averaged compositions of the analyses of glass fragments and spherules from fallback layer sample 117-A1 All data in wt

Tektitesa Microtektitesa MicrotektitesbGlass 117A1average AndashFc

Fallbacksuevited Fallout suevitee

n = 11 n = 4 n = 16 n = 95 n = 20 n = 11

SiO2 6758 6737 664 6747 6316 6358TiO2 056 059 055 065 055 064Al2O3 1674 1707 169 1664 1465 1558FeO 616 640 665 569 549 658MnO 006 007 007 005 006 011MgO 346 370 427 278 250 143CaO 138 122 173 254 267 139Na2O 19 163 192 255 294 173K2O 195 186 160 189 167 127Total 9979 9991 10009 10026 9970 10001

aKoeberl et al (1997)bGlass et al (2004)cThis work average of 95 individual electron microprobe analyses along 10 of the profiles given in Table 5 and shown in Fig 9 (averages of all unaltered glasses

reported as ldquoAvgrdquo in Table 5 profiles A1 A2 B1 B2 C1 C2 D1 D2 F1 and F2)dFerriegravere et al (2007) total includes 525 wt loss on ignitioneBoamah and Koeberl (2003) total includes 768 wt loss on ignition

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

728 C Koeberl et al

N10) the Austrian Academy of Sciences and by theCanadian NSERC This work was supported by the AustrianFWF (P17194-N10 (to C K) We are grateful to M Talbot(University of Bergen Norway) and J Peck (University ofAkron USA) for advice and images and to J Peck andespecially J King (University of Rhode Island USA) forhelp with obtaining samples of the fallback layer from coreLB-05B Thanks also to P Claeys (Free University ofBrussels) for the information about the PGE content of asubsample of 117A1 We appreciate helpful reviews byH Dypvik and P Claeys as well as editorial advice byB Milkereit

Editorial HandlingmdashDr Bernd Milkereit

REFERENCES

Artemieva N A Karp T and Milkereit B 2004 Investigating theLake Bosumtwi impact structure Insight from numericalmodeling Geochemistry Geophysics Geosystems 5 doi1010292004GC000733

Boamah D and Koeberl C 2003 Geology and geochemistry ofshallow drill cores from the Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 381137ndash1159

Bluth G J S and Rose W I 2004 Removal processes of volcanicash particles from the atmosphere Proceedings 2ndInternational Conference on Volcanic Ash and Aviation SafetyOffice of the Federal Coordinator for Meteorology (OFCM)Washington DC pp 51ndash54

Coney L Reimold W U Gibson R L and Koeberl C 2007Geochemistry of impactites and basement lithologies from ICDPborehole LB-07A Bosumtwi impact structure GhanaMeteoritics amp Planetary Science 42 This issue

Dai X Boamah D Koeberl C Reimold W U Irvine G andMcDonald I 2005 Bosumtwi impact structure GhanaGeochemistry of impactites and target rocks and search for ameteoritic component Meteoritics amp Planetary Science 401493ndash1511

Ferriegravere L Koeberl C Reimold W U and Mader D 2007 Drillcore LB-08A Bosumtwi impact structure Ghana Geochemistryof fallback breccia and basement samples from the central upliftMeteoritics amp Planetary Science 42 This issue

French B M 1998 Traces of catastrophe A handbook of shock-metamorphic effects in terrestrial meteorite impact structuresLPI Contribution 954 Houston Texas Lunar and PlanetaryInstitute 120 p

Gentner W Lippolt H J and Muumlller O 1964 Das Kalium-Argon-Alter des Bosumtwi Kraters in Ghana und die chemischeBeschaffenheit seiner Glaumlser Zeitschrift fuumlr Naturforschung19A150ndash153

Glass B P 1968 Glassy objects (microtektites) from deep-seasediments near the Ivory Coast Science 161891ndash893

Glass B P 1969 Chemical composition of Ivory Coast microtektitesGeochimica et Cosmochimica Acta 331135ndash1147

Glass B P and Pizzuto J E 1994 Geographic variation inAustralasian microtektite concentrations Implicationsconcerning the location and size of the source crater Journal ofGeophysical Research 9919075ndash19081

Glass B P and Zwart P A 1979 The Ivory Coast microtektite strewnfield New data Earth and Planetary Science Letters 43336ndash342

Glass B P Swincki M B and Zwart P A 1979 Australasian IvoryCoast and North American tektite strewn field Size mass andcorrelation with geomagnetic reversals and other earth events

Proceedings 10th Lunar and Planetary Science Conferencepp 2535ndash2545

Glass B P Kent D V Schneider D A and Tauxe L 1991 IvoryCoast microtektite strewn field Description and relation to theJaramillo geomagnetic event Earth and Planetary ScienceLetters 107182ndash196

Glass B P Huber H and Koeberl C 2004 Geochemistry ofCenozoic microtektites and clinopyroxene-bearing spherulesGeochimica et Cosmochimica Acta 683971ndash4006

Goderis S Tagle R Schmitt R T Erzinger J and Claeys Ph 2007Platinum group elements provide no indication of a meteoriticcomponent in ICDP cores from the Bosumtwi crater GhanaMeteoritics amp Planetary Science 42 This issue

Huffman A R and Reimold W U 1996 Experimental constraints onshock-induced microstructures in naturally deformed silicatesTectonophysics 256165ndash217

