24. LITHOLOGY, PETROGRAPHY, AND MINERALOGY OF BASALTS FROM DSDP SITES 482,483, 484, AND 485 AT THE MOUTH OF THE GULF OF CALIFORNIA1
Brendon J. Griffin, Electron Optical Center, University of Adelaide, Box 498, G.P.O. Adelaide, South Australia 5001and
Rolf D. Neuser and Hans-Ulrich Schmincke, Institut für Mineralogie, Ruhr-Universitàt, 4630 Bochum,Federal Republic of Germany
ABSTRACT
The majority of the basalts drilled on Leg 65 in the Gulf of California are aphyric to sparsely phyric massive flowsranging in average thickness between 5 meters in the upper part of the sections in Holes 483 and 483B, where they are in-terlayered with sediment, and 14 meters in Hole 485A, where interlayered sediments constitute more than half of thesection. Massive flows interlayered with pillows are generally less than 4 meters thick. The pillow lavas recovered aremore phyric (up to 15 modal%) and contain two to three generations of plagioclase and olivine ± clinopyroxene. Pla-gioclase generally exceeds 60% of any given phenocryst assemblage. Resorbed olivine, clinopyroxene, and plagioclasemegacrysts may reflect a high-pressure stage, the phenocrysts crystallizing in the main magma chamber and the skeletalmicrophenocrysts in dikes. Precise measurements of length/width ratios of different phenocryst types and composi-tions show low aspect ratios and large crystal volumes for early crystals and high ratios and low volumes for late crystalsgrown under strong undercooling conditions. The minerals examined show wide ranges in composition: in particu-lar, plagioclase ranges from An92 to An36; clinopyroxene ranges from Ca41Mg51Feg in the cores of phenocrysts to Ca^óMg^Fef< in the groundmass; and olivine ranges from Fo86 to Fo8,.
The wide range in mineral compositions, together with evidence of disequilibrium based on textures and compari-sons of glass and mineral compositions, indicate complex crystallization histories involving both polybaric crystal frac-tionation and magma mixing.
INTRODUCTION
During Leg 65 of the Deep Sea Drilling Project(DSDP), six major holes were drilled at three principalsites (482, 483, and 485) at the mouth of the Gulf ofCalifornia (23 °N) (Figs. 1 and 2), recovering a total ofabout 420 meters of basalt (Figs. 3 and 4). The holeswere drilled in crust ranging in age from 0.5 m.y. (Site482) to 2.0 m.y. (Site 483) at distances ranging from 12km (Site 482) to 52 km (Site 483) from the spreadingaxis. One additional site (Site 484) was drilled in thenearby Tamayo Fracture Zone (Fig. 1). One of the mainpurposes of this leg was to study magma chamber evolu-tion and crustal construction processes at a ridge with amoderately high spreading rate (about 5 cm/y.) in ayoung ocean basin with a high sediment influx for com-parison with slow spreading ridges (2-3 cm/y.) of theAtlantic type with very low sedimentation rates. Thepresent chapter presents and briefly evaluates lithologic,petrographic, and mineral composition data for basaltsfrom these sites. Major and trace element data are dis-cussed in a companion paper (Flower et al., this volume)and volcaniclastic rocks and glass alteration are discussedby Schmincke (this volume). A fuller discussion of thedata will be presented elsewhere.
GENERAL LITHOLOGYTwo principal types of basalt were recovered in the Leg
65 drill sites: massive basalt (Plate 1, Figs. 1-4; Plate 2,
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Figure 1. Index map of the Gulf of California showing the main sitesdrilled during Leg 65.
Figs. 1-3) and pillow basalt (Plate 2, Fig. 4; Fig. 5; Table1). Most, if not all, of the massive basalts are believed tohave been emplaced as sheet flows rather than as sills.This interpretation is based on several lines of evidence,
527
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
Heat Flow
AgeMagneticAnomaly
D i s t a n c e90 80 70 60 50 40 30 20 10 0 10 20 30 40 50
I • I • I • I i I • i(km)
Figure 2. Cross section from Baja California through the East Pacific Rise showing main drill sites, sedi-ment basins (black), heat-flow values, and magnetic anomalies. (Simplified from Lewis, this volume.)
482C
0
10
20
•2 30
2 40
50
60
70
482D100 m-
D
Figure 3. Stratigraphic correlation of main lithologic and chemicalunits between three holes drilled at Site 482. (A, B, E, and F repre-sent chemical (or magma) types defined by Flower et al., thisvolume.)
including the following: (1) the presence of a distinctlyvesicular zone below the top of the massive basalts(Plate 2, Figs. 1-3); (2) the absence of intrusive relation-ships; (3) the absence of baked contacts (the minor in-duration observed in the sediments immediately overly-ing several of the massive basalt units is attributed tosubsequent diagenetic reactions near the sediment/ba-salt contact) (Plate 1, Figs. 1, 2); (4) the presence in onehole of a bedded hyaloclastite which chemically resem-bles the underlying massive basalt, suggesting derivationfrom the glassy crust of a surface flow (Schmincke, thisvolume); (5) the stratigraphic continuity over distancesof several hundred meters of flows which are verticallyseparated from other massive units by a few centimetersto a few meters of sediment (Fig. 3); (6) the imprint ofbroken vesicles in the sediments lying in contact with theunderlying basalt; and finally (7) the presence of ophi-mottled textures in the basalts themselves (Robinson etal., 1980).
Other lines of evidence, however, such as internal dik-ing in Unit 3, Hole 483 (Plate 1, Fig. 4) and chemical dif-ferences within Unit 5, Hole 485 (Flower et al., this vol-ume) suggest intrusive processes. In lithologic Unit 3
(Hole 483) the diking could have occurred as autointru-sion during the sagging of a thick, liquid-cored lava sheetafter it was emplaced on soft sediment. The chemical di-versity within Lithologic Unit 5 (Hole 485A), however,is probably the result of the intrusion of magma batchesof slightly differing chemistry. Thin stringers of sidero-melane extending into the sediment from the top ofLithologic Unit 8 in Hole 485A also suggest intrusion(Plate 1, Fig. 3). The sites drilled are thus intermediatebetween an environment with a high sedimentation rate,such as the Guaymas Basin, in which many if not all ofthe basalts are emplaced as intrusives (Curray, Moore,et al., in press) and one with a low sedimentation rate,such as the Mid-Atlantic Ridge, where nonintrusive sheetflows are common (e.g., Robinson et al., 1980). A fur-ther indirect but powerful argument for the interpreta-tion of the massive basalts as extrusives is the successionof flows observed in Holes 483 and 483B in which mas-sive flows of more evolved chemistry and mineralogyare underlain by pillow lavas of more mafic chemistryand higher phenocryst content (Flower et al., this vol-ume). The massive basalt flows are from 3 to 7 metersthick in the upper part of Holes 483 and 483B above thepillows, but are much thicker in Hole 485A where theaverage thickness is about 10 to 14 meters (excludingUnit 5, which is 26 m thick). We are not certain whetherthick sheet flows and intrusives grade into each otherwhere surface flows have burrowed into soft sediments.In any case, thick massive flows appear to dominateduring the late and probably waning stage of volcanism.They represent single extrusive events separated fromeach other by a significant hiatus in time. The sheetflows interbedded with pillow lavas in the lower part ofHoles 483 and 483B (Fig. 4) are thinner and morevesicular than the flows higher in the section and areseparated from each other and the overlying pillows bymuch thinner sediment layers. The pillow sequences arealso thin (about 15m). Although sheet flow and pillowunits could represent the distal edges of eruptive units,the similarity in the thickness of the units between Holes
528
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LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
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Table 1. Comparison of characteristic features of massive and pillowbasalt flows drilled on Leg 65.
Basalt
Massive (sheet) flowsPillow (tube) flows
Thickness(m)
1.0-10.00.4-0.5
Pheno-crysts
(vol.%)
25-15
Contacts
FlatCurved
Thicknessof Glassy
Rinds(cm)
0.51-2
Fractures
StraightIrregular
483 and 483B, about 100 meters apart (Fig. 4), as well asbetween Holes 482B, C, and D where the holes areabout 200 meters apart (Fig. 3), suggests that the erup-tive units are relatively thin in this environment. Inother words, the height of sheet flow and pillow volca-noes during terminal activity at the mouth of the Gulf ofCalifornia appears to have been low.
