Erzinger, J., Becker, K., Dick, H.J.B., and Stokking, L.B. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 137/140

29. DATA REPORT: MAGNETIC MINERALOGY, MAJOR- AND TRACE-ELEMENT GEOCHEMISTRY,AND ROCK MAGNETIC PROPERTIES OF HOLE 504B UPPER CRUSTAL ROCKS1

Laura B. Stokking,2 Elizabeth A. Heise,2 Janet E. Pariso,3 and Simon A. Allerton4

ABSTRACT

Leg 140 of the Ocean Drilling Program deepened Hole 504B to a total depth of 2000.4 m below seafloor (mbsf), making it thedeepest hole drilled into ocean crust. Site 504, south of the Costa Rica Rift, is considered the most important in-situ reference sectionfor the structure of shallow ocean crust. We present the results of studies of magnetic mineralogy and magnetic properties of Hole504B upper crustal rocks recovered during Legs 137 and 140. Results from this sample set are consistent with those discussed inPariso et al. (this volume) from Legs 111, 137, and 140. Coercivity (Hc) ranges from 5.3 to 27.7 mT (mean 12 mT), coercivity ofremanence (HCR) ranges from 13.3 to 50.6 mT (mean 26 mT), and the ratio HCRIHC ranges from 1.6 to 3.19 (mean 2.13). Saturationmagnetization (js) ranges from 0.03 to 5.94 × 10"6 Am2, (mean 2.52 × 10~6 Am2), saturation remanence (JR) ranges from 0.01 to0.58 × 10~6 Am2 (mean 0.37 × 10"6 Am2), and the ratio JRIJS ranges from 0.08 to 0.29 (mean 0.16), consistent with pseudo-single-domain behavior. Natural remanent magnetization (NRM) intensity ranges from 0.029 to 7.18 A/m (mean 2.95 A/m), whereas RM10

intensity varies only from 0.006 to 4.8 A/m and has a mean of only 1.02 A/m. Anhysteretic remanent magnetization (ARM) intensityranges from 0.04 to 6.0 A/m, with a mean of 2.46 A/m, and isothermal remanent magnetization (IRM) intensity ranges from 0.5 to1683 A/m, with a mean of 430.7 A/m. Volume susceptibility ranges from 0.0003 to 0.043 SI (mean 0.011 SI). In all samples examined,high-temperature oxidation of primary titanomagnetite has produced lamellae or pods of magnetite and ilmenite. Hydrothermalalteration has further altered the minerals in some samples to a mixture of magnetite, ilmenite, titanite, and a high-titanium mineral(either rutile or anatase). Electron microprobe analyses show that magnetite lamellae are enriched in the trivalent oxides Cr2O3,A12O3, and V2O5, whereas divalent oxides (MnO and MgO) are concentrated in ilmenite lamellae.

INTRODUCTION

During Leg 140 of the Ocean Drilling Program, Hole 504B wasextended to a total depth of 2000.4 m below seafloor (mbsf), makingit the deepest hole ever drilled into ocean crust. Site 504 is located 201km south of the Costa Rica Rift, the easternmost arm of the GalapagosSpreading Center (latitude 1 ° 13.61 l'N; longitude 83°43.818'W; waterdepth 3460 m), in 5.9-m.y.-old crust (Fig. 1). Hole 504B provides ourmost important in-situ reference section for the structure of shallowocean crust. Leg 140 was the seventh leg of DSDP/ODP to occupyHole 504B, which was begun during DSDP Leg 69 and subsequentlydeepened and/or logged during parts of five other legs: Leg 70 (1979),Leg 83 (1981-1982) Leg 92 (1983), Leg 111 (1986), and Leg 137(1991). These legs provided a wealth of scientific results, muchof which is summarized by CRRUST (1982); Cann, Langseth,Honnorez, Von Herzen, White, et al. (1983); Anderson, et al. (1982);Anderson, Honnorez, Becker, et al. (1985); Leinen, Rea, et al. (1986);Becker, Sakai, et al. (1988, 1989); Becker et al. (1989); Alt et al.(1986); Becker, Foss, et al. (1992); and Dick, Erzinger, Stokking, etal., (1992).

We present the results of studies of magnetic mineralogy andmagnetic properties of Hole 504B upper crustal rocks drilled duringLegs 137 and 140. Pariso et al. (this volume) discuss magnetic petrog-raphy in conjunction with magnetic properties, and Allerton, McNeillet al. (this volume) relate structural to magnetic fabrics, and Allerton,Pariso et al. (this volume) address components of natural rema-nent magnetization.

Erzinger, J., Becker, K., Dick, H.J.B., and Stokking, L.B. (Eds.), 1995. Proc. ODP,Sci. Results, 137/140: College Station, TX (Ocean Drilling Program).

2 Ocean Drilling Program, Texas A&M University Research Park, 1000 DiscoveryDrive, College Station, TX 77845-9547, U.S.A.

3 School of Oceanography, University of Washington, Seattle, WA98195, U.S.A.4 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR,

United Kingdom.

BACKGROUND

Understanding the structure, tectonics, mineralogy, and chemicalalteration processes in the oceanic crust is one of the most importantscientific goals of the Ocean Drilling Program and the primary objec-tive of Leg 140. Knowledge of the nature of the oceanic basement isderived mainly from studies of ophiolite complexes, remote geo-physical surveys, and studies made on dredged or cored samples fromthe ocean floor. Only Hole 504B penetrates through the extrusivepillow lavas into the sheeted dike complex to provide a representativein-situ section of the ocean crust.

