7. GEOCHEMISTRY OF THE LOWER SHEETED DIKE COMPLEX, …

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

7. GEOCHEMISTRY OF THE LOWER SHEETED DIKE COMPLEX, HOLE 504B, LEG 1401

Joel W. Sparks2

ABSTRACT

Sixty-three samples representing 379 m of sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program Site 504Bhave been analyzed for major and selected trace elements by X-ray fluorescence. The samples range from microcrystalline aph-yric basalts to moderately phyric (2%-10% phenocrysts) diabase that are typically multiply saturated with plagioclase, olivine,and clinopyroxene, in order of relative abundance. All analyzed samples are classified as Group D compositions with moderateto slightly elevated compatible elements (Mg'-value = 0.65% ± 0.03%; A12O3 = 15.5% ± 0.8%; CaO = 13.0% ± 0.3%; Ni = 114± 29 ppm), and unusually depleted levels of moderate to highly incompatible elements (Nb < 1 ppm; Zr = 44 ± 7 ppm; Rb < 0.5ppm; Ba 1 ppm; P2O5 = 0.07% ± 0.02%). These compositions are consistent with a multistage melting of a normal oceanridge basaltic mantle source followed by extensive fractionation of olivine, plagioclase, and clinopyroxene.

Leg 140 aphyric to sparsely phyric (0%-2% phenocrysts) basalts and diabases are compositionally indistinguishable fromsimilarly phyric samples at higher levels in the hole. An examination of the entire crustal section, from the overlying volcanicsthrough the sheeted dikes observed in Leg 140, reveals no significant trends indicating the enrichment or depletion of CostaRica Rift Zone source magmas over time. Similarly, significant trends toward increased or decreased differentiation cannot beidentified, although compositional patterns reflecting variable amounts of phenocryst addition are apparent at various depths.Below 1700 mbsf to the bottom of the Leg 140 section, there is a broadly systematic pattern of Zn depletion with depth, theresult of high-temperature hydrothermal leaching. This zone of depletion is thought to be a significant source of Zn for thehydrothermal fluids depositing metal sulfides at ridge-crest hydrothermal vents and the sulfide-mineralization zone, located inthe transition between pillow lavas and sheeted dikes.

Localized zones of intense alteration (60%-95% recrystallization) are present on a centimeter to meter scale in many litho-logic units. Within these zones, normally immobile elements Ti, Zr, Y, and rare-earth elements are strongly depleted comparedwith "fresher" samples centimeters away. The extent of compositional variability of these elements tends to obscure primaryigneous trends if the highly altered samples are not identified or removed. At levels up to 40% (or possibly 60%) recrystalliza-tion, Ti, Zr, and Y retain their primary signatures. Although the mechanisms are unclear, it is possible that these intense alter-ation zones are a source of Y and rare-earth elements for the typically rare-earth-element-enriched hydrothermal vent fluids ofmid-ocean ridges.

INTRODUCTION AND GEOLOGIC SETTING

Leg 140 marked the sixth excursion of the Deep Sea DrillingProject/Ocean Drilling Program (DSDP/ODP) to core Hole 504B, lo-cated approximately 200 km south of the Costa Rica Rift Zone(1°13.61 l'N, 83°43.818'W). The 379 m drilled during this leg deep-ened the hole to a total depth of 2000.4 meters below seafloor (mbsf),making it the deepest penetration into oceanic crust (1725.9 m). Hole504B has already provided an unprecedented view of the oceaniccrust as it has penetrated 572 m of extrusive volcanics (Layer 2A),209 m of a mixed extrusive/sheeted dike transition zone (Layer 2B),and 945 m of sheeted dikes (Layer 2C). The main intent of Leg 140was to drill through and sample the Layer 2C-3 transition, from sheet-ed dikes to layered gabbros, the existence of which is inferred fromanalogies to ophiolite complexes (e.g., Coleman, 1977) and to plu-tonic rock sequences sampled in submarine fracture zones (e.g., VonHerzen, Robinson, et al., 1991).

The Costa Rica Rift Zone (CRRZ) is an intermediate-rate spread-ing ridge on the easternmost extremity of the Cocos-Nazca SpreadingCenter, bounded by the Ecuador Fracture Zone to the west, and thePanama Fracture Zone to the east (Hey et al., 1977; Lonsdale andKlitgord, 1978). The asymmetric spreading rates of 3.6 cm/yr to the

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

department of Geology, Stanford University, Stanford, CA 94305, U.S.A.

south and 3.0 cm/yr to the north (Hey et al., 1977) are consistent withthe characteristically uniform seafloor topography of an intermedi-ate-rate ridge. Hole 504B is situated between magnetic Anomalies3A and 4, in 5.9-m.y.-old oceanic crust.

The basalts and diabases from Hole 504B are typically multiplysaturated with respect to olivine, plagioclase, and clinopyroxene, andrange from 0% to 10% phenocrysts, of which plagioclase is the mostcommon. Considerable work from previous legs has focused on char-acterizing Hole 504B igneous compositions based on bulk-rock ma-jor and trace element analyses (Autio and Rhodes, 1983; Etoubleauet al., 1983; Hubberten et al., 1983; Marsh et al., 1983; Sharaskin etal., 1983;Emmermann, 1985; Kemptonet al., 1985;Tualetal., 1985;Autio et al., 1989), isotopic determinations (Barrett and Friedrichsen,1982; Barrett, 1983; Friedrichsen, 1985; Kusakabe et al., 1989;Shimizu et al., 1989), and electron microprobe analyses of freshglasses and selected mineral phases (Natland, et al., 1983; Kempton,1985; Kempton et al., 1985). Using the classification scheme of Au-tio and Rhodes (1983) and Kempton et al. (1985), over 98% of thesamples analyzed from this location are Group D (depleted) compo-sitions. The remaining 2% belong to either Groups M (moderatelyevolved), E (enriched), or T (transitional). Groups M and E corre-spond to normal Type I and Type II mid-ocean-ridge basalt (MORB)compositions (e.g., Bryan et al., 1976), whereas Group T is transi-tional between them. Group D bulk-rock and glass compositions varyfrom slightly olivine- to quartz-normative tholeiites, and are similarin many respects (compatible major- and trace-element abundances;Sr and Nd isotopes) to moderately evolved, normal Type I MORB.

81

J.W. SPARKS

Despite these similarities, Group D compositions exhibit marked lev-els of depletion in moderate to highly incompatible element abun-dances relative to Type I MORB. These characteristics have beenmodeled as either unusual primitive melt compositions (Natland etal., 1983), or the result of multistage melting of a normal MORBmantle source, followed by extensive crystal fractionation (Autio andRhodes, 1983; Autio, 1984; Kempton et al., 1985; Autio et al., 1989;Kusakabe et al., 1989; Shimizu et al., 1989).