One of the factors determining volcano height ap-pears to be spreading rate; volcano heights range from
529
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
about 250 meters in the FAMOUS area to 50 meters inthe Galapagos Rift (Ballard et al., 1979). Eruptive units,as well as eruptive cycles combining several petrologicallyrelated eruptive units, appear to be thicker in the Atlan-tic than in the Pacific, judging from the deepest holesdrilled on Legs 37 and 51-53 (Flower and Robinson,1979, 1981; Robinson et al., 1980).
Robinson et al. (1980) restrict the term "flow" to cool-ing units more than 3 meters thick. While such thick-nesses hold for flows in the upper part of Holes 482, 483,483B, and all of Hole 485A, several massive basalt unitsinterlayered with pillows in the lower part of Holes 483and 483B have been interpreted as flows even thoughthey are less than three meters thick. We have observedflows of similar thickness to be common in the Troodosextrusive section (Schmincke et al., in press).
The reason why submarine lavas erupt as pillow andsheet flows is not clear, but high eruptive rates arethought to be an important factor (Ballard, 1979; Rob-inson et al., 1980). The lateral continuity of individualflows over at least 200 meters at two sites (482 and 483)also suggests that these flows represent high eruptiverates.
PETROGRAPHYMETHODS
About 60 thin sections, representing all holes and all major basalttypes, were first examined qualitatively using a petrographic micro-scope. Twenty thin sections, chiefly of glassy to tachylitic basalts,were then studied quantitatively using the semi-automatic VIDEO-PLAN picture-analysis system, which allows precise measurement ofthe volume, size, and form of phenocryst phases. Two methods ofmeasuring crystal dimensions were employed in this study. In the firstmethod, thin sections were projected onto a digitizing tablet and thephenocryst perimeters were then traced with an electronic pen. In thesecond method, the light from a light-emitting diode (LED) attachedto an electronic cursor is projected into the measuring field of themicroscope. Thus the movements of the cursor on the digitizing tabletcan be followed while observing the thin section. The longest andshortest dimensions were then calculated for each crystal from the ma-jor (A) and minor (B) axes of the ellipse with the same moment of in-ertia as that of the measured grain. In each thin section, up to six areasabout 0.8 cm in diameter were measured, covering from 50% to 80%of the area.
Pillow basalts were selected for more detailed examination in thisstudy because they tend to be more phyric than massive basalts andbecause phenocrysts can be more easily identified in rocks with aglassy or fine-grained groundmass. The data from this analysis arereported in Table 2 and in Figures 5 through 9.
General Petrographic OverviewThe basalts drilled on Leg 65 show the entire range of
submarine basalt textures, from thick, fresh, glassy pil-low rinds (Plate 2, Fig. 4) and thin, mostly devitrifiedmargins of massive flows (Plate 1, Figs. 1, 2) to coarse-grained gabbros in the 10- to 20-meter-thick cooling unitsrecovered at Site 485. Most of the basalts are aphyric tosparsely phyric, but phyric basalts with 5% to 15% totalphenocrysts are common in the pillowed units of Holes483 and 483B (Table 2) and in the lowermost massiveflows in Hole 483. Like most ocean floor basalts, themost common phenocryst phase is plagioclase, whichgenerally composes from 60% to 90% of the phenocrystassemblage. Olivine is generally less abundant ( 3% in
volume), and clinopyroxene is usually sparse (< 1%) toabsent. Clinopyroxene is more common than olivine,however, in Cores 483B-22 through 25, where it occursprincipally in glomerophyric intergrowths with plagio-clase, indicating more advanced crystallization of themagma prior to eruption.
Vesicles are rare to absent in the chilled margins ex-amined but increase in size and abundance toward theinteriors of pillows and the tops of sheet flows, averag-ing less than 2% by volume. Segregation vesicles, on theother hand, are very common in both types of basalt(Plate 2, Figs. 1-3). These become increasingly filled to-ward the interiors of pillows and may be rimmed withplagioclase.
Pervasive alteration is generally restricted to the fill-ing of vesicles (Plate 2, Figs. 1-3) and fractures, and tothe replacement with smectite of groundmass glass andmost of the olivine in the more coarse-grained basalts.Higher-grade alteration occurs near a hydrothermal veinin Hole 482C and in the interior of several thick coolingunits at Site 485 (Morrison and Thompson, this volume).
Phenocrysts
PlagioclasePlagioclase occurs in four texturally and composi-
tionally distinct forms in the Leg 65 basalts. These con-sist of megacrysts, normal phenocrysts, micropheno-crysts, and microlites.
Plagioclase I (megacrysts): (Plate 3, Figs. 1,2). Theplagioclase megacrysts observed occur almost exclusive-ly in the phyric pillow basalts of Holes 483 and 483Bwhere they generally constitute less than 1 % of the rock.They are classified as megacrysts not because of theirabnormal size (54 mm) but because they are larger thanthe other plagioclase types, occur mostly as single crys-tals, are moderately to strongly rounded, may have coresfilled with, or delineated by, devitrified melt inclusions,and show complex, in part oscillatory zoning with oneor more major breaks in composition. The crystals varyin area from about 5 to 20 mm2 with a mean value of10.5 mm2 and have an aspect ratio, A/B, which is al'ways less than 3 (Figs. 6, 7).
Plagioclase II (normal phenocrysts): (Plate 3, Fig. 3).Normal phenocrysts constitute between 6% and 8% ofthe most phyric basalts and range in length from 0.1 to0.3 mm, with most averaging 0.7 mm. The phenocrystsvary in area from 0.3 to 10 mm2 with a mean of 1.5 mm2
and have a maximum aspect ratio of 8 (Figs. 6 and 7).The crystals are invariably euhedral and commonly dis-play synneusis twinning. Zoning, however, is not pro-nounced. The plagioclase in this group may occur inglomerocrysts with olivine but crystals intergrown withclinopyroxene are also included in this group, thoughthey are mostly sub- to anhedral. In some basalts, phe-nocrysts grade irregularly into microphenocrysts.
Plagioclase III (microphenocrysts or pre-emptionmicrolites): Plate 3, Fig. 4; Plate 4, Fig. 1). The plagio-clase crystals belonging to Group III are generally eu-hedral and range up to 1.2 mm in length, with most fall-ing between 0.2 and 0.25 mm in length. The crystals are
530
Table 2. Textures and phenocryst abundances in Leg 65 basalts.
Sample(interval in cm)
RockType
LithologicUnit
CoolingUnit
Chemicala
Type
Phenocrysts(vol. Vo, modal)
Glomerocrysts(vol.% modal)
Ol I Ol II Cpx I Cpx II PI I PI II PI III Total Ol-Pl Ol-Pl-Cpx Pl-Cpx VesiclesCounted (C);Estimated (E) Predominant Texture
Hole 482D
11-1, 24-28
Hole 483
21-1,49-5521-1, 57-6121-1, 78-8221-3, 6-921-2, 68-7222-2, 85-8822-2, 88-9322-2, 94-99
Hole 483B
20-2, 37-4220-2, 50-5920-2, 62-6522-2, 63-6722-2, 77-8225-2, 39-4527-1, 12-1830-3, 13-1632-1, 1-632-1, 27-3132-1, 44-5032-1, 90-94
Hole 485A
11-3, 57-6011-3, 116-12112-1, 10-1612-1, 76-8213-1, 110-11730-3, 38-4130-4, 8-1336-3, 65-6939-1, 26-32
PT
PTPCPBPBPTPCPCPB
SBSBSBPCPBSTSTPBPTPCPCPB
PTPC
PC?PC?scscsc
PB?SC
2626265858788394
104104104104
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0.50.7
0.51.0
1.82.03.02.02.23.21.11.6
3.32.5
1.2
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0.7
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Vitrophyric-variolitic
C Variolitic with segregation vesiclesC Tachylitic with segregation vesiclesC Vitrophyric-tachyliticE Vitrophyric-varioliticC Vitrophyric-tachyliticC Tachylitic with filled segregation vesiclesC Tachylitic with open segregation vesiclesC Tachylitic with filled segregation vesicles
E IntergranularE IntergranularE Vitrophyric-intersertalE TachyliticC Tachylitic-varioliticE IntersertalE IntersertalE Vitrophyric-varioliticC Vitrophyric-varioliticC Intersertal with segregation vesiclesC TachyliticC Variolitic
C Vitrophyric-varioliticE IntersertalC Tachylitic-ophimottled
IntersertalIntersertalOphiticOphitic
C Tachylitic-vitrσphyricSubophitic
Note: PT = pillow top; PC = pillow center; PB = pillow bottom; ST = sheet flow top; SC = sheet flow center; SB = sheet flow bottom; d = Dominant type of glomerocryst; ma After Flower et al. (this volume).