Although the magnetic structure of the extrusive rocks of theoceanic crust is well studied, little is known about the magnetic struc-ture of intrusive oceanic rocks or their contribution to marine mag-netic anomalies. Studies of rock magnetic properties and the potentialcontribution to marine magnetic anomalies have been performed onupper crustal rocks drilled during Legs 111 (upper sheeted dike sec-tion, Hole 504B; Pariso and Johnson, 1989, 1991) and 118 (gabbros,Atlantis II Fracture Zone, Hole 735B; Kikawa and Pariso, 1991). Thepotential contribution of the material to a magnetic anomaly is evalu-ated by the measurement of magnetic susceptibility and remanencecharacteristics, such as natural remanent magnetization (the sum ofall types of remanence acquired by the sample, including any thermalremanence retained by primary phases, chemical remanence acquiredby any secondary minerals produced during alteration or weathering,and viscous remanence), stability of remanence, and the Koenigsber-ger ratio: the ratio of remanent to induced magnetization. Pariso andJohnson (1991) demonstrated that the upper portion of the sheeteddike complex at Hole 504B drilled during Leg 111 may contributesubstantially to the magnetic anomaly measured at the sea surface.

The magnetic properties of rocks depend on many variables: thetypes of magnetic minerals present in the rock (which themselvesreflect the initial rock composition and any alteration, thermal orchemical, that the rock has undergone), the concentrations and thegrain volumes of the various magnetic minerals in the rock, and themagnetic domain state of those minerals. The predominant magneticmineral in ocean crustal rocks is titanomagnetite, a spinel that is a solid

327

DATA REPORT

10° N

- 5°

- 0c

90° W - 5°S

Figure 1. Location of Hole 504B (after Hobart et al., 1985).

solution between iron and titanium, which may also contain aluminum,chromium, magnesium, vanadium, manganese, and nickel. Secondaryphases, such as titanomaghemite, ilmenite, magnetite, hematite, andtitanite, are produced by thermal and chemical alteration of titanomag-netite. In some cases, secondary phases and alteration textures (i.e., theex solution of primary titanomagnetite into low-titanium titanomag-netite and high-titanium hemoilmenite; Haggerty, 1976; Smith et al.,1991) can be observed using reflected light microscopy (Ade-Hall etal., 1971; Johnson and Hall, 1978). In other cases, however, the exso-lution lamellae are too fine to be observed optically but can be detectedby the disagreement between the bulk composition of a grain (deter-mined by electron microprobe analysis) and the Curie temperature (thetemperature above which a mineral behaves paramagnetically and

cannot retain a magnetic remanence) predicted on the basis of that bulkcomposition (Evans and Wayman, 1974; Smith and Banerjee, 1986;Smith etal., 1991).

Magnetic minerals are identified and characterized on the basis ofreflected and transmitted light microscopy in conjunction with elec-tron microprobe analysis, alternating field demagnetization behavior,anhysteretic and isothermal remanence acquisition, and hysteresisparameters. Alternating field (AF) demagnetization yields the coer-civity spectrum, (a property that, assuming the sample contains asingle type of magnetic carrier, is itself dependent on the grain-volume spectrum in the rock) and indicates whether the coercivitydetermined by measurement of hysteresis represents that of the car-rier of remanence. Measurement of magnetic hysteresis (the magneti-

328

DATA REPORT

zation behavior exhibited by a sample in the presence of an increasingmagnetic field) will determine coercivity, saturation remanence, andsaturation magnetization. These parameters, in conjunction with co-ercivity of remanence (a measurement of the reverse field required toremove saturation isothermal remanence), will then provide con-straints on the domain state of the magnetic minerals in the sample(Dunlop, 1969).

LITHOLOGY AND ALTERATION

The Hole 504B basement section includes 571.5 m of pillow lavasand minor flows, underlain by a 209-m transition zone of mixed pillowlavas, thin flows and dikes, and 945.4 m of sheeted dikes and massiveunits (Fig. 2). Coring during Leg 140 recovered 47.7 m, of which11.4% was aphyric, 18.6% sparsely phyric, and 70% moderately phyricplagioclase-pyroxene-olivine diabase, which was divided into 59 lith-ological units (Dick, Erzinger, Stokking, et al., 1992). The coarsest unitidentified has an average grain size of 1.5 mm, but in terms of textureand grain size, it is a diabase and not a gabbro. Phenocrysts includeplagioclase, augite, olivine, and Cr-rich augite. The groundmass isdominated by plagioclase, augite, and titanomagnetite. In a few unitsCr-spinel is present as inclusions in olivine and plagioclase. Most ofthe rocks are seriate porphyritic, a texture in which there is a continu-ous range of grain sizes from the phenocrysts down to the groundmass(Dick, Erzinger, Stokking, et al., 1992).

All of the recovered rocks are mineralogically and chemicallyaltered to some extent and exhibit a pervasive slight "background"alteration (Fig. 2). Locally, more extensively altered zones occuraround veins and in centimeter-sized patches. The background altera-tion is characterized by a 10%-20% replacement of primary mineralsby secondary phases. Olivine in most of the rocks is completelyaltered, and pseudomorphs have been interpreted to reflect multiplestages of alteration, consisting of early talc + magnetite followed bylater chlorite or mixed-layer clay formation. Fresh olivine is presentin some samples. Clinopyroxene in the rocks is partly replaced byactinolite. Plagioclase is generally only slightly altered to albite andchlorite along fractures and grain boundaries. In Leg 137/140 rocks,centimeter-sized, green to light gray "patches" of alteration make upabout 8% of the core recovered and are similar to those identified onprevious legs. Amygdules containing actinolite and chlorite are sur-rounded by alteration haloes (2-10.0 mm wide) where the rock isextensively altered (about 80%) to actinolite, chlorite, albite, andtitanite (Dick, Erzinger, Stokking, et al., 1992).