Nearly all samples recovered from the sheeted dike section showmineralogical and compositional signs of subsolidus alteration rang-ing from slight (<10%) to extensive (95%) recrystallization. Thesechanges result from a complex interplay between alteration undergreenschist facies conditions (250°-380°C) near the ridge axis, pre-cipitation of anhydrite from deep penetrating seawater as the crustmoved off-axis into recharge zones, and low-temperature formationof zeolites (<150°C) as the section moved further off-axis (Alt, 1984;Alt et al., 1985, 1986a, 1989b; Shipboard Scientific Party, 1992).Bulk-rock compositional changes that take place during alterationhave been documented at Hole 504B for major elements (Honnorezet al., 1983; Hubberten et al., 1983; Noack et al., 1983; Alt and Em-mermann, 1985; Emmermann, 1985; Kawahata and Furuta, 1985; Altet al., 1989b), selected trace elements (Belyi et al., 1983; Hubberten,1983; Hubberten et al., 1983; Alt and Emmermann, 1985; Kawahataand Furuta, 1985; Alt et al., 1989a, 1989b), as well as stable and ra-diogenic isotopes (Barrett, 1983; Barrett and Friedrichsen, 1982,1983; Belyi et al., 1983; Honnorez et al., 1983; Friedrichsen, 1985;Mitchell and Terrell, 1985; Alt et al., 1986a, 1986b, 1989a, 1989b;Alt and Chaussidon, 1989; Kusakabe et al., 1989; Shimizu et al.,1989; Ishikawa and Nakamura, 1992). These compositional varia-tions are controlled by the style of alteration, which in turn, is depen-dent upon the given section of oceanic crust that is of interest. Low-temperature (0°-100°C), high water/rock ratio (10-100) seawater al-teration dominates the highly porous pillow lava section down to thebeginning of the transition into the sheeted dikes. Below the transi-tion, higher temperatures (200°-380°C; greenschist facies mineralo-gy) and significantly lower water/rock ratios (1-5), characterize thehydrothermal alteration in the sheeted dikes. Basalts in this sectionare heterogeneously altered (on a scale of centimeters to tens or hun-dreds of meters), exhibiting trends toward losses of SiO2, CaO, TiO2;variable changes in A12O3; gains in MnO, Na2O, CO2, H2O

+, S, MgO(slight), and oxidation of Fe; and some mobility of light rare-earth el-ements (Alt and Emmermann, 1985).

Ti, Zr, and Y are generally considered to be relatively immobileelements and extremely useful for evaluating igneous processes inbasalts that have been exposed to a range of subsolidus alteration pro-cesses since initial emplacement (e.g., Cann, 1970; Winchester andFloyd, 1976; Humphris and Thompson, 1978; Pearce and Norry,1979). However, recent work on the secondary alteration of basalts(e.g., Bienvenu et al., 1990; Price et al., 1991) has shown that theseelements are not always immobile, and the preliminary findings ofTi, Zr, and Y depletion in localized areas of intense alteration (Ship-board Scientific Party, 1992), appear to support this notion.

The main goals of this study are to document and discuss the vari-ations in primary igneous chemistry of the sheeted dike complex, asthe hole approaches the Layer 3 gabbros, and to document the varia-tions of Ti, Zr, Y, and selected rare-earth elements (REE) due to hy-drothermal alteration. Particular emphasis is placed on how thesevariations affect normal primary igneous trends. Finally, a discussionis presented concerning the presence and implications of a Zn deple-tion zone near the base of the Leg 140 section.

SAMPLING AND ANALYTICAL METHODS

Most samples used in this study were chosen as being representa-tive of the lithologic units described by the Shipboard Scientific Party

(1992), including three samples from the Leg 137 core (8.8 m totalrecovery). In addition, several lithologic units displaying a widerange of hydrothermal alteration were sampled in detail to documentthe compositional variability of Ti, Zr, Y, and REE resulting fromsubsolidus alteration.

Prior to analysis, samples were trimmed and ground to removeany large (>0.5 mm wide) hydrothermal veins and outside surfacesthat may have been contaminated by exposure to the coring bit. Sawmarks were removed on a silicon carbide grinding wheel, and sam-ples were cleaned with distilled water in an ultrasonic bath. Cleanedrock chips were reduced in a tungsten carbide (WC) jaw crusher, andground to a powder (<200 mesh) in a 20 mL WC-shatterbox (gra-ciously lent to the author by J.M. Rhodes, University of Massachu-setts). The amount of powder produced for each sample ranged from8 to 43 g, with most samples over 15 g. Powders to be used for majorelement analysis were ignited for 4 to 6 hr at 1000°C to determineloss on ignition and to oxidize iron to the trivalent state. Duplicateglass discs for each sample were prepared for X-ray fluorescence(XRF) major element analysis by fusing ignited sample powder witha lanthanum-bearing lithium borate flux (Norrish and Hutton, 1969).Samples for XRF trace analysis were prepared with approximately 5g of unfired sample powder pressed into a boric acid backing using ahydraulic press (Norrish and Chappell, 1977). Major and trace ele-ment (Nb, Zr, Y, Sr, Rb, Ga, Zn, Cu, Ni, Cr, V, Cl, and S) abundanceswere analyzed at Stanford University using an automated Rigaku S-MAX wavelength-dispersive spectrograph, equipped with an end-window, Rh-target X-ray tube. Major element matrix correctionswere made following the method of Norrish and Hutton (1969). Be-cause ignited powders were used, all major element analyses are re-ported on an anhydrous basis, with total iron as Fe2O3 (Fe2O3*). Losson ignition (LOT) is corrected for the oxidation of FeO to Fe2O3 (as-suming the average FeO content of a typically altered sample is 85%of the total iron content). Trace element analyses were corrected fornonlinear backgrounds and spectral interferences after Norrish andChappell (1977). Matrix absorption effects for all XRF trace ele-ments except Cl and S were measured using a modified Comptonscattering method (Reynolds, 1963; 1967; Walker, 1973; Willis,1989). Mass absorption coefficients for S and Cl were calculatedfrom major element analyses using mass absorption data tables (e.g.,Norrish and Chappell, 1977). All XRF analyses are presented in Ta-ble 1. Estimates of accuracy and precision (Table 2) were determinedby analyzing well-characterized basalt and diabase standards (USGSstandards BIR-1 and DNC-1, and ODP Leg 111 inter-laboratory stan-dard BAS- 111) together with the unknowns. These standards werechosen for their broad compositional similarities to the highly deplet-ed Leg 140 diabase to provide a realistic assessment of analytical pre-cision.