Minor type of glomerocryst.
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
Olivine I (•)Ol iv ine II (•)0 1 2 3 4
50 F"~
Clinopyroxene I (•)Clinopyroxene II ( )
0 1 2 3
Megacrysts I (•)Phenocrysts 11 ( )
Microphenocrysts III (O)0 2 4 6 8
Vesicles0 2
0
,O
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
80
60
II 40
20
PI I (6)PI II (203)PI III (2303)
\
0.01 0.1 1
Phenocryst Area (mm2)
Figure 6. Relative frequency vs. cross-sectional area of phenocrystsbelonging to different generations of plagioclase found in Leg 65basalts. (PI I, megacrysts; PI II, phenocrysts; and PI III, microphe-nocrysts. Numbers in parentheses represent numbers of measure-ments.)
0.01 100.0
Figure 8. Aspect ratio vs. cross-sectional area of clinopyroxene crys-tals in Leg 65 basalts. (Note the relative abundance of small crys-tals with high length/width ratios. The low ratios shown forcrystals with small areas result from crystals with long axes in-clined to the plane of the thin section.)
PI l (7)Pi l l (203)PI III (2286)
6 9 12Aspect Ratio of Phenocrysts (A/B)
15
Figure 7. Relative frequency vs. aspect ratio of phenocrysts belongingto different generations of plagioclase found in Leg 65 basalts. (PII, megacrysts; PI II, phenocrysts; and PI III, microphenocrysts.The peaked distribution for PI I indicates that the megacrystsgenerally have low length/width ratios while the flatness of the PIIII curve reflects the high length/width ratios of acicular plagio-clase. The plagioclase phenocrysts (PI II) show an intermediatedistribution of aspect ratios. Numbers in parentheses representnumbers of measurements.)
ly rounded, and occur as rare, single crystals up to 12mm in length. The more common euhedral phenocrystsmay be present in clots with plagioclase and, more rare-ly, with clinopyroxene phenocrysts.
Olivine II (microphenocrysts): The crystals belongingto Group II are generally less than 2 mm long, showskeletal growth forms (Plate 3, Fig. 4; Plate 4, Fig, 1)
0.01 100.0
Figure 9. Aspect ratio vs. cross-sectional area of blivine crystals in Leg65 basalts.
and occur in glomerophyric clots with plagioclase or,more rarely, as single crystals.
Olivine III (groundmass olivine): Groundmass oliv-ine is common in the highly crystallized interiors of mas-sive basalts, but it is generally replaced by alterationproducts.
Plagioclase/Clinopyroxene Glomerophyric ClotsThree types of plagioclase/clinopyroxene clots with
gradations between all types are recognized in the Leg65 basalts. These include: (1) small clots, generally 1.0mm in length, of strained anhedral clinopyroxene andacicular plagioclase (Plate 4, Fig. 4); (2) intergrowths ofmoderately coarse-grained, subhedral to euhedral clino-
533
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
pyroxene and plagioclase (Plate 4, Fig. 3), and rare clotsof rounded clinopyroxene and plagioclase; and (3) sub-ophitic to ophitie clots of anhedral clinopyroxene andplagioclase ranging up to 5 mm in length. These are dis-tinct from the clinopyroxene/plagioclase clots that formin situ in the zone between the chilled margin and thefully crystallized interior of the thicker cooling units.
Petrographic RelationshipsA number of conclusions may be drawn from the ob-
servations that have been presented. For example, it isclear that plagioclase phenocrysts are much more abun-dant in the Leg 65 basalts than are olivine and clinopy-roxene phenocrysts but that a wide spectrum of assem-blages is observed, including plagioclase + olivine, pla-gioclase + clinopyroxene, plagioclase + olivine + clino-pyroxene, and olivine + spinel. It is also evident thateach of the major phenocryst phases may appear in sev-eral different forms, such as megacrysts, normal pheno-crysts, microphenocrysts, and microlites. Finally, it isapparent that the pillow units at Site 483 are distinctlymore porphyritic than the massive basalts but that intra-cooling-unit variation in phenocryst volumes and pro-portions within the pillow basalts are negligible.
The occurrence of different assemblages and amountsof phenocrysts is clear evidence that crystal fractiona-tion is likely to have played a major role in the evolutionof these magmas. However, the diversity in texturaltypes within a given rock indicates that single-stage,closed-system fractionation is not a realistic model fortheir generation. We interpret the phenocryst texturesand different types of clots in terms of several differentsites and dynamic conditions of crystallization. Of these,the cooling history appears to be the most importantcontrolling factor. Crystals grown at slow cooling ratesare larger and more equant (Figs. 7, 8), a physical rela-tionship that is accentuated by the relatively high degreeof resorption of these crystals occurring under disequi-librium conditions subsequent to their crystallization.Possible mechanisms for the resorption of early formedcrystals include either a decrease in pressure accompa-nied by contraction of the primary plagioclase field oran influx of hot magma into a more evolved, coolermagma and the selective concentration of plagioclase asa result of flotation (see also Flower et al., this volume).The last generation of microphenocrysts, on the otherhand, commonly displays a skeletal habit (Plate 4, Fig.1), suggesting crystallization under rapid cooling condi-tions high in the section.
Apart from the high-pressure(?) megacrysts, the ma-jority of phenocrysts are interpreted as having crystal-lized in the central magma chamber under the ridge. Theoccurrence of gabbroic clots indicates the disruptionand incorporation of slowly cooled parts of the chamberinto ascending magma. Although the phenocryst assem-blages, amounts, and textures reflect a wide range in thedegree of crystallization, the highly porphyritic varietiesencountered in several holes in the Atlantic were notfound, possibly indicating more rapid replenishment ofthe magma reservoirs.
The general lack of significant variation in pheno-cryst abundance within the pillows (Fig. 5) indicates theintrusion of well-mixed, homogeneous magmas. Evenwhere such variation is significant, as in the base of sev-eral of the massive sheet flows in Hole 483B, it is inter-preted as being the result of posteruptive crystal settling.
The more porphyritic nature of the pillows as a wholeis not unique to Site 483 but has also been observed inseveral holes in the Atlantic. A systematic relationship isalso suggested by the observation that thin sheet flowsof a more fractionated, but related chemical composi-tion underlie the pillow lavas (Flower et al., this vol-ume). In such eruptive cycles, the sheet flows possiblyrepresent the upper part of zoned magma chambers. Pil-low sequences have also been observed to overlie sheetflows in the Galapagos Rift zone (Ballard, 1979) and agradation toward more mafic composition is commonin the eruptive cycles observed in the Atlantic (Flowerand Robinson, 1979, 1981).
GEOCHEMISTRYSAMPLE SELECTION AND METHODS
After the petrographic studies had been completed, 15 representa-tive basalt samples were chosen for detailed microprobe studies ofmineral composition and three additional samples were chosen forglass analysis. About 600 analyses were obtained on these samples atthe Max-Planck Institute für Chemie in Mainz using a KEVEX 5100energy dispersive analytical system attached to an ARL-SEMQ micro-probe. An additional 80 analyses were obtained using an automatedCAMEBAX microprobe. Glass analyses were carried out using a de-focused beam. Particular attention was paid to compositional varia-tions within single crystals. Representative mineral and glass analysesare given in Tables 3 through 9, and the average mineral and glasscompositions are summarized for selected basalt samples in Table 10.
Mineral Chemistry
OlivineThe olivine phenocrysts and microphenocrysts ana-
lyzed from Holes 483 and 483B are chemically homoge-neous within each sample. The compositions range fromFo813 for the microphenocrysts examined in Section483-21-3 to Fo86 2 for the phenocrysts in Section 483B-30-3 (Table 3). These olivine crystals are neither in equi-librium (Roedder and Emslie, 1970) with the glass in thegroundmass (where present) (Table 10), nor with pos-sible "primary" magma compositions. This indicatesthat at the time of olivine crystallization, these lavas hadalready undergone some degree of fractionation.