The predominant magnetic oxide mineral in Leg 137 and Leg 140rocks is low-titanium magnetite, which formed as a result of high-temperature (greater than 500°-600°C) deuteric oxidation of pri-mary titanomagnetite to magnetite plus ilmenite (Pariso and Johnson,1991; Pariso et al., this volume). According to Pariso et al. (thisvolume), titanomagnetite grains from Leg 137/140 rocks have under-gone greater high-temperature deuteric oxidation than rocks from theupper sheeted dikes. Leg 137/140 rocks were then hydrothermallyaltered to varying degrees, which resulted in the replacement of someor all ilmenite lamellae by titanite and/or a high-titanium mineral(anatase or rutile) accompanied by the loss of iron.

METHODS

Hysteresis behavior of 15 samples was measured using a Prince-ton Applied Research vibrating sample magnetometer housed in thePaleomagnetics Laboratory at Scripps Institution of Oceanography.Hysteresis measurements are performed by applying an increasingmagnetic field to a sample, then reversing the applied field, whilecontinually measuring the magnetization of the sample as it respondsto the changing applied field. A plot of applied field vs. sample mag-netization results in a loop, the shape of which is determined by the

chemical composition, microstructure, and particle orientation of themagnetic material within the sample (Stacey and Banerjee, 1974; Dayet al., 1977; Cisowski, 1980). The acquisition of isothermal remanentmagnetization (IRM) and the coercivity of remanence (HCR) weredetermined using the vibrating sample magnetometer on the sampleswhose hysteresis behavior was studied.

Intensities of natural remanent magnetization (NRM) of sampleswere measured on board the JOIDES Resolution and in the Paleo-magnetics Laboratory at Texas A&M University (TAMU), prior to AFdemagnetization. On the Resolution, both a Molspin spinner magne-tometer and a 2-G Enterprises (Model 760R) pass-through cryogenicrock magnetometer were used. At TAMU, NRM intensities weremeasured on a three-axis CTF cryogenic magnetometer housed in ashielded room. The magnetic susceptibilities of discrete samples weremeasured at TAMU using a Bartington Instruments SusceptibilityMeter at a frequency of 0.47 kHz.

On the ship, an AF demagnetizer (Model 2G600) capable ofproducing an alternating field up to 20 mT was used on-line with thepass-through cryogenic magnetometer. AF demagnetization at higherfields was performed using a single-axis Schonstedt GeophysicalSpecimen Demagnetizer (Model GSD-1) capable of producing alter-nating fields up to 100 mT. At TAMU, samples were AF demagne-tized using Schonstedt Geophysical Specimen Demagnetizer (ModelGSD-1). Samples were analyzed using the cryogenic magnetometersbetween AF steps.

Exposure to strong magnetic fields in the core barrel, in the steeldrill string, and on the drill-rig floor imparts a steep negative inclina-tion to cores, particularly those containing low-coercivity material. Thisspurious magnetization is removed from most samples by AF demag-netization at a peak field of 10 mT. Hence, both NRM intensity andintensity remaining after 10-mT (RM10) demagnetization are reported.

After AF demagnetization, the acquisition behavior of anhyster-etic remanent magnetization (ARM) was studied for a suite of 11 pilotsamples. The Schonstedt AF demagnetizer and an anhysteretic mag-netizer constructed at TAMU by W. Sager were used to produce theARM. The alternating field was progressively increased from 0 toabout 100 mT in a direct current (DC) bias field of 0.05 mT. Acquiredremanence was measured between steps using the cryogenic magne-tometer. The ARM was then AF demagnetized so the demagnetizationbehavior could be compared with that of NRM and IRM. A maximumARM was subsequently imparted to the 27 samples in the study usingan alternating field of 100 mT and a DC bias field of 0.05 mT.

An IRM was applied at a saturating field of about 1.0 T to allsamples in the study using an ASC, Inc., impulse magnetizer (ModelIM-10) and measured using the DIGICO spinner magnetometer at theUniversity of Rhode Island. Eleven pilot samples were given an IRMat LOT and AF demagnetized so the demagnetization of IRM couldbe compared with that of NRM and ARM.

After all magnetic analyses were performed, polished thin sec-tions were made from 15 representative samples. Electron micro-probe analyses were performed to determine the chemical compo-sition (major and trace elements) of the opaque minerals. Opaqueminerals were analyzed using a Cameca SX-50 automated electronmicroprobe in the TAMU Geology Department, using a finely fo-cused ( l µm), nonrastered, spot-fixed electron beam. During analy-sis, the beam current was 30 nA at 15 kev. For ilmenite and magnetiteanalyses, Si and Ca were each counted for 10 s; Ti and Fe were eachcounted for 20 s; V, Ni, and Zn were each counted for 30 s; Mn andCr were each counted for 40 s; Mg was counted for 100 s; and Al wascounted for 110 s. For the titanite analyses, Ca and Ti were eachcounted for 20 s, Mn and Fe were each counted for 60 s, and Mg andSi were each counted for 80 s. Natural mineral standards used werediopside, spinel, FeTiO3, chromite, ilmenite, spessartine, and gahnite.Metal standards used were nickel and vanadium. Ferric/ferrous cal-culations were performed using the Cameca program, which uses theDeer, Howie, and Zussman (1962) formulas.

DATA REPORT

.52 «Φcoco

Φ

O .ti

§ gLL *t

Φ

'E

lado

Φ

ü

Φ

Illip

s

sza.