Using compositions determined by XRF as the basis for selection,rare-earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, and Lu) and Ba were determined on a subset of samples by in-ductively coupled plasma mass spectrometry (ICPMS) at Union Col-lege, Schenectady, New York, by J. Tepper. Unfired sample powderswere digested using an HNO3-HF digestion technique, followed bydilution with 0.1N HNO3. Internal standards (Cs and Ta) were addedto the solutions for monitoring instrumental drift. All samples wereanalyzed in duplicate, and the resulting concentrations were calibrat-ed against basalt standard NBS-688. Estimates of precision weremade using the formula

s = A1Σ(X~X)2/(Π-\)

where x and x are the observation and mean for replicate analyses ofthe same element in one sample, and n is the number of samples ana-lyzed (e.g., Autio and Rhodes, 1983). This method produces error

GEOCHEMISTRY, HOLE 504B

estimates that reflect the true errors associated with the concentra-tion levels observed in the Leg 140 diabases. Basalt and diabasestandards BIR-1 and DNC-1 were analyzed together with theunknowns as a measure of accuracy. The ICPMS analyses and ana-lytical statistics are presented in Table 3.

Various authors have reported significant sample contaminationfor W, Co, Ta, and to a lesser extent, Nb in samples that are groundin a WC shatterbox (Thompson and Bankston, 1970; Joron et al.,1980; Hickson and Juras, 1986). Joron et al. (1980) observed an av-erage 2 ppm increase in Nb content of basalts ground in WC vs. thoseground in agate, whereas Hickson and Juras (1986) suggest a smallerincrease ( 0.5 ppm) under similar conditions. In the present study,Nb contamination does not appear to be significant ( « 1 ppm), be-cause the maximum measured concentration is only 1.0 ppm, andmany of the analyzed samples are close to the lower limit of detection(0.3 ppm; 2 σ counting error). This reduced level of contaminationmay be due, in part, to decreased sample hardness resulting from sub-solidus alteration, relatively short grinding times ( 90 s), or the useof a "cleaner" tungsten carbide in the fabrication of the shatterboxused in this study (Spex Industries, Inc., pers. comm., 1992).

Two terms referred to throughout this study are the "degree of al-teration" and Mg'-value. The degree of alteration is the total percent-age of secondary mineralization as determined by either thin-sectionpoint counts or visual inspection of sliced core samples (ShipboardScientific Party, 1992). A rough estimate of accuracy and precision isapproximately ± 10% (absolute). Mg'-value is the molar ratio MgO/(MgO + FeO), where Fe2+/(Fe2+ + Fe3+) is equal to 0.90 (Basaltic Vol-canism Study Project, 1981).

IGNEOUS CHEMISTRY

The mineralogy, water content, and the abundance of normallymobile elements (Na, K, Rb, Sr, Cu, S, and Cl) indicate that all sam-ples selected for this study have undergone varying degrees of hydro-thermal alteration. The effects of this alteration must be quantified toassess primary igneous trends, many of which could be partially orfully obscured by compositional variations. To this end, samples thathave undergone more than 60% recrystallization of their groundmassphases are arbitrarily classified as "highly altered," and will not beused in discussions of igneous compositions unless specifically men-tioned. The selection of this alteration indicator is discussed later inthis study.

The following descriptions and discussion of igneous composi-tions include 74 samples analyzed by the Shipboard Scientific Party(1992), using analytical methods and calibration standards similar tothis study. For comparison, 10 of the analyzed shipboard sampleswere measured at Stanford, and the mean differences between thetwo data sets are listed in Table 2. Only Zn and Cr exhibit significantdeviations (11 and 12 ppm lower than this study, respectively), andeach shipboard analysis has been adjusted accordingly for more pre-cise comparisons. Data selected from previous legs to Hole 504Bwere produced at the University of Massachusetts (Autio and Rhodes1983; Kempton, et al., 1985; Autio et al., 1989). The Leg 111 inter-laboratory standard, analyzed at the University of Massachusetts(Autio et al., 1989) and in this study, is listed in Table 2 for compar-ison.

All Leg 137/140 dike samples can be classified as Group D com-positions (Autio and Rhodes, 1983; Kempton et al., 1985), with mod-erate to slightly elevated levels of compatible elements (MgO =7.7%-10.0%; Fe2O3* = 8.1%-11.7%; CaO = 11.7%-13.8%; A12O3 =14.2%-17.9%; Ni = 74-189 ppm; Cr = 127-456 ppm; V = 174-374ppm), unusually high CaO/Na2O ratios (up to 9.6), and highly deplet-ed concentrations of moderately to highly incompatible elements(TiO2 = 0.67%-1.14%; K2O 0.01%; P2O5= 0.04%-0.13%; Nb < 1ppm; Zr = 31-62 ppm; Sr = 43-77 ppm; Rb < 0.5 ppm).

Analyzed samples typically fall into three petrographic classes:microcrystalline to fine-grained aphyric basalt (<1% phenocrysts),fine- to medium-grained sparsely phyric diabase (l%-2% phenoc-rysts), and fine- to medium-grained, moderately phyric diabase (2%-10% phenocrysts). Although there is significant compositional over-lap, from aphyric to porphyritic samples (Table 4), small increases inA12O3 (15.0%-15.9%), Ni (98-119 ppm), Cr (271-337 ppm), and de-creases in TiO2 (0.99%-0.86%), Fe2O3* (10.5%-9.7%), Zr (51-42ppm), Y (26-22 ppm), and V (314-274 ppm) reflect the addition ofolivine, plagioclase, (Cr-rich) clinopyroxene, and Cr-spinel compo-nents.

The average composition of aphyric to sparsely phyric samplesfrom Leg 137/140 is nearly identical to that of the Group D basaltsfrom Layer 2B-2C transition zone-sheeted dikes (Legs 83 and 111),and the Layer 2A volcanic section of Legs 69 and 70, provided nor-mally mobile elements, K and Rb, are discounted (Table 5). The aph-yric samples are olivine-normative tholeiites with Mg'-values (0.60-0.70) and Ni abundances (74-143 ppm) similar to most moderatelyevolved mid-ocean-ridge basalts (Figs. 1 and 2). In contrast to mostMORB, however, these compositions are markedly lower in TiO2

and Zr at a given level of evolution such as Mg'-value or Ni abun-dance. Included in Figures 1 and 2 is Sample 111-504B-159R-1, 52-56 cm (Piece 9), the most evolved Group D composition (Mg'-value= 0.56; Ni = 55 ppm) observed at Hole 504B (Autio et al., 1989). A1-atm fractionation path calculated using the computer programCHAOS (e.g., Nielsen, 1985, 1990) is shown for comparison. Thestarting composition was primitive aphyric Sample 69-504B-35R-1,100 cm (Mg' value = 0.70; Autio and Rhodes, 1983), and fraction-ation increments of 2% were used (see also, Naslund et al., this vol-ume). Based on this path, the range of observed compositionsrepresents roughly 35% fractionation of olivine + plagioclase, fol-lowed by olivine + plagioclase + clinopyroxene. These calculationsagree closely with fractionation paths based on experimental (1 atm)melts of Group D Costa Rica Rift Zone compositions of Autio(1984).