ClinopyroxeneMost of the clinopyroxenes examined consist of augite
(Tables 4, 5). The more primitive phenocryst cores fall inthe endiopside field while the groundmass crystals rangefrom endiopside toward ferroaugite and subcalcic augitein composition (Tables 4, 5; Fig. 10). The phenocrystcore compositions are notably uniform in their majorelement chemistry (100 Mg/Mg + Fe2 += 84.86; Ca41Mg51Fe8); in accordance with their relatively primitivemajor element chemistry, they contain minor Cr2O3 (1.0-1.3%) and relatively little TiO2 (0.3%).
534
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Table 3. Composition of selected olivine phenocrysts.
Component
Major oxide (wt.(
SiO2FeO«a
MnOMgOCaONiO
Total
483-21-3,6-9
39.2917.570.22
42.410.25n.a.
99.73
Cation proportions
SiFeMnMgCaNi
Total
Fo (mole
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
Table 4. Composition of selected clinopyroxene phenocrysts, Holes 483, 483B, and 485A.
Component
483-21-3,6-9
Major oxide (wt.%)
Siθ2TiO 2A1 2O 3FeO* a
MnOMgOCaONa 2 OC r 2 O 3
Total
53.930.321.467.740.13
20.0316.32
0.25
100.18
Cation proportions"
SiAlΣTiFeMnMgCaNaCrΣ
Total
M g N o .
19.5940.627
20.2210.0882.3520.039
10.8456.352
0.07219.747
39.968
82.2
Normative composition
NACJdCCRCTiCATsWoEnFs
0.000.000.720.881.90
30.1154.3911.99
483-21-3,6-9
51.660.993.777.22
17.1819.460.140.40
100.81
18.8331.621
20.4540.2722.201
2.3347.6010.0990.115
19.622
40.076
80.9
0.980.000.162.705.27
33.6546.3210.92
483B-20-2,16-21(rim)
53.690.212.344.83
18.0421.14
0.73
100.98
19.3730.994
20.3670.0581.456
9.7048.175
0.20919.601
39.968
86.9
0.000.002.020.583.40
38.0248.68
7.30
483B-20-2,37-42(core)
53.570.232.194.77
18.1320.58
0.170.74
100.38
19.4260.936
20.3620.0621.445
9.7977.9960.1210.213
19.635
39.997
87.1
1.210.000.920.623.60
37.4348.99
7.23
483B-20-2,37-42
53.570.441.289.560.20
20.5414.34
0.11
100.02
19.5600.552
20.1120.1202.9200.061
11.1775.609
0.03119.917
40.029
79.3
0.000.000.311.201.40
26.5155.7214.86
Sample(interval in cm)
483B-20-2,37-42
50.021.152.81
11.900.14
16.1416.10
98.26
18.9811.256
20.2370.3293.7750.0969.1316.545
19.825
40.062
70.7
0.000.000.003.272.97
29.4045.3718.99
483B-20-2,50-59(core)
52.010,423.635.21
17.3219.990.141.30
100.01
18.9911.560
20.5510.1161.591
9.4247.8210.0970.375
19.189
39.98
85.6
0.970.002.791.165.26
34.6047.24
7.98
483B-20-250-59(rim)
50.960.933.938.39
16.2319.10
99.53
18.8821.715
20.5970.2602.598
8.9637.582
19.403
40.000
77.5
0.000.000.002.605.97
33.6244.8112.99
483B-20-2,50-59
52.960.372.556.08
17.9119.64
0.57
100.09
19.3181.098
20.4160.1021.854
9.7397.676
0.16319.534
39.950
84.0
0.000.001.641.033.67
35.4148.94
9.32
483B-20-2,50-59(core)
53.070.423.036.03
17.5020.32
0.65
101.03
19.2031.292
20.4950.1151.826
9.4407.880
0.18719.447
39.942
83.8
0.000.001.881.164.40
35.9147.47
9.18
483B-20-2,50-59(rim)
49.651.303.89
13.090.17
14.1117.550.30
100.05
18.6961.726
20.4220.3674.1230.0537.9197.0820.217
19.760
40.182
65.8
0.002.130.003.603.81
31.0738.8820.51
Note: NAC = NaCr (Si 2O 6); Jd = NaAl (Si 2O 6); CCR = CaCr (AlSiO6); CTi = Tp = CaTi (A12O6); CATs = CaAl (AlSiO6).a FeO* represents total iron as FeO." Proportions shown on a basis of 60 oxygen atoms.
the calcic plagioclase and magnesian clinopyroxene ob-served in ocean floor basalts. Such an origin requiresmagma mixing at oceanic spreading ridges. Rhodes andDungan (1979) discuss this latter question in some detailbased on residual glass compositions and compositionalvariations in olivine and plagioclase phenocrysts. Theysuggest that mixing and crystal contamination betweenupwelling primary magmas and evolved magmas in asteady-state magma chamber beneath the ridge give riseto a "buffered" ocean floor basalt magma. However,since none of the residual or included glass composi-tions match the composition suggested by Green et al.(1979) a primary magma origin is considered unlikelyfor the calcic phenocrysts considered in this study. It isalso interesting to note that while Rhodes and Dungan(1979) found plagioclase values as high as An85 in liq-uidus runs on Leg 45 basalts, they did not find plagio-clase as calcic as the natural phenocryst compositions.
The disparities observed between experimental workand natural rocks demonstrate the existence of a funda-mental problem with respect to the petrogenesis ofocean floor basalt. Clearly an understanding of the
genesis of highly calcic plagioclase phenocrysts is vitalto determining the origin and evolution of ocean floorbasalts. The Leg 65 results demonstrate the existence ofa continuum of feldspar compositions to values as highas An92 within samples from one area. The presence ofthis compositional continuum is inconsistent with theexistence of a uniform steady-state magma chambersince it demands wide variations in the conditions ofcrystallization. Since experimental studies on primitivebasalts have failed to reproduce the range of phenocrystcompositions seen in submarine basalts and studies ofnatural glass inclusions (Rhodes and Dungan, 1979) failto support the existence of magmas other than those ob-served on the ocean floor, it is clear that the experimen-tal results must be used with caution until the methodol-ogy of experimental petrology has been improved.
ACKNOWLEDGMENTS
We are grateful to the Deutsche Forschungsgemeinschaft for pro-viding financial support for this research (Schm 250/24 and 26). Wealso wish to thank G. Pritchard for providing a number of mineralanalyses; the Max Planck Institut für Kosmochemie (Mainz) and K.Abraham (Ruhr-Universitàt Bochum) for allowing us to use their
536
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Table 4. (Continued).
483B-22-1,28-32(core)
52.770.422.846.15
17.5720.16
0.61
100.52
19.2071.218
20.4250.1151.872
9.5327.863
0.17619.557
39.982
83.6
0.000.001.781.154.07
35.8947.78
9.38
483B-22-1,26-36
52.190.953.787.17
16.8519.47
0.28
100.70
19.0021.620
20.6220.2612.184
9.1947.583
0.08219.264
39.886
80.7
0.000.000.832.645.14
34.1046.2511.05
483B-22-2,63-67
53.670.251.536.33
18.7318.34
0.33
99.78
19.6150.659
20.2740.0682.118
10.2057.181
0.09519.667
39.941
82.8
0.000.000.960.682.15
34.2251.3310.65
483B-22-2,63-67
52.000.512.538.55
17.2018.27
99.06
19.3051.109
20.4140.1432.655
9.5187.268
19.583
39.997
78.2
0.000.000.001.434.12
33.5847.6013.28
483B-22-2,77-82
51.690.843.358.070.25
16.3719.320.28
100.17
19.0381.454
20.4920.2332.4860.0788.9897.6240.200
19.610
40.102
78.3
0.001.980.002.313.90
34.6344.4912.69
Sample(interval in cm)
483B-22-2,77-82
53.210.471.877.080.20
18.0318.470.22
99.55
19.5380.809
20.3470.1302.1740.0629.8707.2660.157
19.569
40.006
81.9
0.001.570.001.301.96
34.6849.3211.17
483B-25-2,39-45
50.381.442.90
11.950.24
14.3418.85
100.11
18.9121.282
20.1940.4083.7520.0778.0267.583
19.845
40.039
68.1
0.000.000.004.062.32
34.5739.9719.07
483B-25-2,39-45(core)
52.890.473.296.27
17.6220.08
0.70
101.31
19.1011.399
20.5000.1281.893
9.4837.768
0.20019.471
39.971
83.4
0.000.002.011.284.73
34.9447.55
9.49
483B-25-2,39-45
52.860.311.798.91
18.7416.67
0.11
99.39
19.4910.777
20.2680.0862.747
10.2976.587
0.03219.750
40.018
78.9
0.000.000.320.862.86
30.8651.3913.71
483B-27-1,12-18(core)
52.060.553.336.15
17.1320.080.240.67
100.22
19.0381.436
20.4740.1531.882
9.3377.8700.1700.193
19.605
40.079
83.2
1.690.000.231.525.49
35.4246.32
9.34
483B-30-3,13-16
53.440.421.796.83
19.7417.05
0.60
99.86
19.4590.766
20.2250.1152.079
10.7126.653
0.17319.732
39.957
83.7
0.00
o.oo1.741.151.82
31.0553.7910.44
483B-32-1,1-6
(core)
54.180.251.856.23
19.7118.12
0.77
101.10
19.4770.785
20.2620.0671.873
10.5586.977
0.21919.693
39.955
84.9
0.000.002.200.672.17
32.5253.039.41
microprobe facilities; A. Fischer and K. Bauszus for drafting andphotography; and V. Helmke for typing the manuscript,facilities; A. Fischer and K. Bauszus for drafting and photography;and V. Helmke for typing the manuscript.