Φ

"ICO

CO

-Co

ii

Φ

E

chl•

_ i

2 ü i-

Φ

Φ

9-Φ

n

iteeo

l

N

•z.

Φ

> ,

Φ

-drii

1—

c<

tθ

oCO

ü

CO3

σ

ΦO-oα

LU

itθ

oΦ

NCOü

szΦ

ct

olit

c

<

Φ

<

itθ

cCO

F

Φ

ietii

O)CO

Φ

1σ>d

•cQ

400 -

600 -

800 -

1000 -

<D 1200O

1400 -

1600 -

1800 -

2000

2a

2b

2c

I I

' I

69

-200

70-400

-600

83

111

137

140

-800 _

Φ

Φ

I-1000 A

Q.ΦQ

-1200

-1400

-1600

Figure 2. Distribution of secondary mineralogy with depth in Hole 504B. From Alt et al., (1986), Becker, Sakai, et al., (1989).

RESULTS

Pariso et al. (this volume) present the magnetic properties andpetrography of magnetite, ilmenite, and titanite. The causes of vari-ability in the rock-magnetic data are discussed in Pariso et al. (thisvolume) and reflect variability in alteration. Detailed discussion ofthese results, in addition to results of analyses of additional samplesfrom Hole 504B collected during Leg 148, will be presented in theLeg 148 Scientific Results volume.

Figure 3 illustrates the hysteresis behavior of representative sam-ples: a relatively unaltered moderately plagioclase-olivine-clinopy-

roxene phyric diabase (Sample 140-504B-191R-1, 23-25 cm), an alter-ation halo in a moderately olivine-plagioclase-clinopyroxene phyricdiabase (Sample 140-504B-202R-1,56-58 cm), and a relatively unal-tered highly clinopyroxene-olivine-plagioclase phyric diabase (Sam-ple 140-504B-227R-1,97-99 cm). Figure 4 illustrates the IRM acqui-sition behavior of those samples. Table 1 summarizes the hysteresisparameters of the samples studied. In this sample set, coercivityranges from 5.3 to 27.7 mT (mean 12 mT), coercivity of remanenceranges from 13.3 to 50.6 mT (mean 26 mT), the ratio HCRIHC rangesfrom 1.6 to 3.19 (mean 2.13). Saturation magnetization (Js) rangesfrom 0.03 to 5.94 × 10~6 Am2, (mean 2.52 x 10"6 Am2), saturation

330

DATA REPORT

B

-5E-3

1 M (emu)

: J-r ;

H ( O θ ) -

'_ M (emu)

j

: Jr :

H(Oe) -

1 M (emu)

JY !

H(θe) :

-5E+3 0 +5E+3 -5E+3 0 +5E+3 -5E+3 0 +5E+3

Figure 3. Hysteresis loops. A. Sample 140-504B-191R-1, 23 cm. B. Sample 140-504B-202R-1, 56 cm. C. Sample 140-504B-227R-1, 97 cm.

A

+5E-3

-5E-3

'. M (emu). Isothermal rβmanβncβ

- Field increment: 99.β Oβ-

H ( o β ) :

B

+2E-3

-2E-3

. M (emu)

. Isothermal remanenc

• Field Increment: 100 Oe-

H(0β) -

c+2E-3

-2E-3

1 M (emu). Isothermal remanence

- Field Increment: 99.6 Oe

-

H ( 0 θ ) -

-2E+3 0 +2E+3 -5E+3 0 +5E+3 -5E+3 0 +5E+3

Figure 4. IRM acquisition curves. A. Sample 140-504B-191R-1, 23 cm. B. Sample 140-504B-202R-1, 56 cm. C. Sample 140-504B-227R-1, 97 cm.

Table 1. Hysteresis data, Leg 137/140. Table 2. Rock-magnetic data.

SampleDepth(mbsf)

/s(mT) (mT) HCR/HC

JR

(10"6 Am2) JRUS (mT) SampleDepth(mbsf)

NRM(A/m)

RM1 0

(A/m)

ARM(A/m)

IRM(A/m) Unit Lith.

137-504B-180M-2.96

140-504B-189R-1,96191R-1,23193R-1,48200R-2, 140202R-1.56204R-1.8206R-1,19208R-2,103209R-2, 9214R-2, 130221R-1.44222R-1.37227R-1.97236R-1, 19

1620.53 13.8 22.1

1651.961661.631674.981731.51747.761756.581760.891780.481789.091821.41875.041884.971925.471980.89Max.Min.Mean

5.4 13.35.3 13.527.7 50.69.78.79.1

17.416.429

15.6 38.59.2 179.1 16.217.8 40.79.4 18.415 337.7 16.121.5 43.527.7 50.65.3 13.312.3 25.7

1.6

2.492.541.831.791.893.192.471.851.772.291.962.22.092.023.191.62.132

9.56

5.942.610.0713.951.060.540.043.672.080.033.041.054.090.075.940.032.52

0.21 500

0.470.230.030.580.170.050.010.540.340.0040.450.180.470.020.580.0040.37

0.080.090.290.150.160.090.210.150.170.160.150.170.120.230.290.080.162

200500500500200500500500500500500500200500200

Note: Abbreviations are defined in the text.

remanence (JR) ranges from 0.01 to 0.58 × 10~6 Am2 (mean 0.37 ×

10~6 Am2), and the ratio JRIJS ranges from 0.08 to 0.29 (mean 0.16).