Group D compositions, characterized by Zr and Nb concentra-tions that are extremely depleted relative to Type I MORB (i.e.,Group M) at similar stages of differentiation, have Zr/Nb ratios thatare similar to Type I basalts (Autio and Rhodes, 1983; Kempton etal., 1985; Autio et al., 1989). Figure 3 shows that all Leg 140/137dike samples fall into the same category—depleted incompatible el-ement signature and Zr/Nb ratios greater than 30. Figure 4 depictsvariations in Zr and Zr/Y for the basalts and diabases of Hole 504Bcompared with other mid-ocean-ridge basalts. Most Hole 504B sam-ples are highly depleted in Zr and Zr/Y with respect to typical mid-ocean-ridge basalts, and this level of depletion is not significantly af-fected by simple multiphase fractionation paths of Autio et al. (1989).

COMPOSITIONAL VARIATIONS WITH DEPTH

The incompatible element signatures of the Leg 137/140 dikesamples are relatively uniform between 1620 mbsf and 2000 mbsf(Fig. 5). Levels of Nb (< 1 ppm), Zr (44 ± 7 ppm), and Zr/Y (1.9 ± 0.2)are well within the typical ranges defined by previous legs to this site.Moreover, when samples from the entire hole are considered, thereare no significant systematic trends toward enrichment or depletionof incompatible elements with depth. Similarly, there are no system-atic differentiation trends with depth, as indicated by compatible ele-ments (Figs. 6 and 7). Instead, there are regions of increasedcompositional variability (at approximately 1300 and 1700 mbsf)separated by a section of minimal variation, near 1550 mbsf. Thispattern is consistent with variable addition of olivine, plagioclase,and clinopyroxene components through influxes of slightly moreprimitive magmas, or simple crystal settling within individual dikes.These patterns may be exaggerated by the unusually low rate of re-

J.W. SPARKS

Table 1. X-ray fluorescence analyses of Leg 137/140 basalts and diabases.

Core, section:Interval (cm):Piece:Depth (mbsf):Lithologic unit:Rock name:Alteration (%):

SiO, (wt%)TiO,Al,0,Fe,O,*MnOMgOCaONa,0

p2b,Total (anhyd.)

LOI* (wt%)Cl (ppm)S

Nb

ZrYSrRbGaZn

CuNiCrV

Mg -valueCaO/‰OZr/YTi/Zr

174R-1108-110

171577.4

195OPCn.d.

50.60.90

15.2910.500.188.16

12.901.67n.d.0.06

100.2

1.6297

776

<0.342.123.546.6<0.517.1708296

249298

0.6317.721.79

128

I74R-254-58

101578.3

195OPCn.d.

50.50.91

15.5710.420.187.95

12.861.72n.d.0.06

100.2

1.5682

789

0.443.624.746.7<0.515.5717995

262313

0.6277.481.77

125

177R-129-32

91604.8

202OPCn.d.

50.71.03

14.5311.330.208.03

12.431.81n.d.0.07

100.1

1.5273

1023

0.746.627.045.8<0.516.28381

142335

0.6096.871.73

132

186R-1116-119

I6B1627.5213A-D50

47.91.00

16.0810.210.189.28

13.461.62n.d.0.08

99.8

2.27416100

1.059.526.264.9<0.515.55727

131382290

0.6678.312.27

101

186R-218-21

61628.0

213A-D50

47.91.04

16.599.920.179.34

13.281.83n.d.

100.2

2.54408143

0.761.525.667.5<0.514.95831

143374285

0.6757.262.40

101

186R-250-53

101628.3

213A-D50

47.11.00

16.3610.690.189.09

13.561.42n.d.0.08

99.5

2.47436

28

0.659.225.S63.6<0.516.05613

137378299

0.6529.552.29

101

187R-15-9

21632.1

214A-D

10

50.21.09

14.5911.160.208.25

12.492.13n.d.0.07

100.1

1.51110

1115

0.955.727.560.6<0.516.3879085

186339

0.6195.862.03

117

187R-138^0

91632.4

215A-D50

51.00.92

14.4610.260.178.38

12.772.36n.d.0.06

100.3

2.21583100

<0.342.324.456.7<0.514.8591284

210320

0.6435.411.73

131

189R-170-73

161651.7218

M-POC10

50.40.94

15.1110.420.188.32

12.721.88n.d.0.07

100.1

1.58115

1146

0.744.424.24X.7<0.516.071

10695

273314

0.6376.771.83

127

189R-2110-113

161653.6

218M-POC

70

51.10.68

15.299.630.178.71

12.341.90n.d.0.05

99.9

3.44356153

0.530.117.749.7<0.512.96910

109367262

0.6666.481.70

135

190R-15-9

21655.2

218M-POC

70

53.70.60

14.0910.070.178.21

12.191.13n.d.0.04

100.2

5.09283364

<0.327.116.827.6<0.512.471

998

313239

0.64210.79

1.61134

I93R-117-20

61674.7

220M-POC

50

50.20.90

15.0210.480.198.50

12.502.01n.d.0.06

99.9

1.85262438

0.540.424.049.1<0.515.373

10391

268320

0.6416.221.68

134

I93R-128-31

91674.8

220M-POC

80

51.00.46

15.928.950.159.22

12.542.04n.d.0.03

100.3

3.51270108

<0.318.212.75 1.5<0.512.071

9113

406232

0.6946.151.43

151

193R-144-46

13A1675.0

220M-POC

56

50.50.97

15.499.870.188.33

13.041.94n.d.0.07

100.4

2.15236584

0.646.225.748.4<0.515.072

10399

291314

0.6506.721.80

125

193R-160-63

141675.1220

M-POC95

54.30.34

13.588.660.147.40

13.450.80n.d.0.02

98.7

7.46337

14132

<0.314.712.33 1.3<0.511.1537

83320156

0.65316.811.20

139

Table 1 (continued).

Core, section:Interval (cm):Piece:Depth (mbsf):Lithologic unit:Rock name:Alteration (%):

SiO, (wt%)TiO,A1,O,FeX>,*MnOMgOCaONa,O

p,b5Total (anhyd.)

LOI* (wt%)Cl (ppm)S

NbZrYSrRbGaZnCuNiCrV

Mg -valueCaO/Na,OZr/YTi/Zr

208R-139-41

101778.4

238S-PCO

10

49.90.76

15.679.310.179.40

13.251.71n.d.0.05

100.2

1.3173

661

<0.335.020.248.8<0.514.76399

141432261

0.6907.751.73

130

2O8R-1114-118

231779.1

239M-OPC

10

50.00.85

15.929.620.178.68

13.131.80n.d.0.06

100.2

1.3180

836

0.342.421.758.8<0.514.56085

113364260

0.6657.291.95

120

208R-269-72

121780.1

239M-OPC

15

49.70.82

16.359.330.178.6?