REFERENCES
Ballard, R. D., 1979. The Galapagos Rift at 86°W: 3. Sheet flows,collapse pits, and lava lakes of the rift valley. J. Geophys. Res.,84:5407-5422.
Flower, M. F. J., and Robinson, P. T., 1979. Evolution of the FA-MOUS ocean ridge segment: Evidence from submarine and deepsea drilling investigations. In Talwani, M., Harrison, C. G., andHayes, D. E. (Eds.), Deep Drilling Results in the Atlantic Ocean:Ocean Crust. Sec. Maurice Ewing Series: Washington (Am. Geo-phys. Union), 314-330.
! , 1981. Basement drilling in the western Atlantic Ocean 2. Asynthesis of construction processes at the Cretaceous ridge axis. /.Geophys. Res., 86:6299-6309.
Green, D. H., Hibberson, W. O., and Jaques, A. L., 1979. Petrogen-esis of Mid-ocean Ridge Basalts. In McElhinny, M. W. (Ed.), TheEarth: Its Origin, Structure and Evolution: London (AcademicPress) pp. 265-299.
Kirkpatrick, R. J., 1979. Processes of crystallization in pillow basalts,Hole 396B, DSDP Leg 46. In Dmitriev, L., Heirtzler, J., et al., In-
it. Repts. DSDP, 46: Washington (U.S. Govt. Printing Office),271-282.
Kuo, L.-C, 1980. Morphology and zoning patterns of plagioclase inphyric basalts from DSDP Legs 45 and 46, Mid-Atlantic Ridge[M.A. dissert.]. University of Illinois.
Lofgren, G., 1974. An experimental study of plagioclase crystal mor-phology: Isothermal crystallization. Am. J. Sci., 274:243-273.
Natland, J. H., 1978. Crystal morphologies in basalts from DSDPSite 395, 23°N, 46°W, Mid-Atlantic Ridge. In kelson, W. G.,Rabinowitz, P. D., et al., Init. Repts. DSDP, 45: Washington(U.S. Govt. Printing Office), 423-445.
Rhodes, J. M., and Dungan, M. A., 1979. The evolution of ocean-ridge basaltic magma. Deep Drilling Results in the Atlantic Ocean:Ocean Crust. Sec. Maurice Ewing Series: Washington (Am. Geo-phys. Union), pp. 262-272.
Robinson, P. T., Flower, M. F. J., Swanson, D. A., et al., 1980. Li-thology and eruptive stratigraphy of Cretaceous oceanic crust,western Atlantic Ocean. In Donnelly, T., Francheteau, J., Bryan,W., Robinson, P., Flower, M., Salisbury, M., et al., Init. Repts.DSDP, 51, 52, 53, Pt. 2: Washington (U.S. Govt. Printing Office),1535-1556.
Roedder, P. L., and Emslie, R. F., 1970. Olivine-liquid equilibrium.Contrib. Mineral. Petrol., 29:275-289.
Schmincke, H.-U., Rautenschlein, M., Robinson, P. T., and Me-hegan, J., in press. Accretionary processes inferred from theTroodos Extrusive Series, Cyprus. Earth Planet. Sci. Lett.
537
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
Table 4. (Continued).
Component
483B-32-1,1-6
(rim)
Major oxide (wt. Vo)
SiO 2TiO 2A12O3FeO* a
MnOMgOCaONa 2 OC r 2 O 3
Total
51.670.593.835.37
16.6520.890.281.39
100.68
Cation proportions"
SiAlΣTiFeMnMgCaNaCrΣ
Total
MgNo.
18.8321.645
20.4770.1621.638
9.0488.1580.1970.401
19.604
40.081
84.7
Normative composition
NACJdCCRCTiCATsWoEn
Fs
1.950.002.021.615.54
35.8844.88
8.12
483B-32-1,1-6
(core)
51.710.643.865.78
16.9320.26
0.87
100.05
18.9171.665
20.5820.1761.717
9.2317.942
0.25119.367
39.949
83.9
0.000.002.521.775.34
35.1046.39
8.88
483B-32-1,1-6
(rim)
51.510.934.556.13
17.2420.03
0.291.20
101.89
18.5671.935
20.5020.2531.849
9.2647.7340.2020.342
19.644
40.146
83.4
1.990.001.382.496.35
33.0145.66
9.11
485A-12-1,76-82
51.021.163.737.460.20
15.9019.930.21
99.64
18.8911.628
20.5190.3232.3100.0638.7777.9600.151
19.584
40.103
79.2
0.001.500.003.214.12
35.8443.5611.78
485A-13-1,110-117
52.020.732.797.520.22
17.1018.760.23
99.37
19.2241.215
20.4390.2032.3240.0699.4217.4280.165
19.610
40.049
80.2
0.001.640.002.023.20
34.3546.8811.91
Sample(interval in cm)
485A-3O-3,38-41
52.800.631.598.680.27
17.5017.790.21
99.46
19.5310.693
20.2290.1752.6850.0859.6517.0510.151
19.798
40.022
78.2
0.001.510.001.750.96
33.8248.1513.82
485A-30-4,8-13
53.030.551.916.340.19
17.2120.28
0.22
99.73
19.3510.837
20.1880.1541.9270.0609.5428.0810.159
19.923
40.111
83.2
0.001.570.001.521.83
38.2047.08
9.80
485A-30-4,8-13
53.860.841.72
10.890.33
16.0118.040.25
101.94
19.6030.738
20.3410.2303.3150.1028.6877.0350.176
19.545
39.886
72.4
0.001.780.002.330.52
34.1643.9417.28
485A-30-4,8-13
51.780.711.699.720.16
15.8118.75
98.62
19.4740.749
20.2230.2013.0570.0518.8647.555
19,728
39.951
74.4
0.000.000.002.021.74
36.0844.5415.62
485A-39-1,26-32
50.751.083.778.03
15.5920.38
0.33
99.94
18.7991.647
20.4460.3022.488
8.6068.088
0.09719.581
40.027
77.6
0.000.000.973.014.72
35.9842.9112.41
485A-39-1,26-32
52.470.652.01
11.070.27
17.6316.46
100.49
19.3370.871
20.2080.1813.3920.0849.6816.501
19.839
40.047
74.1
0.000.000.001.802.53
30.1948.1817.30
538
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Table 5. Composition of selected clinopyroxene groundmass crystals.
Component483B-20-2,
16-21
Major Oxide (wt.%)
SiO2T1O2A12O3FeO*a
MnOMgOCaONa2OCr 2O 3
Total
50.561.152.02
15.570.25
12.7417.73
100.0
Cation Proportions
SiAl
tTiFeMnMgCaNaCr£Total
Mg No.