Rock magnetic data from all samples in the study are presented in

Table 2. NRM intensity ranges from 0.029 to 7.18 A/m (mean 2.95

A/m), whereas RM10 intensity varies only from 0.006 to 4.8 A/m and

has a mean of only 1.02 A/m. ARM intensity ranges from 0.04 to 6.0

A/m, with a mean of 2.46 A/m, and IRM intensity ranges from 0.5 to

1683 A/m, with a mean of 430.7 A/m. Volume susceptibility ranges

137-504B-180M-2, 96

140-504B-189R-1.96189R-2, 39190R-l,95191R-1.25193R-1,48197R-1, 131200R-1, 128200R-2, 140200R-3, 55202R-1, 56204R-l,8206R-1, 19208R-2, 103209R-2, 9209R-2, 16210R-1.91211R-1, 129214R-1, 132214R-2, 130221R-1.44222R-1.37225R-2, 37226R-2, 103227R-1.97227R-1, 101236R-1, 19

1620.53 1.06

1651.961652.891656.051661.631674.981704.111729.881731.51732.151747.761756.581760.891780.481789.091789.11795.811799.791819.921821.41875.041884.971914.071922.531925.471925.511980.89Max.Min.Mean

2.892.61.033.880.0295.824.84.215.371.110.732.363.540.0870.716.517.186.896.23.490.823.170.114.80.190.047.180.032.95

1.14

0.4190.4260.2320.740.0061.244.84.212.020.430.430.551.160.0220.332.061.251.141.011.040.531.160.051.160.080.034.800.011.02

1683.02 0.043 205 PCO

3.3 436.001.88 194.660.43 71.993.96 608.170.04 0.534.66 707.663.45 531.753.84 618.453.86 568.960.88 47.871.68 161.222.55 533.702.83 574.360.14 3.540.53 84.27

510.711260.651337.54

0.15 2.213.59 401.941.81 173.302.91 418.780.24 5.43

677.2812.43

0.19 2.406.00 1683.020.04 0.532.46 430.70

3.214.925.48

3.710.29

0.0050.0070.0050.0270.00030.0280.0160.0110.0140.0020.0040.0070.0110.00040.0010.0100.0180.0250.00030.0110.0040.0200.0010.0170.0010.0010.040.000.01

218 POC218 POC218 POC218 POC220 POC223 POC227 POC227 POC227 POC230 OPC232 PCO235 COP239 OPC240 OPC240 OPC241 POC241 POC244 POC244 POC254 COP254 COP260 COP260 COP260 COP260 COP269 POC

Notes: Lith. = lithology; PCO = plagioclase-clinopyroxene-olivine phyric diabase; POC =plagioclase-olivine-clinopyroxene diabase; OPC = olivine-plagioclase-clinopyroxenephyric diabase; COP = clinopyroxene-olivine-pyroxene phyric diabase. Other abbrevia-tions are defined in the text.

331

DATA REPORT

oARM• AFARM• AFNRMα AFIRM

20 40 60Alternating field (mT)

80 100

Figure 5. ARM acquisition curves (open circles), AF demagnetization of NRM(diamonds), ARM (filled circles), and IRM (squares). A. Sample 140-504B-191R-1, 23 cm. B. Sample 140-504B-202R-1, 56 cm. C. Sample 140-504B-227R-l,97cm.

from 0.0003 SI to 0.043 SI (mean 0.011 SI). Figure 5 illustrates repre-sentative ARM acquisition curves and AF demagnetization behaviorof NRM, IRM, and ARM for Samples 140-504B-191R-1,23-25 cm,140-504B-202R-1, 56-58 cm, and 140-504B-227R-1, 97-99 cm.

Major- and trace-element geochemical analyses are presented inTable 3. Back-scattered electron images of two grains from Sample140-504B-191R-1,23-25 cm, and one grain from Sample 140-504B-227R-1, 97-99 cm, are illustrated in Figure 6. In all samples exam-ined, high-temperature oxidation of primary titanomagnetite has pro-duced lamellae or pods of magnetite and ilmenite. Hydrothermalalteration has further altered the minerals to a mixture of magnetite,ilmenite, titanite, and a high-titanium mineral (either rutile or ana-tase), as discussed in Pariso et al. (this volume). Table 3 shows thatmagnetite lamellae tend to be enriched in trivalent elements: Cr2O3

averages 0.12 wt% in magnetite relative to 0.02 wt% in ilmenite,A12O3 averages 0.91 wt% in magnetite and only 0.16 wt% in ilmenite,and V2O5 averages 1.36 wt% in magnetite and 0.50 wt% in ilmenite.Divalent elements are partitioned into ilmenite lamellae: MnO aver-

ages 2.96 wt% in ilmenite and 0.06 wt% in magnetite, and MgOaverages 0.14 wt% in ilmenite and only 0.01 wt% in magnetite.

DISCUSSION AND CONCLUSIONS

The predominant magnetic oxide mineral in Leg 137 and Leg 140rocks is low-titanium magnetite, produced during high-temperature(greater than 500°-600°C) deuteric oxidation of primary titanomag-netite to magnetite plus ilmenite (Pariso and Johnson, 1991; Pariso etal., this volume). Subsequently, Leg 137/140 rocks were hydrother-mally altered to varying extents: some or all ilmenite lamellae werereplaced by titanite and/or a high-titanium mineral (anatase or rutile)and some iron was lost.