13.301.83n.d.0.06

100.2

1.3483

777

0.540.420.859.4<0.514.858

120375257

0.6717.271.94

122

209R-113-17

31787.6

240M-OPC

15

49.90.81

15.809.340.178.94

13.251.78n.d.0.06

100.0

1.2769

741

0.339.720.657.9<0.514.76385

114426256

0.6787.441.93

122

209R-1135-139

161788.9

240M-OPC

50

49.10.74

17.758.360.158.85

13.481.74n.d.0.05

100.2

1.81127571

<0.336.018.057.5<0.514.454

158134389229

0.7007.752.00

123

209R-2102-105

17A1790.0

240M-OPC

80

52.30.37

13.5711.250.179.58

11.281.56n.d.0.03

100.1

4.05510896

<0.318.012.448.4<0.510.469

8130458187

0.6527.231.45

124

211R-183-87

181799.3241

S-POC15

49.30.80

16.529.550.159.08

13.041.79n.d.0.06

100.3

1.73146282

0.338.621.653.9<0.514.053

115144358247

0.6777.281.79

124

213R-110-11

41812.6

242A-B60

50.81.04

14.4210.790.178.19

12.752.07n.d.0.07

100.3

1.82169112

0.748.627.354.5<0.516.7495674

169337

0.6266.161.78

129

213R-122-25

81812.7

242A-B60

50.51.09

14.5910.810.178.15

12.762.06n.d.0.07

100.2

1.79178210

0.348.426.254.4<0.515.150

10575

160347

0.6246.191.85

135

213R-179-83

211813.3

243S-POC

30

49.80.99

14.9210.470.188.43

12.812.04n.d.0.07

99.7

1.2489

838

0.549.425.654.7<0.516.1799095

264317

0.6396.281.93

120

213R-1121-123

28A1813.7

243S-POC

90

48.90.50

17.318.800.129.57

12.682.08n.d.0.03

100.0

2.66506

62

<0.319.814.254.9<0.513.85111

142409214

0.7056.101.39

150

214R-158-61

61819.2

244M-POC

90

49.10.84

16.249.250.169.14

13.391.80n.d.0.06

99.9

2.05143591

0.540.422.753.3<0.516.058

194147370254

0.6857.441.78

124

214R-195-101

121819.6

244M-POC

90

49.00.56

16.469.720.149.79

12.291.79n.d.0.04

99.8

2.46630

18

<0.328.015.751.7<0.513.253

9164446211

0.6896.871.78

119

214R-258-64

141820.6

244M-POC

90

49.20.56

16.888.670.139.57

12.971.86n.d.0.04

99.9

2.05602

66

<0.325.515.353.7<0.513.24811

160477222

0.7086.971.67

132

218R-117-19

51847.1

247A-D70

49.41.06

15.0610.710.208.61

12.662.33n.d.0.07

100.1

2.09411100

0.650.427.055.5<0.516.1514488

252346

0.6395.431.87

126

Notes: Rock name: A = aphyric (%); S = sparsely phyric (%); M = moderately phyric (%); D = diabase; B = aphanitic groundmass. Phenocrysts are listed in order of abundance foreach unit. P = plagioclase; O = olivine; C = clinopyroxene (Shipboard Scientific Party, 1992). OPC = phenocrysts as just described, but with no relation to relative abundance (firstthree samples only, from Leg 137). Alteration % = percentage of secondary minerals in the groundmass. n.d. = not determined.



194R-15-9

21680.5

220M-POC

15

49.80.84

16.129.560.178.58

13.081.85n.d.0.05

100.0

1.77158603

<0.339.422.246.7<0.515.36891

110348291

0.6647.071.77

128

196R-10-3

11696.5

221A10

50.51.11

14.4711.200.198.27

12.292.10n.d.0.08

100.2

1.71113933

0.654.329.349.1<0.516.1

1097988

172353

0.6195.851.85

122

197R-192-95

201703.7222

M-POC15

50.81.02

14.6311.030.198.14

12.611.86n.d.0.07

100.3

1.3382

843

0.347.226.846.1<0.516.0778286

228322

0.6196.781.76

130

197R-I105-107

231703.9

223S-POC

15

50.71.06

14.5311.270.208.31

12.371.89n.d.0.07

100.4

1.44119622

<0.350.128.643.7<0.516.4798291

220358

0.6196.541.75

127

198R-14-62

1712.2223

S-POC15

49.20.79

15.829.770.179.56

12.921.68n.d.0.05

100.0

1.5792

729

<0.336.921.754.4<0.514.26888

153371247

0.6837.691.70

128

198R-175-78

191713.0224AD60

50.81.01

14.6510.180.188.48

12.872.07n.d.0.07

100.3

1.68543

43

0.548.926.059.1<0.515.9591390

270336

0.6476.221.88

124

199R-171-76

171720.1226

M-PCO15

49.40.78

16.419.100.169.69

13.051.76n.d.0.05

100.4

1.5876

838

0.339.819.964.2<0.514.67088

168432242

0.7017.412.00

117

199R-181-84

191720.2226

M-PCO15

49.40.77

16.239.160.169.59

13.011.69n.d.0.05

100.1

1.5482

776

0.538.919.363.8<0.513.67084

171430232

0.6977.702.02

119

200R-153-57

121729.1227

M-POC15

49.30.75

16.539.090.159.43

13.101.75n.d.0.05

100.2

1.5053

780

<0.337.918.767.0<0.514.86082

170415218

0.6957.492.03

119

200R-I86-88

17A1729.5227

M-POC15

49.50.75

16.518.930.169.35

13.201.74n.d.0.05

100.2

1.3958

805

0.439.119.065.4<0.513.76487

161437232

0.6977.592.06

115

201R-15-9

21737.9228A-D60

51.20.73

14.8310.530.198.69

12.052.00n.d.0.05

100.2

1.80482

21

0.433.419.760.2<0.514.8681093

250287

0.6456.031.70

131

203R-14-7

21749.1

231A-D15

50.20.92

15.0110.500.188.78

13.031.89n.d.0.06

100.6

1.35n.d.n.d.

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

0.6486.89n.d.n.d.

203R-119-21

61749.2231A-D15

50.10.93

14.8210.350.198.61

13.271.96n.d.0.06

100.3

1.2093

687

0.343.024.852.6<0.514.97790

104377310

0.6476.771.73

130

203 R-l23-25

71749.2231A-D15

49.80.90

15.1510.450.188.86

12.871.81n.d.0.06

100.0

1.1884

951

0.444.125.355.0<0.514.78196

107367290

0.6517.111.74

123

205R-178-79

181757.8

234A-B20

49.91.10

14.4411.190.198.15

12.632.17n.d.0.07

99.8

1.25122950

0.755.627.959.9<0.515.7848881

181336

0.6165.821.99

118

205R-191-92

221757.9235

M-COP10

50.11.08

14.3911.000.198.32

12.822.22n.d.0.07

100.2

1.31139426

0.855.127.760.2<0.515.4759981

181341

0.6255.771.99

118

207R-10-4

11768.4236

A60

50.61.01

14.4210.700.208.58

12.771.96n.d.0.07

100.3

1.36235259

0.549.426.455.2<0.515.2596795

276341

0.6386.521.87

122


220R-10-2

11865.5

251A-D30

50.91.07

14.9410.160.187.92

13.062.31n.d.0.07

100.6

1.34449

39

0.450.826.861.2<0.516.0422679

231339

0.6325.671.90

126

222R-1100-104

191885.6

255S-OCP

60

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

n.d.184344

0.348.325.950.6<0.515.7569481

187324

n.d.n.d.1.86n.d.