19.2460.906
20.1520.3284.9570.0807.2277.230
19.821
39.973
59.3
Normative Composition
NACJDCCRCTiCATsWoEnFs
0.000.000.003.291.25
33.9836.2325.25
483B-20-2,16-21
50.881.232.52
13.380.21
15.6416.02
99.88
19.0931.113
20.2060.3464.2000.0668.7456.442
19.798
40.004
67.6
0.000.000.003.462.10
29.4143.7021.32
483B-20-2,50-59
50.101.154.378.37
16.8917.780.150.38
99.18
18.6161.916
20.5320.3202.601
9.3547.0790.1050.112
19.570
40.102
78.2
1.040.000.073.176.28
30.2846.2912.87
483B-20-2,50-59
52.620.683.246.29
17.6719.740.170.51
100.93
19.0751.383
20.4580.1861.908
9.5467.6680.1210.146
19.576
40.034
83.3
1.210.000.251.854.91
34.7047.57
9.51
483B-20-2,50-59
50.581.263.05
14.490.22
14.3316.76
100.69
18.9601.346
20.3060.3554.5410.0718.0076.731
19.706
40.012
63.8
0.000.000.003.553.18
30.2639.9923.03
483B-20-2,50-59
48.822.185.67
11.460.13
19.9817.33
100.58
18.1332.482
20.6150.6103.5580.0428.2956.896
19.401
40.016
70.0
0.000.000.006.096.30
28.2341.4117.97
Sample(interval in cm)
483B-22-2,63-67
49.451.434.778.840.12
15.6119.070.290.11
99.68
18.4202.094
20.5140.4002.7530.0398.6667.6110.2070.031
19.707
40.221
75.9
0.301.720.003.915.47
32.5442.3913.66
483B-22-2,63-67
50.581.183.55
10.360.27
17.7715.42
0.10
99.23
18.8231.557
20.3800.3303.2230.0859.8566.149
0.03119.673
40.053
74.4
0.000.000.313.284.31
26.6349.0216.45
483B-27-1,12-18
50.321.062.10
15.010.35
14.3016.280.27
99.69
19.1310.941
20.0720.3034.7730.1148.1036.6310.196
20.120
40.192
62.9
0.001.920.002.970.68
30.7039.7523.97
483B-27-1,12-18
50.291.443.99
10.20
15.0918.760.220.21
100.20
18.6971.749
20.4460.4023.170
8.3617.4710.1620.062
19.630
40.076
72.5
0.620.990.003.994.19
32.9841.4915.73
485A-39-1,26-32
50.790.882.819.14
16.2318.38
0.35
98.60
19.0671.244
20.3110.2502.869
9.0837.393
0.10419.698
40.009
76.0
0.000.001.042.503.20
33.5645.3714.33
485A-39-1,26-32
53.211.461.28
11.350.21
18.0815.91
100.50
19.5820.555
20.1370.1283.4940.0659.9166.273
19.875
40.012
73.9
0.000.000.001.281.49
29.9449.5217,77
485A-39-1,26-32
51.481.021.89
15.070.34
15.6415.260.17
100.87
19.2390.831
20.0700.2874.7100.1088.7136.1110.120
20.049
40.119
64.9
0.001.190.002.840.68
28.4443.0523.81
Note: NAC = NaCr (Si2O6); Jd = NaAl (Si2C>6); CCR = CaCR (AlSiO6); CTi = Tp = CaTi (A12O6); CATs Ca* FeO' represents total iron as FeO.
Proportions shown on a basis of 60 oxygen atoms.
539
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
Table 6. Composition of selected plagioclase phenocrysts, Holes 483, 483B, and 485A.
Component
Major oxides (v
SiO2TiO 2A12O3FeO* a
M n O
MgO
C a O
Na 2 OK 2 O
SrO
483-21-3,6-9
(rim)
rt.%)
48.15
33.510.52
0.1816.55
1.92
483-21-3,6-9
(core)
52.44
30.010.52
0.2513.223.940.05
483-21-3,6-9
(core)
49.69
32.500.40
0.2415.822.510.07
483B-8-1,24-27
47.830.04
33.320.38
0.1817.17
1.74
483B-8-1,24-27
. 47.52
32.990.39
0.1717.11
1.740.01
483B-13-1,60-64
48.030.04
32.630.520.020.17
16.492.040.01
Sample(interval in cm)
483B-13-1,60-64
49.350.04
31.960.520.020.11
15.602.560.010.06
483B-20-2,16-21
(rim)
53.62
28.570.88
0.2711.674.900.08
483B-20-2,16-21
48.13
34.090.29
0.1717.34
1.68
483B-20-2,16-21(core)
49.75
31.710.37
0.1314.753.02
483B-20-2,37-42(core)
50.41
32.510.45
0.2615.582.650.06
483B-20-2,37-42(core)
52.40
30.380.44
0.3013.563.950.06
483B-20-2,37-42(rial)
58.73
25.760.66
0.168.067.220.12
Cation proportions
Si
ΛIΣTiFé
MgCaNaKSrE
Total
An(mole%)
8.7632.188
15.951
0.079
0.0493.2260.678
4.031
19.982
82.6
9.4946.404
15.898
0.078
0.0672.5641.3820.012
4.103
20.001
64.8
8.9856.926
15.911
0.061
0.0663.0650.8810.017
4.089
20.000
77.3
8.7317.169
15.9000.0050.058
0.0493.3580.616
4.086
19.986
84.5
8.7417.152
15.893
0.060
0.0473.3720.6210.002
4.102
19.995
84.4
8.8247.065
15.8890.0060.0800.0030.0473.2460.7270.0020.0044.115
20.004
81.7
9.0186.883
15.9010.0050.0790.0030.0303.0540.9070.0020.0064.086
19.987
77.1
9.7386.116
15.845
0.134
0.0732.2711.7260.019
4.223
20.077
56.5
8.6907.255
15.945
0.043
0.0463.3540.587
4.031
19.976
85.1
9.1086.842
15.950
0.057
0.0352.8941.071
4.057
20.007
73.0
9.0446.875
15.919
0.067
0.0692.9940.9230.014
4.067
19.986
76.2
9.4356.448
15.883
0.066
0.0802.6161.3780.130
4.153
20.036
65.3
10.4715.412
15.883
0.098
0.0431.5402.4940.027
4.201
20.084
37.9
a FeO* represents total iron as FeO.Proportions shown on a basis of 32 oxygen atoms
Table 6. (Continued).
Component
Major oxides (v
SiO2TiO2A12O3FeO aMnOMgOCaONa2OK 2 OSrO
483B-25-2,39-45
rt.%)
55.49
27.460.87
0.1410.265.700.09
483B-25-2,39-45(core)
48.77
31.540.32
0.2615.142.67
483B-25-2,39-45
52.06
30.210.49
0.2313.373.880.06
483B-27-1,12-18
50.15
31.190.41
0.2814.603.040.06
483B-27-1,12-18
48.22
32.530.31
0.1315.912.09
483B-27-1,12-18
52.20
30.350.54
0.2713.463.840.06
Sample(interval in cm)
483B-30-3,13-16(core)
46.33
33.890.33
0.3517.54
1.25
483B-30-3,13-16
51.61
30.480.59
0.2913.843.66
483B-3O-3,13-16(core)
48.62
32.640.21
0.2115.992.36
483B-32-1,1-6
51.99
30.170.56
0.3413.593.710.10
483B-32-1,1-6
(core)
47.26
33.290.32
0.2217.16
1.53
483B-32-1,1-6
48.63
32.340.31
0.2215.772.25
483B-32-1,1-6
(rim)
53.67
28.860.83
0.4512.494.550.18
Cation proportions''
SiAlETiFeMnMgCaNaKSr
Total
An(mole%)
10.0295.849
15.878
0.135
0.0371.9871.9960.022
4.177
20.055
49.6
9.0346.885
15.919
0.050
0.0713.0040.959
4.084
20.003
75.8
9.4446.458
15.902
0.074
0.0622.5991.3650.013
4.114
20.016
65.4
9.1806.72915.90
0.063
0.0762.8631.0780.014
4.093
20.002
72.4
8.8957.071
15.966
0.048
0.0373.1440.748
1977
19.943
80.8
9.4426.449
15.891
0.082
0.0722.6081.3460.015
4.123
20.014
65.7
8.5517.373
15.924
0.051
0.0973.4690.448
4.063
19.987
88.6
9.3636.517
15.880
0.089
0.0772.6891.287
4.142
20.022
67.6
8.8987.039
15.937
0.032
0.0593.1350.838
4.064
20.001
78.9
9.4266.446
15.872
0.085
0.0932.6391.3020.023
4.142
20.014
66.6
8.6997.221
15.920
0.049
0.0623.3850.547
4.043
19.963
86.1
8.9387.005
15.943
0.048
0.0613.1050.802
4.017
19.960
79.5
9.6696.128
15.797
0.125
0.1202.4111.5890.041
4.285
20.082
59.7
540
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Table 6. (Continued).