The ranges of NRM intensity and magnetic susceptibility measuredin this suite of samples are consistent with those reported in Pariso etal. (this volume), which reflect a general slight increase in both prop-erties with depth in the sheeted dike section. The shapes of ARM andIRM acquisition curves are consistent with the predominant magneticcarrier being low-titanium magnetite. The mean ratio JR/JS is 0.16,consistent with pseudo-single-domain behavior, and close to the meanvalue reported for the entire sheeted dike section (0.19; Pariso et al.,this volume). Magnetic remanence (natural and laboratory-induced)and magnetic susceptibility reflect the concentration of magnetic car-riers in the rock. Samples whose magnetic intensity is low also havelow magnetic susceptibility (Table 2) and low saturation magnetization(Table 1), as discussed in Pariso et al. (this volume) and implying thatthe low remanence and susceptibility values reflect a decrease in theamount of magnetite. This variation in concentration of magnetite inturn reflects hydrothermal alteration, which causes loss of iron andreplacement of ilmenite by anatase and/or rutile (Pariso and Johnson,1992; Pariso et al., this volume). As observed by Pariso and Johnson(1992) and Pariso et al. (this volume), samples whose magnetic inten-sity and susceptibility are low show evidence of high degrees of hydro-thermal alteration, whereas samples whose magnetic intensity and sus-ceptibility are moderate to high are less altered. Thus, variability in theamount of magnetizable material produced by differing degrees ofhydrothermal alteration of the samples results in the wide variation inrock magnetic properties observed in this suite of samples.

Deer, Howie, and Zussman (1962) and Haggerty (1976) report thepartitioning of divalent ions (Mn and Mg) into ilmenite and trivalentions (Cr, Al, V) into magnetite when the two minerals are associated.This distribution was observed in altered and unaltered magnetite andin primary and secondary ilmenite from Hole 504B extrusives andupper sheeted dikes drilled during ODPLeg 83 (Kempton et al., 1985):A12O3 and Cr2O3 concentrations are higher in both altered and unal-tered magnetite than in ilmenite, whereas MgO and MnO are higher inboth primary and secondary ilmenite than in associated magnetite ortitanomagnetite. Vanadium concentrations were not reported. Thus, thevariations in major and trace elements observed in the lower sheeteddikes drilled during Legs 137 and 140 are consistent with trendsobserved in magnetite-ilmenite associations in general, as well as in theextrusives and upper sheeted dikes from Hole 504B. Work in progressinvolves correlating rock magnetic properties and the degree of oxidealteration with ratios of trivalent trace elements to trivalent iron andwith ratios of divalent trace elements to divalent iron.

ACKNOWLEDGMENTS

We thank J. Aguiar for assistance with data processing. We thankL. Tauxe for permitting us to use the vibrating sample magnetometerin the Paleomagnetics Laboratory at the Scripps Institution of Ocean-ography, and we thank J. King for allowing us to use the spinnermagnetometer at the University of Rhode Island. We are grateful to R.Guillemette for his guidance and assistance with the electron micro-probe. This research was supported by a grant to L.S. from the U.S.Science Support Program of the Joint Oceanographic Institutions, Inc.

332

DATA REPORT

Table 3. Major- and trace-element microprobe analyses.

Analysis

140-504B-191R-l,25-bl-magn-l191R-l,25-a3-magn-l191R-l,25-a3-magn-2200R-3,55-bl-magn-l213R-l,75-a2-magn-l213R-l,75-a2-magn-2213R-l,75-al-magn-l213R-l,75-al-magn-2227R-1, 97-b-magn-l227R-l,97-a-magn-l227R-l,97-a-magn-2227R-l,97-d-magn-l227R-l,97-d-magn-2MaximumMinimumMean

191R-1, 25-a3-ilme-l191R-l,25-bl-ilme-l200R-3, 55-al-ilme-l200R-3, 55-al-ilme-2200R-3, 55-bl-ilme-l202R-l,56-a-üme-l202R-l,56-a-ilme-2213R-l,75-a2-ilme-l213R-l,75-a4-ilme-l213R-1, 75-a4-ilme-2213R-l,75-al-ilme-l213R-1,75-al-ilme-2227R-l,97-b-ilme-l227R-l,97-a-ilme-l227R-1,97-a-ilme-2227R-l,97-a-ilme-3227R-l,97-a2-ilme-l227R-1,97-d-ilme-l227R- l,97-d-ilme-2MaximumMinimumMean

227R-1, 97-b-hiTi(?)-l227R-1, 97-b-hiTi(?)-2191R-l,25-al-hiTi-l202R-l,56-a-hiTi-lMaximumMinimumMean

Analysis

140-504B-202R-1,56-a-tita-l202R-1, 56-a-tita-22O2R-1,56-a-tita-3191R-l,23-a3-tita-l191R-l,23-a3-tita-2191R-1, 25-bl-tita-l213R-l,75-a2-tita-l213R-1, 75-a2-tita-2213R-1,75-al-tita-l227R-1, 101-d-tita-l227R-1, 101-d-tita-2227R-1, 101-d-tita-3MaximumMinimumMean

Notes: magn = magnetite; i

siθ2 :MgO iU2O3

(wt%) (wt%) (wt%)

0.190.700.110.600.170.660.120.320.470.110.150.120.120.700.110.30

2.700.410.330.480.050.060.163.180.080.050.080.063.900.150.120.171.790.090.083.900.050.73

25.7814.03

1.520.136.514.63

10.37

SiO2

(wt%)

29.4230.2430.6926.2329.4821.9727.5628.0430.5228.8715.5928.8630.6915.5927.29

0.000.000.000.070.000.000.000.000.010.000.020.000.000.070.000.01

0.140.000.060.050.050.030.040.540.030.050.030.041.120.060.080.070.110.050.061.120.000.14

0.000.070.010.000.030.010.02

MgC

2.300.801.061.040.820.131.430.620.110.342.590.070.492.590.070.91

0.370.070.040.040.030.030.080.160.020.040.030.041.370.160.130.050.210.100.091.370.020.16