222R-1121-125

231885.8

256A-D70

49.20.96

15.6910.210.159.02

12.821.67n.d.0.08

99.8

2.67348

27

0.657.325.165.6<0.515.54510

137462290

0.6607.682.28

101

224R-178-82

171904.5259A-D30

50.81.14

14.1711.740.198.01

11.692.25n.d.0.08

100.1

1.53538504

0.553.930.353.6<0.516.6578976

127374

0.6005.201.78

127

225R-113-16

31912.3259A-D50

49.80.98

15.299.680.188.81

13.201.92n.d.0.06

99.9

1.34359187

<0.356.724.277.3<0.515.340

107112416278

0.6676.892.34

103

225 R-l103-106

261913.2259A-D50

49.70.92

15.469.650.178.98

13.131.85n.d.0.06

99.9

1.50406

95

0.552.123.277.0<0.514.84058

115414174

0.6727.102.25

106

226R-318-24

21923.1260

M-COP60

49.20.77

16.919.340.169.10

13.321.76n.d.0.05

100.6

1.74153333

<0.335.920.054.7<0.515.248

148151363244

0.6827.571.80

129

227R-226-30

51926.1261

M-COP30

50.50.88

15.7210.000.178.28

13.091.77n.d.0.06

100.5

1.4049

735

<0.343.123.254.0<0.515.37680

101278279

0.6467.401.86

122

228R-10-5

11934.0262

M-OCP20

49.60.82

16.039.880.188.85

13.061.79n.d.0.06

100.2

1.8094

1063

0.639.021.952.7<0.515.26294

133394261

0.6637.291.78

125

230R-15-8IB

1953.1265

S-POC50

50.91.01

14.7910.760.188.29

12.182.15n.d.0.07

100.3

1.48446

90

<0.346.126.151.6<0.515.0444584

178327

0.6295.671.77

131

232R-14-6

11957.3265

S-POC5

50.50.98

14.9110.830.198.33

12.162.28n.d.0.07

100.2

0.6988

159

<0.347.726.052.5<0.516.4908688

168347

0.6295.331.83

123

233R-14-7

21967.5266A60

50.30.89

14.8510.130.158.90

13.241.80n.d.0.06

100.3

1.57647

23

<0.341.224.547.8<0.515.1392092

207323

0.6597.361.68

129

235R-10-3

11976.1267

M-PCO40

50.50.92

15.609.970.188.48

13.151.96n.d.0.07

100.8

1.1682

826

<0.344.723.460.2<0.515.4588697

358276

0.6526.711.91

123

235R-118-20

61976.3268

S-PC40

50.80.94

14.7510.350.158.46

12.871.88n.d.0.07

100.3

1.68183167

0.444.423.848.9<0.516.0429889

266324

0.6436.851.87

127

236R-10-4

11980.7269

M-POC30

49.50.78

17.028.820.158.55

13.591.85n.d.0.06

100.3

1.52117599

0.638.920.061.2<0.515.449

114125369243

0.6817.371.95

120

238R-I22-25

81992.3269

M-POC30

50.10.86

15.659.600.178.53

13.001.91n.d.0.06

99.9

1.14108795

<0.341.622.159.3<0.515.96384

105377273

0.6626.811.88

124

J.W. SPARKS

Table 2. X-ray fluorescence analytical statistics and comparisons.

SiO, (wt%)TiO,A1,0,FejO,*MnOMgOCaONa,0K20p2ö,LÖITotal

Cl (ppm)SNbZrYSrRbGaZnCuNiCrV

BIR-1

Mean

47.80.96

15.6511.380.1749.65

13.311.830.0280.022

-0.75100.1

n.d.n.d.0.5

15.614.9

108<0.516.373.0

129159380311

Std. dev.(iσ>

0.20.010.090.050.0030.090.050.030.0020.002

n.d.n.d.0.10.40.310.40.20.41124

DNC-

Mean

46.90.49

18.5810.040.151

10.1211.33

1.900.2330.066

-0.1099.7

60648

1.336.916.6

1433.3

14.570.689

246276149

!

Std. dev.(1CJ)

0.20.000.090.070.0030.070.050.040.0020.002

420.10.40.200.10.40.41113

BAS-111

Mean

49.50.79

16.439.040.1568.56

13.221.820.0110.0520.42

100.0

79729

0.539.620.159.9<0.514.668.387

127370245

Std. dev.

(iσ)0.20.010.070.040.0020.070.030.030.0010.002

350.10.20.20.40.20.4

0.51122

BAS-140

50.60.99

14.5811.080.198.14

12.411.85n.d.0.065n.d.

99.9

87

<0.344.826.645.5<0.515.6858485

192342

Detectionlimits (2<3)

n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

420.30.50.50.50.50.8

11113

Mean difference

(Stanford-Shipboard)

-0.2-0.02-0.17

0.10-0.002-0.21-0.03-0.01

n.d.0.014

n.d.n.d.

-0.10.3

-0.1-0.4

n.d.n.d.

-110

-1-12

-2

Std. dev.

(iσ)0.250.010.130.150.010.120.080.06n.d.0.03

n.d.n.d.0.20.50.70.8

n.d.n.d.32366

Note: BAS-140 was ignited before major element analysis; therefore, its total is reported on an anhydrous basis.

Table 3. Barium and rare-earth element abundances, lithologic Unit 220.

Core, section:Interval (cm):Piece number:Depth (mbsf):

Ba (ppm)LaCePiNdSinEuGdTbDyHoErTinYbLu

I93R-117-20

61674.7

1.00.863.480.734.101.750.712.500.533.510.812.440.352.410.33

193R-128-31

91674.8

1.50.761.670.332.090.880.441.440.302.200.471.520.201.440.21

193R-14 4 ^ 6

13A1675.0

1.40.923.920.814.821.870.742.940.604.040.902.680.402.630.37

193R-160-63

141675.1

0.70.301.500.332.250.920.671.450.291.960.421.310.181.180.16

194R-15-9

21680.5

0.70.773.200.654.161.670.652.490.483.390.742.260.332.060.31

Precision(±lGppm)

0.250.030.100.010.090.060.040.050.020.070.020 . 1 2

0.010.080.01

BIR-1

210.772.590.462.781.310.582.110.473.030.742.080.302.240.33

Standards

DNC-1

102

3.608.651.125.181.570.672.180.432.830.702.100.332.060.30

NBS-688

1804.98

12.351.788.282.400.992.900.503.250.712.030.312.010.31

N

-

0.330.880.1120.600.1810.0690.2490.0470.3220.0700.2000.0300.2000.034

Notes: Precision = calculated precision (G) for Unit 220 (see text). Chondrite-normalizing value (N) for Dy is from Boynton (1989); all others from Haskin et al. (1968).

covery around 1550 mbsf, due to numerous problems with coringduring Legs 111, 137, and 140 (e.g., Shipboard Scientific Party,1992).