483B-20-2,50-59(rim)
53.39
29.410.71
0.2512.564.40
483B-20-2,50-59
51.38
30.420.49
0.3013.833.48
483B-20-2,50-59
46.58
33.590.33
17.091.15
483B-20-2,50-59
49.68
32.420.39
15.412.63
483B-20-2,50-59
50.29
31.570.40
0.2714.962.910.06
483B-20-2,50-59
47.52
33.040.35
0.2416.621.820.06
483B-20-2,50-59(core)
49.10
32.400.45
0.2015.942.27
Sample(interval in cm)
483B-20-2,50-59(rim)
58.70
25.330.70
7.637.260.12
483B-22-1,26-32(rim)
53.76
28.490.72
0.3211.984.650.05
483B-22-1,26-32
50.95
30.060.55
0.2913.503.430.06
483B-22-1,26-32
48.47
31.920.35
0.1815.432.340.05
483B-22-2,63-67(rim)
54.46
29.100.86
0.3012.254.790.08
483B-22-2,63-67
49.65
31.850.34
0.2515.262.600.06
483B-22-2(63-67(core)
51.37
30.540.51
0.2313.853.650.10
483B-22-2,77-82
52.140.08
29.570.700.020.20
13.643.570.010.06
99.99
9.6286.521
16.149
0.107
0.0662.4261.540
3.68
20.017
61.2
9.3646.533
15.897
0.075
0.0822.7001.231
4.088
19.985
68.7
8.6547.354
16.008
0.052
3.4010.413
3.867
19.875
89.2
9.0296.945
15.974
0.059
3.0010.927
3.987
19.961
76.4
9.1436.766
15.909
0.061
0.0722.9141.0260.013
4.085
19.994
73.7
8.7537.174
15.927
0.053
0.0673.2800.6490.015
4.064
19.991
83.2
8.9566.965
15.921
0.068
0.0553.1140.805
4.043
19.964
79.5
10.5495.366
15.915
0.105
1.4702.5280.028
4.131
20.046
36.5
9.7566.093
15.849
0.109
0.0872.3301.6360.012
4.173
20.022
58.6
9.3836.525
15.908
0.085
0.0792.6631.2240.014
4.065
19.973
68.3
8.9776.969
15.946
0.054
0.0503.0620.8390.013
4.018
19.964
78.2
9.7176.119
15.836
0.128
0.0812.3411.6560.019
4.225
20.061
58.3
9.0706.857
15.927
0.051
0.0692.9870.9190.014
4.041
19.968
76.2
9.3446.547
15.891
0.077
0.0612.6991.2860.023
4.143
20.034
67.3
9.4976.348
15.8450.0110.1070.0030.0542.6621.2610.0020.0064.006
19.951
67.8
Table 6. (Continued).
485 A-11-3,68-73
50.160.08
31.080.59
0.2015.093.04
0.04
100.28
9.1566.686
15.8420.0110.090
0.0542.9511.076
0.0044.186
20.028
73.3
485A-12-1,76-82
51.820.08
29.550.670.020.25
13.873.690.010.06
100.02
9.4526.353
15.8050.0110.1020.0030.0682.7111.3050.0020.0064.208
20.013
67.5
485A-13-1,110-117
51.870.06
30.070.630.020.18
14.243.46
0.07
100.60
9.4076.427
15.8340.0080.0960.0030.0492.7671.217
0.0074.147
19.981
69.5
485A-13-1,110-117
56.150.15
27.121.100.020.11
11.065.360.010.06
101.14
10.0575.725
15.7820.0200.1650.0030.0292.1221.8610.0020.0064.208
19.990
53.2
485A-3O-3,38-41
52.060.06
29.080.72
0.0913.903.850.01
99.77
9.5206.268
15.7880.0080.110
0.0252.7231.3650.002
4.233
20.021
66.6
485A-30-4,8-13
53.340.11
29.010.760.030.22
12.794.320.020.07
100.67
9.6406.179
15.8190.0150.1150.0050.0592.4771.5140.0050.0074.197
20.016
62.0
Sample(interval in cm)
485A-30-4,8-13
51.220.06
30.160.600.020.17
14.143.570.020.06
100.02
9.3516.490
15.8410.0080.0920.0030.0462.7661.2640.0050.0064.190
20.031
68.6
485A-30-4,8-13
50.11
29.830.55
14.462.74
97.69
9.3466.558
15.904
0.086
2.8900.991
3.967
19.871
74.5
485A-39-1,26-32
50.89
30.850.45
0.2814.193.34
100.0
9.2766.628
15.904
0.069
0.0752.7721.181
4.096
20.000
70.1
485A-39-1,26-32
57.06
26.560.62
0.119.286.44
100.07
10.2615.629
15.890
0.094
0.0291.7872.247
4.157
20.047
44.3
485A-39-1,26-32(core)
45.38
34.660.26
0.1818.270.99
99.74
8.3917.553
15.944
0.040
0.0503.6190.355
4.065
20.009
91.1
485A-39-1,26-32(rim)
53.71
28.970.80
0.1912.124.69
100.49
9.7046.169
15.873
0.120
0.0502.3471.644
4.161
20.034
58.8
485A-39-1,26-32
48.69
32.710.37
0.2416.172.14
100.32
8.8907.037
15.927
0.057
0.0663.1630.757
4.043
19.970
80.7
485A-39-1,26-32
53.36
29.160.67
0.2212.344.630.05
100.43
9.6526.217
15.869
0.101
0.0592.3911.6260.012
4.189
20.058
59.3
541
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
Table 7. Composition of selected plagioclase groundmass crystals.
Component
Major Oxides (\
SiO2A12O3
F eO aMgOCaONa2OK2O
Total
483B-20-2,16-21
vt.%)
53.8429.030.780.22
11.864.67
100.40
Cation Proportions
SiAlΣFeMgCaNaKE
Total
An mole%
9.7216.179
15.9000.1170.0602.2951.636
4.107
20.007
58.4
483B-20-2,16-21
56.2927.070.77
9.798.84
99.76
10.1635.761
15.9240.116
1.8932.044
4.054
19.978
48.1
483B-20-2,37-42
52.7729.590.810.27
12.734.320.07
100.56
9.5506.313
15.8630.1230.0722.4691.5150.0164.196
20.059
61.7
Sample(interva
483B-20-2,50-59
56.7127.47
1.050.059.905.950.07
101.20
10.1175.775
15.8920.1560.0221.8912.0570.0124.138
20.030
47.8
il in cm)
483B-20-2,50-59
52.7229.38
1.100.35
12.634.18
100.34
9.5656.281
15.8460.1660.0942.4551.469
4.183
20.029
62.6
483B-20-2;50-59
54.1928.190.830.17
12.154.47
100.73
9.7546.133
15.8870.1260.0462.3421.559
4.072
19.959
60.0
483B-2O-2,50-59
57.2427.040.790.089.636.170.09
101.04
10.2095.683
15.8920.1180.0221.8402.1330.0214.134
20.026
46.1
483B-27-1,12-18
52.3929.610.590.26
13.084.10
100.04
9.5256.346
15.8710.0900.0712.5481.446
4.154
20.025
63.8
a FeO* represents total iron as FeO.Proportions shown on a basis of 32 oxygen atoms.
Table 8. Composition of selected Fe/Ti-oxides.