Jones W B 1985 Chemical analyses of Bosumtwi crater target rockscompared with the Ivory Coast tektites Geochimica etCosmochimica Acta 482569ndash2576

Jones W B Bacon M and Hastings D A 1981 The LakeBosumtwi impact crater Ghana Geological Society of AmericaBulletin 92342ndash349

Karp T Milkereit B Janle P Danuor S K Pohl J Berckhemer Hand Scholz C A 2002 Seismic investigation of the LakeBosumtwi impact crater Preliminary results Planetary andSpace Science 50735ndash743

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small samples analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

Koeberl C 1994 Tektite origin by hypervelocity asteroidal orcometary impact Target rocks source craters and mechanismsIn Large impacts and planetary evolution edited by DresslerB O Grieve R A F and Sharpton V L GSA Special Paper293 Boulder Colorado Geological Society of Americapp 133ndash151

Koeberl C and Reimold W U 2005 Bosumtwi impact crater Ghana(West Africa) An updated and revised geological map withexplanations Jahrbuch der Geologischen Bundesanstalt Wien(Yearbook of the Austrian Geological Survey) 14531ndash70(+1 map 150000)

Koeberl C Bottomley R J Glass B P and Storzer D 1997Geochemistry and age of Ivory Coast tektites and microtektitesGeochimica et Cosmochimica Acta 611745ndash1772

Koeberl C Reimold W U Blum J D and Chamberlain C P 1998Petrology and geochemistry of target rocks from the Bosumtwiimpact structure Ghana and comparison with Ivory Coasttektites Geochimica et Cosmochimica Acta 622179ndash2196

Koeberl C Milkereit B Overpeck J T Scholz C A Peck J andKing J 2005 The 2004 ICDP Bosumtwi impact crater GhanaWest Africa drilling project A first report (abstract 1830) 36thLunar and Planetary Science Conference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A ReimoldW U Amoako P Y O Boamah D Claeys P Danuor SDeutsch A Hecky R E King J Newsom H Peck J andSchmitt D R 2006 An international and multidisciplinarydrilling project into a young complex impact structure The 2004ICDP Bosumtwi impact crater Ghana drilling projectmdashAnoverview (abstract 1859) 37th Lunar and Planetary ScienceConference CD-ROM

Koeberl C Milkereit B Overpeck J T Scholz C A AmoakoP Y O Boamah D Danuor S Karp T Kueck J Hecky R EKing J W and Peck J A 2007 An international andmultidisciplinary drilling project into a young complex impactstructure The 2004 ICDP Bosumtwi Crater Drilling ProjectmdashAn overview Meteoritics amp Planetary Science 42 This issue

Uppermost impact fallback layer in the Bosumtwi crater (Ghana) 729

Lacroix A 1934 Sur la deacutecouverte de tectites agrave la Cocircte drsquoIvoireComptes Rendus Academie des Sciences Paris 1991539ndash1542

Lippolt H J and Wasserburg G J 1966 Rubidium-Strontium-Messungen an Glaumlsern vom Bosumtwi-Krater und anElfenbeinkuumlsten-Tektiten Zeitschrift fuumlr Naturforschung 21A226ndash231

Littler J Fahey J J Dietz R S and Chao E C T 1961 Coesitefrom the Lake Bosumtwi crater Ashanti Ghana GSA SpecialPaper 68 Boulder Colorado Geological Society of America218 p

Montanari A and Koeberl C 2000 Impact stratigraphy The Italianrecord Lecture Notes in Earth Sciences vol 93 HeidelbergSpringer-Verlag 364 p

Poag C W 2002 Synimpact-postimpact transition insideChesapeake Bay crater Geology 30995ndash998

Reimold W U Brandt D and Koeberl C 1998 Detailed structuralanalysis of the rim of a large complex impact crater Bosumtwicrater Ghana Geology 26543ndash546

Schnetzler C C Pinson W H and Hurley P M 1966 Rubidium-strontium age of the Bosumtwi crater area Ghana comparedwith the age of the Ivory Coast tektites Science 151817ndash819

Schnetzler C C Philpotts J A and Thomas H H 1967 Rare earth

and barium abundances in Ivory Coast tektites and rocks from theBosumtwi crater area Ghana Geochimica et CosmochimicaActa 311987ndash1993

Scholz C A Karp T Brooks K M Milkereit B Amoako P Y Oand Arko J A 2002 Pronounced central uplift identified in theBosumtwi impact structure Ghana using multichannel seismicreflection data Geology 30939ndash942

Shaw H F and Wasserburg G J 1982 Age and provenance of thetarget materials for tektites and possible impactites as inferredfrom Sm-Nd and Rb-Sr systematics Earth and PlanetaryScience Letters 60155ndash177

Storzer D and Wagner G A 1977 Fission track dating of meteoriteimpacts Meteoritics 12368ndash369

Stoumlffler D and Langenhorst F 1994 Shock metamorphism of quartzin nature and experiment I Basic observation and theoryMeteoritics 29155ndash181

Wagner R Reimold W U and Brandt D 2001 Bosumtwi impactcrater Ghana A remote sensing investigation In Meteoriteimpacts in Precambrian shields edited by Plado J and PesonenL J Impact Studies vol 2 Heidelberg Springer-Verlagpp 189ndash210