A significant trend of Zn depletion with depth (Fig. 8A) occurs inthe bottom 350 m of core, and is observed in all samples ranging from5% to 95% alteration (Shipboard Scientific Party, 1992; this study).With the exception of the sulfide mineralization zone (910-928 mbsf)and other transition zonebasalts, typical Zn concentrations for 504Bvolcanics and sheeted dikes vary between 60 and 80 ppm. However,beginning at about 1650 mbsf, the mean Zn concentration ( 70 ppm)decreases to about 42 ppm at 2000 mbsf. Similarly, the minimum Znabundance decreases (55-30 ppm) through the same depth interval.The relative Zn depletion as depicted in Figure 8A, is not as pro-nounced as the one reported by the Leg 140 Scientific Party. The dif-ference is that the Zn data produced by the shipboard XRF lab,although precise, was found to be consistently 12 ppm lower thanthose determined by this study and from previous legs, and has beencorrected for a more precise comparison. The resulting depletion pat-tern is different in one significant aspect only—the depth at which the

trend begins is very likely between 1700 and 1800 mbsf, and nothigher up in the sheeted dike section ( 1300 mbsf), as it may have ap-peared before (i.e., Shipboard Scientific Party, 1992; Dick et al.,1992). A comparable depletion trend for Mn or Cu has not been iden-tified (Fig. 8B, -C). Below 1600 mbsf, the pattern of MnO concentra-tions is similar to that of Fe2O3*, and Cu is highly scattered aroundtypical values of 75 to 110 ppm.

VARIABILITY OF "IMMOBILE" ELEMENTSTI, ZR, AND Y

As previously noted, all samples recovered from Leg 140 haveundergone minor to extensive levels of subsolidus alteration (Ship-board Scientific Party, 1992), as indicated by mineralogy (5%-95%secondary minerals), water content (LOF = 0.7%-7.5%), and con-centrations of relatively mobile elements (Cu = 7-194 ppm; S = 18-14132 ppm; Sr = 28-77 ppm; and Na2O = 0.8%-2.4%). The extent of


Table 4. Comparison of samples with variable phenocryst content.

SiCX (wt%)TiO,A1203

Fe.O,*MnOMgOCaO‰ OK,0p,ö,LOI* (wt%)Cl (ppm)S

NbZrYSrRbGaZnCuNiCrV

Mg -valueCaO/‰OZr/YTi/Zr

Mean

50.00.99

14.9810.520.188.59

12.861.98n.d.0.07

1.67335342

0.550.525.059.5<0.515.5625798

271314

0.646.611.95

120

Aphyric( 1% phenocrysts)

1 std. dev.

1.10.100.670.540.010.420.470.23n.d.0.01

0.41188383

0.27.32.38.0n.d.0.7

1936219843

0.021.030.24

12

Min.

47.10.73

14.179.650.157.92

11.691.42n.d.0.05

1.188421

0.333.419.747.8

n.d.14.7391074

127174

0.605.201.68

101

Max.

51.21.14

16.5911.740.209.34

13.562.36n.d.0.08

2.54647

1115

1.061.530.377.3n.d.

16.7109107143416374

0.679.552.40

135

Mean

50.20.93

15.1710.370.178.67

12.691.92n.d.0.06

1.39150474

0.344.424.650.5<0.515.56588

105267308

0.656.691.80

127

Sparsely phyric(l%-2% phenocrysts)

1 std. dev.

0.70.120.680.690.020.530.380.20n.d.0.01

0.31112295

0.15.52.73.7n.d.0.9

1618299141

0.030.840.074

Min.

49.20.76

14.539.310.158.14

12.161.68n.d.0.05

0.697390

0.335.020.243.7

n.d.14.0424581

168247

0.625.331.70

120

Max.

50.91.06

16.5211.270.209.56

13.252.28n.d.0.07

1.73446843

0.550.128.654.7n.d.

16.490

115153432358

0.697.751.93

131

Mean

49.90.86

15.919.730 . 1 7

8.6913.051.83n.d.0.06

1.54109739

0.441.622.255.8<0.515.26595

119337274

0.667.171.88

123

Moderately phyric(2%-10% phenocrysts)

1 std. dev.

0.50.090.800.740.010.490.280.12n.d.0.01

0.2555

200

0.24.22.76.8n.d.0.89

21298337

0.030.510.115

Min.

49.10.74

14.398.360.157.95

12.431.67n.d.0.05

1.1449

333

0.335.918.045.8

n.d.13.64879

142218

0.615.771.68

115

Max.

50.71.08

17.7511.330.209.69

13.592.22n.d.0.07

2.15262

11460.8

55.127.767.0n.d.

17.183

158171437341

0.707.752.06

134

this alteration is extremely heterogeneous on a scale of centimeters tometers, commonly ranging from slight, to nearly complete recrystal-lization within 10 cm of core. Localized areas of extensive alteration,dubbed alteration "patches" or "halos," typically represent 70% to90% replacement of primary minerals by actinolite, chlorite, albite,titanite, and trace epidote, prehnite, and anhydrite, whereas, sulfideminerals are virtually absent (Alt et al., 1989b; Shipboard ScientificParty, 1992). For detailed descriptions and compositions of alterationminerals in the sheeted dike complex, see Alt et al. (1985, 1986a,1989b), Ishizuka (1989), Kempton et al. (1985), Kusakabe et al.(1989), and various authors, this volume. At high degrees of alter-ation (>70%), elements that are normally considered to be relativelyimmobile during hydrothermal alteration (i.e., TiO2, Zr, Y) becomehighly variable, and exhibit even greater levels of depletion than haspreviously been observed at this site (as noted earlier, the degree ofalteration is simply the total percentage of secondary mineraliza-tion—predominantly chlorite and actinolite—as determined by eitherthin-section point counts or visual inspection of sliced core samples).Comparing TiO2, Zr, and the ratio Zr/Y with highly mobile Cu, illus-trates this point (Figs. 9-11). For samples with less than 50% alter-ation of the groundmass, Cu is typically between 75 and 115 ppm,with minimal variation in TiO2 (0.9% ± 0.1%), Zr (43 ± 5 ppm), andZr/Y (1.9 ± 0.1). With increased alteration Cu becomes highly vari-able, with the addition or removal of Cu-Fe sulfides, ranging from 10to 158 ppm at 50% to 60% alteration, and 7 to 194 ppm at greater than60% alteration. In contrast, the values of TiO2, Zr, and Zr/Y in sam-ples that contain up to 60% secondary minerals, are more variable,but they are well within typical ranges for Hole 504B. At greater lev-els of alteration, however, even these elements exhibit marked varia-tion, particularly toward depletion (TiO2 = l.l%-0.3%; Zr = 66-15ppm; Zr/Y = 2.6-1.2). For this reason, an arbitrary limit of 60%groundmass alteration is used by this study to filter out "altered" sam-ples when describing variations in igneous chemistry. Alternatively,a minimum limit of 35 to 60 ppm Cu could be used to achieve thesame purpose. Under this scheme, however, some of the units withZr/Y ratios greater than 2.2 may be unduly omitted.