Component
Major oxides
Siθ2Tiθ2A12O3FeO* a
FeOF e 2 O 3MnOMgOCaO
V 2 O 3
Total
483-22-2,89-83
(wt.%)
0.5222.93
5.0868.34
(51.13)(19.13)
0.730.650.330.20
98.78
(100.7)
Cation proportions"3
SiAlT i oF e 2 +
Fe 3 +
MnMgCaV
Total
0.1872.1576.211
15.4035.1960.2230.3490.1270.058
29.911
Sample(interval in cm)
483-22-2,89-83
23.945.16
67.48(52.57)(16.57)
0.480.480.17
97.99
(99.37)
2.2306.601
16.1194.5720.1490.2620.067
30.0
483-22-2,89-83
21.776.67
70.47(51.87)(20.67)
0.420.410.110.37
100.22
(101.92)
2.7885.807
15.3865.5170.1260.2170.0420.105
29.988
485A-30-4,8-13
0.5521.15
1.9971.28
(49.74)(23.93)
0.820.53
96.32
(98.71)
0.2070.8815.972
15.6216.7620.2610.297
30.001
Note: Recalculated values shown in parentheses.j* FeO* represents total iron as FeO.k Proportions shown on a basis of 40 oxygen atoms.
542
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Table 9. Selected glass compositions.
Major Oxides(wt.%)
SiO2TiO2A12O3FeO*b
MnOMgOCaONa2OK2OP2O5SO3
Total
483-21-3,6-9a
50.512.04
14.3511.51
—4.39
12.602.580.120.140.45
98.70
483-21-3,6-9
50.102.00
13.9111.400.177.04
11.222.850.080.200.55
99.53
483-21-3,6-9
51.692.11
14.0712.18—6.90
11.190.670.080.140.53
99.57
Sample(interval in cm)
483B-22-1,26-32
50.471.94
14.0011.470.137.11
11.372.910.090.170.48
100.16
483B-22-2,77-82
51.951.90
13.3811.430.236.45
10.862.720.07——
98.99
483B-3O-3,13-16
50.241.86
13.9811.37
—
7.1511.392.630,08—
0.29
99.00
483B-32-1,1-6
50.791.73
14.3510.48
7.6211.962.850.120.110.47
100.48
a Glass inclusion in olivine." FeO represents total iron as FeO.
Table 10. Average mineral and glass compositions for selected basalts from Holes 483 and 483B.
Sample(interval in cm)
Hole 483
21-3, 6-9
22-2, 85-88
26-1, 39-40
Hole 483B
20-2, 16-2120-2, 37-42
20-2, 50-59
20-2, 50-59
22-1, 26-3222-2, 63-6725-2, 39-4527-1, 12-18
30-3, 13-16
30-3, 18-2232-1, 1-6
39-1, 26-32
Olivine
CrystalType
MP
P
—
—
-
—
P——
1
PP
MP, P
—
Mg No.a
81.3 (10)
83.3 (7)
—
—
-
-
81.6 (7)———
86.2 (4)82.9 (6)84.5 (2)84.5 (11)
—
CrystalType
Gl
P
—
MPMP
GL, MP
MMP
MP, GLMP, GLMP, GLMP, GL
MP, GL
—MP, GL
MP, GL
Phenocrysts
Clinopyroxene
Mg No.a
82.8 (4)
85.6 (6)
—
86.9 (4)87.1 (5)
85.6 (9)
84.1 (7)84.7 (7)84.0 (14)83.1 (15)84.7 (23)83.1 (4)
84.2 (8)
—84.9 (12)
77.7 (6)
Ca:Mg:Fe
37:52:11
41:51:8
—
42:50:842:51:7
42:50:8
40:50:1041:50:940:50:1040:50:1040:50:941:49:10
42:49:9
—36:54:10
42:45:13
Plagioclase
CrystalType
IMP
IMPP
pP
IM\P
P
PPPP
!GPLPP
| M
IGL
An(mole
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
• Megacrysts
o Phenocrysts, groundmass
Ca
Figure 10. Quadrilateral diagram showing compositions of clinopy-roxene megacrysts, phenocrysts, and microphenocrysts in Leg 65basalts.
100
90-/'
• Plagioclase phenocrysts .
+ Plagioclase microlites jL
80-/
40 -
6V
50 -/.
10
20
30-/10
Or
20
Figure 11. Ternary plot showing selected plagioclase compositions forLeg 65 basalts.
544
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Plate 1. Massive basalts recovered on Leg 65. 1. Sample 483B-20-2, 62-65 cm; thin layer of sandstone adhering to the devitrified base of a massivelava flow (Lithologic Unit 6) (see also Fig. 2.). 2. Sample 483B-20-2, 62-65 cm; basal contact of massive sheet flow(?) (Unit 6) against underlyingsiltstone (light-colored); glass shows dark alteration against silt and on both sides of straight fracture filled with smectite; glass rinds on massiveflows are thinner than on pillows (width of photomicrograph, 5 mm). 3. Sample 485A-38-2, 5-10 cm; cross section through top of deepestmassive (intrusive?) basalt drilled in Hole 485A (Lithologic Unit 8) showing devitrified glass stringers in siltstone (width of photograph, 2mm). 4. Sample 483-16-2, 64-70 cm; fine-grained basalt intruding (self-intruding) medium-grained, massive basalt (Unit 3) (width of photomi-crograph, 5 mm).
545
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
35 rW
Plate 2. Pillow basalts recovered on Leg 65. 1. Sample 483B-18-2, 35-40 cm; highly vesicular basalt 1.5 meters below top of 3.5-meter-thick massiveflow (Lithologic Unit 6) (see also Fig. 3). 2. Sample 483B-26-2, 42-47 cm; massive, vesicular basalt with a coarse-grained, subophitic to interser-tal groundmass (Lithologic Unit 8); vesicles to right and center lined with carbonate and filled with smectite; vesicle on left filled with chlorite(?);vesicle to the upper right is a filled segregation vesicle (width of photomicrograph, 5 mm). 3. Sample 483B-18-2, 18-22 cm; partially filledsegregation vesicle subsequently lined with smectite(?) showing contraction cracks; note plagioclase laths aligned tangential to vesicle (see Fig. 1)(width of photomicrograph, 5 mm). 4. Sample 482D-11-1, 24-28 cm; cross section through thick, fresh glassy margin of porphyritic pillow lava(50 cm thick) (see also Plate 4, Fig. 1 for photomicrograph of olivine and plagioclase microphenocrysts) (width of photomicrograph, 2 cm).
546
LITHOLOGY, PETROLOGY, MINERALOGY OF BASALTS
Plate 3. Plagioclase phenocrysts. 1. Sample 483B-7-1, 7-12 cm; plagioclase megacryst in massive flow (Lithologic Unit 3) showing inclusion-rich,partially resorbed core (light gray), an oscillatory zoned mantle, and a dark outer rim separated from the mantle by a sharp compositional break(width of photomicrograph, 5 mm). 2. Sample 483-23-1, 10-16 cm; glomerocryst in pillow basalt (Unit 6) consisting of three partially resorbedplagioclase xenocrysts with numerous glass inclusions and two subhedral plagioclase phenocrysts (width of photomicrograph, 5 mm). 3. Sample483-21-1, 49-55 cm; euhedral to subhedral clot of regularly zoned plagioclase phenocrysts in tachylitic matrix of pillow basalt (Unit 6) (width ofphotomicrograph, 5 mm). 4. Sample 485A-36-3, 65-69 cm; clot of skeletal olivine and weakly skeletal plagioclase microphenocrysts in fine-grained pillow basalt(?) (Unit 7) with a tachylitic groundmass containing skeletal plagioclase microlites (width of photomicrograph, 2 mm).
547
B. J. GRIFFIN, R. D. NEUSER, H.-U. SCHMINCKE
~/ r
IP
Plate 4. Phenocrysts and clots. 1. Sample 482D-11-1, 24-28 cm; clot of skeletal plagioclase and olivine microphenocrysts in a matrix of fresh sidero-melane from the margin of a pillow; microphenocrysts show minor terminal in situ growth (width of photomicrograph, 2 mm). 2. Sample483-22-2, 68-72 cm; partially resorbed clinopyroxene megacryst enclosing euhedral plagioclase phenocrysts in a tachylitic pillow margin(Lithologic Unit 6) (width of photomicrograph, 5 mm). 3. Sample 483-22-2, 77-82 cm; clot of subhedral to euhedral clinopyroxene and plagio-clase crystals in a tachylitic pillow margin (Lithologic Unit 6) (width of photomicrograph, 5 mm). 4. Sample 483B-29-1, 55-57 cm; clot ofstrained anhedral clinopyroxene and plagioclase in an intersertal pillow basalt matrix (Lithologic Unit 9) (width of photomicrograph 5 mm).
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