0.680.310.120.070.200.170.30

TiO2 Cr2O, CaO

(wt%) (wt%) (wt%;

2.853.581.843.311.512.351.701.783.595.105.580.624.255.580.622.93

45.3892.8548.9148.7449.0649.3048.8244.0345.6645.8445.5145.6933.5144.8948.7149.3451.0345.6648.9692.8533.5149.05

38.8653.1190.8992.6176.3782.1868.87

• CaO 1

0.140.080.110.150.140.110.190.120.130.070.170.070.080.190.070.12

0.010.030.000.000.010.030.020.040.040.000.040.000.020.030.030.030.050.060.030.060.000.02

0.060.120.030.050.070.050.07

VlnO(wt%) (wt%) (wt%)

0.000.000.030.030.360.000.000.000.000.010.042.802.800.000.27

27.3627.7828.8724.2526.5220.4025.4426.2628.4127.6716.3923.0528.8716.3925.20

0.040.110.000.010.050.230.010.050.000.101.630.061.630.000.19

0.480.880.330.460.431.260.370.820.600.080.120.230.121.260.080.48

2.990.910.350.700.170.300.621.430.430.320.490.372.170.150.090.221.590.150.162.990.090.72

25.0010.172.360.715.704.589.56

TiO2

MnC1 v2o :

1 (wt%) (wt%

0.010.060.050.140.040.090.040.030.020.050.130.020.050.140.010.06

3.600.002.092.072.095.675.052.742.802.813.312.822.672.913.823.513.722.092.465.670.002.96

0.000.030.030.020.030.020.02

FeO

1.481.161.371.541.481.252.071.501.401.041.680.730.922.070.731.36

0.370.610.310.290.330.360.360.400.550.450.530.370.690.640.630.620.730.620.560.730.290.50

0.900.940.790.820.850.830.86

H2O

(wt%) (wt%) (wt%)

39.4438.6137.6344.0430.9341.3535.6239.0038.9531.3036.85 :29.9844.04 :29.9836.98

1.982.330.351.715.69

16.057.393.341.434.28

29.509.32

29.500.356.95

4.975.014.974.874.644.724.744.865.044.604.344.655.044.344.78

lme = ilmenite; hiTi = high-titanium; tita = titanite analyses.

REFERENCES* Alt. J.C.. Honnore;

FeO) (wt%)

4.2234.4532.9433.8932.4432.3732.4632.52

3.324.796.76

31.6135.2035.20

3.3224.38

37.193.79

41.7841.6747.5438.5838.7935.9938.1938.3137.5638.1926.4037.4339.9040.7540.7338.9641.4947.54

3.7937.01

4.4416.183.123.357.365.156.77

Totals

(wt%)

103.21104.08102.54101.1497.67

104.72100.76101.55104.3596.83

104.3498.72

104.7296.83

101.66

NiO

(wt%)

0.070.030.000.040.020.020.000.020.000.020.020.020.030.070.000.02

0.000.000.000.000.000.000.000.030.000.000.020.010.020.030.010.030.000.000.000.030.000.01

0.000.010.000.000.000.000.00

'. J.. Laverne. C

ZnO

(wt%)

0.010.160.050.070.040.040.050.090.000.050.190.000.000.190.000.06

0.000.020.000.010.030.000.000.070.010.000.100.000.060.030.050.070.010.040.030.100.000.03

0.060.010.010.000.010.010.02

Fe2O3

(wt%)

93.2561.0664.2260.9064.7662.4863.0264.1896.1993.8987.1967.6760.5496.1960.5472.26

5.450.005.855.960.005.135.19

12.1611.8611.7411.9311.4725.2515.076.175.220.00

11.236.59

25.250.008.22

0.000.000.000.000.000.000.00

Totals(wt%)

105.00102.96102.08102.21101.85100.76101.45102.00105.84105.54104.60101.16101.80105.84100.76102.87

98.2098.6999.72

100.0199.3699.4999.13

100.7799.6799.6199.6399.0697.18

101.5599.74

100.0899.9799.05

100.51101.5597.1899.55

95.7894.9898.8897.7697.1297.6596.85

.. and Emmermann. R..

Ade-Hall, J.M., Palmer, H.C., and Hubbard, T.P., 1971. The magnetic andopaque petrological response of basalts to regional hydrothermal altera-tion. Geophys. J. R. Astron. Soc, 24:137-174.

Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).

alteration of a 1 km section through the upper oceanic crust, Deep Sea

Drilling Project Hole 504B: mineralogy, chemistry, and evolution of

seawater-basalt interactions. J. Geophys. Res., 91:10309-10335.

Anderson, R.N., Honnorez, J., Becker, K., Adamson, A.C., Alt, J.C., Emmer-

mann, R., Kempton, P.D., Kinoshita, H., Laverne, C, Mottl, M.J., and

Newmark, R.L., 1982. DSDP Hole 504B, the first reference section over

1 km through Layer 2 of the oceanic crust. Nature, 300:589-594.

Anderson, R.N., Honnorez, J., Becker, K., et al., 1985. Init. Repts. DSDP, 83:

Washington (U.S. Govt. Printing Office).

333

DATA REPORT

Becker, K., Foss, G., etal., 1992. Proc. ODP, Init. Repts., 137: College Station,TX (Ocean Drilling Program).

Becker, K., Sakai, H., Adamson, A.C., Alexandrovich, J., Alt, J.C., Anderson,R.N., Bideau, D., Gable, R., Herzig, P.M., Houghton, S., Ishizuka, H.,Kawahata, H., Kinoshita, H., Langseth, M.G., Lovell, M.A., Malpas, J.,Masuda, H., Merrill, R.B., Morin, R.H., Mottl, M.J., Pariso, J.E., Pezard,P., Phillips, J., Sparks, J., and Uhlig, P., 1989. Drilling deep into the oceaniccrust at Hole 504B, Costa Rica Rift. Rev. Geophys., 27:79-102.

Becker, K., Sakai, H., etal., 1988. Proc. ODP, Init. Repts., 111: College Station,TX (Ocean Drilling Program).

, 1989. Proc. ODP, Sci. Results, 111: College Station, TX (OceanDrilling Program).

Cann, J.R., Langseth, M.G., Honnorez, J., Von Herzen, R.P., White, S.M., etal., 1983. Init. Repts. DSDP, 69: Washington (U.S. Govt. Printing Office).

Cisowski, S., 1980. The relationship between the magnetic properties ofterrestrial igneous rocks and the composition and internal structure of theircomponent Fe-oxide grains. Geophys. J. R. Astron. Soc, 60:107-122.

CRRUST (Costa Rica Rift United Scientific Team), 1982. Geothermal regimesof the Costa Rica Rift, east Pacific, investigated by drilling, DSDP-IPODLegs 68, 69, and 70. Geol. Soc. Am. Bull, 93:862-875.

Day, R., Fuller, M., and Schmidt, V.A., 1977. Hysteresis properties of ti-tanomagnetites: grain-size and compositional dependence. Phys. EarthPlanet. Inter., 13:260-267.

Deer, W. A., Howie, R.A., and Zussman, J., 1962. Rock-forming Minerals (Vol.5): Non-silicates: London (Longman Group Ltd.).

Dick, H.J.B., Erzinger, J., Stokking, L.B., et al., 1992. Proc. ODP, Init. Repts.,140: College Station, TX (Ocean Drilling Program).

Dunlop, D.J., 1969. Hysteretic properties of synthetic and natural monodomaingrains. Philos. Mag., 19:329-338.

Evans, M.E., and Wayman, M.L., 1974. An investigation of the role ofultra-fine titanomagnetite intergrowths in paleomagnetism. Geophys. J. R.Astron. Soc, 36:1-10.

Haggerty, S.E., 1976. Opaque mineral oxides in terrestrial igneous rocks. InRumble, D., Ill (Ed.), Oxide Minerals: Mineral. Soc. Am., Short CourseNotes, 3:101-277.

Hobart, M.A., Langseth, M.G., and Anderson, R.N., 1985. A geothermal andgeophysical survey on the south flank of the Costa Rica Rift: Sites 504 and505. In Anderson, R.N., Honnorez, J., et al., Init. Repts. DSDP, 83:Washington (U.S. Govt. Printing Office), 379^04.

Johnson, H.P., and Hall, J.M., 1978. A detailed rock magnetic and opaquemineralogy study of the basalts from the Nazca Plate. Geophys. J. R.Astron. Soc, 52:45-64.

Kempton, P.D., Autio, L.K., Rhodes, J.M., Holdaway, M.J., Dungan, M.A.,and Johnson, P., 1985. Petrology of basalts from Hole 504B, Deep SeaDrilling Project, Leg 83. In Anderson, R.N., Honnorez, J., Becker, K., etal., Init. Repts. DSDP, 83: Washington (U.S. Govt. Printing Office),129-164.

Kikawa, E., and Pariso, J.E., 1991. Magnetic properties of gabbros from Hole735B, Southwest Indian Ridge. In Von Herzen, R.P., Robinson, P.T., et al.,Proc. ODP, Sci. Results, 118: College Station, TX (Ocean Drilling Pro-gram), 285-307.

Leinen, M., Rea, D.K., et al., 1986. Init. Repts. DSDP, 92: Washington (U.S.Govt. Printing Office).

Pariso, J.E., and Johnson, H.P., 1989. Magnetic properties and oxide petrog-raphy of the sheeted dike complex in Hole 504B. In Becker, K., Sakai, H.,et al., Proc. ODP, Sci. Results, 111: College Station, TX (Ocean DrillingProgram), 159-167.

, H.P., 1991. Alteration processes at Deep Sea Drilling Project/OceanDrilling Program Hole 504B at the Costa Rica Rift: implications formagnetization of oceanic crust. /. Geophys. Res., 96:11703-11722.

Smith, G.M., and Banerjee, S.K., 1986. The magnetic structure of the upperkilometer of the marine crust at Deep Sea Drilling Project Hole 504B,Eastern Pacific Ocean. J. Geophys. Res., 91:10337-10354.

Smith, G.M., Gee, J., and Klootwijk, C.T., 1991. Magnetic petrology of basaltsfrom Ninetyeast Ridge. In Weissel, J., Peirce, J., Taylor, E., Alt, J., et al.,Proc. ODP, Sci. Results, 121: College Station, TX (Ocean Drilling Pro-gram), 525-545.

Stacey, F.D., and Banerjee, S.K., 1974. Developments in Solid Earth Geophys-ics (Vol. 5): The Physical Principles of Rock Magnetism: Elsevier (NewYork).

Date of initial receipt: 28 July 1993Date of acceptance: 3 November 1993Ms 137/140SR-029

334

DATA REPORT

140-504B-191R-1,23-a1

Figure 6. Back-scattered electron images. Labels correspond to analyses in Table 3.

335

DATA REPORT

140-504B-191R-1,23-a3

Figure 6 (continued).

336

DATA REPORT

Figure 6 (continued).

337