The largest TiO2, Zr, and Y anomalies were recorded in lithologicUnit 220, a unit in which the average groundmass alteration is ap-

proximately 10%, but in localized patches reaches 60% to 95%(Shipboard Scientific Party, 1992). The composition of these patchesrelative to less altered samples in the unit is shown in Figure 12.Three samples (140-504B-193R-1, 17-20 cm; -193R-1, 44-46 cm;and -194R-1, 5-9 cm) represent slightly to moderately altered (LOI*= 1.9% ± 0.2%; Cu = 99 ± 7 ppm) sections of Unit 220. These sampleswere averaged and used as a "fresh" composition for normalizationagainst two highly altered samples (140-504B-193R-1, 28-31 cm;and -193R-1, 60-63 cm). Mobile elements in the highly altered sam-ples are exceedingly variable and/or depleted with respect to theslight to moderately altered samples (Na2O = 0.4-1.1 times that of theaverage "fresh" composition; S = 0.2-26'; Cu = 0.1'; Sr = 0.7-1.1').Concentrations of TiO2, Zr, Y, P, and most rare-earth elements, onthe other hand, are all depleted the same amount—roughly one-halfthose observed in the fresher samples—resulting in Zr/Y and Ti/Zrvariations that are more limited than the overall changes in abun-dance. Zr is more depleted than Ti, Y, and the middle to heavy rare-earth elements in the altered samples, producing a small degree of de-coupling. La and Eu exhibit behavior slightly different from the restof the rare-earth elements (Figs. 12 and 13). Both altered sampleshave small, positive Eu anomalies (Eu/Eu* < 2), and one sample(140-504B-193R-1, 28-31 cm) displays an unusual enrichment in Larelative to the trend of other light REEs. Positive Eu anomalies couldbe attributable to the presence of secondary albite (Humphris et al.,1978), or the interaction with seawater (e.g., Michard et al., 1983).However, the relative level of Na abundance in the two altered sam-ples is the opposite of what would be expected if Eu was indicativeof albite concentration—the sample with the strongest Eu anomalycontains the least Na. It is not known if the REE pattern for Sample140-504B-193R-1, 28-31 cm, represents a La enrichment, or a nega-tive Ce anomaly associated with the interaction of seawater. If it isgenuine La enrichment, the light rare-earth pattern differs from thepatterns associated with palagonitization of glassy volcanic pillows(e.g., Ludden and Thompson, 1979; Alt and Emmermann, 1985). Inthese cases, the light rare-earth elements from La to Sm are all en-riched relative to the fresh, glassy interiors, in contrast to the singularincrease in La as seen here.

87

J.W. SPARKS

Table 5. Hole 504B interleg comparisons-aphyric and sparsely phyric (0%-2%) samples only.

Legs 69/70 Leg 83 Leg 111

SiO, (%)TiO,A 1 2 O 3

Fe^O,*MnOMgOCaONa,OK,Op 2 bj

Nb (ppm)ZrYSrRbGaZnCuNiCrV

Mg -valueCaO/Na,OZr/Y

Mean

50.30.94

15.619.970.188.28

12.681.990.140.08

0.547.523.366.9

2.216.078

n.d.106362279

0.6476.392.04

1 std. dev.

0.30.040.560.560.020.180.240.130.090.01

0.13.01.46.02.00.67n.d.

194528

0.0120.430.09

Min.

49.80.88

14.908.830.167.99

12.171.800.040.06

0.443.02 1 . 5

57.00.4

15.266

n.d.76

282231

0.6285.66

Max.

50.71.00

16.7010.690.238.55

12.982.200.360.10

0.753.025.678.0

7.316.890

n.d.144463347

0.6717.092.18

Mean

50.11.00

14.8310.680.218.49

12.861.990.020.09

0.750.624.753.4

0.515.485

n.d.93

274292

0.6366.54

1 std. dev.

0.50.080.380.600.020.370.250.19

<O.Ol0.01

0.25.52.26.10.10.5

11n.d.7

5924

0.0210.61

Min.

48.80.84

14.319.670.177.72

12.251.68

<O.Ol

0.08

0.438.020.543.0

0.414.567

n.d.76

168245

0.5945.021.60

Max.

50.81.14

16.0811.620.269.09

13.182.440.030.11

1.159.028.066.0

0.916.4

104n.d.

102355326

0.6667.732.84

Mean

50.30.96

15.2310.460.188.33

12.721.900.010.06

0.848.324.557.9

0.115.874

n.d.104252271

0.6376.751.97

1 std. dev.

0.60.120.850.840.010.410.400.16

<O.Ol0.01

0.35.92.66.40.10.9

10n.d.

2810636

0.0290.740.10

Min.

48.90.75

13.809.260.157.16

11.701.59

<O.Ol0.05

0.338.420.143.7

0.014.155

n.d.5526

213

0.5585.271.78

Max.

51.11.24

16.9012.500.219.18

13.172.220.020.08

1.564.430.466.4

0.418.499

n.d.164388345

0.6868.212 . 1 2

+ Legs 69, 70, 83, and 111• Legs 137 and 140Δ Calculated fractionation path

0.65 0.70 0.750.50 0.55 0.60Mg'-value

Figure 1. Mg'-value vs. TiO2. Compared with over 1300 aphyric to sparsely phyric MORB compositions, the Group D basalts and diabases of Hole 504B (>270analyses) are unusually low in TiO2 for a given Mg'-value. The calculated liquid line of descent (e.g., Nielsen, 1985, 1990) represents 38% fractionation fromthe starting composition, and does not result in more "normal" MORB compositions after extensive differentiation. MORB field constructed after Autio et al.(1989). Dashed field boundary represents 95% of the MORB samples. Data from Legs 137 and 140 exclude highly (>60%) altered samples. Analytical error isapproximately the width of each plot symbol.

DISCUSSION

Primary Igneous Compositions

Discounting the mobile elements K, Rb, Sr, Cu, Mn, and Zn, the127 basalts and diabases examined in this study and by the Leg 140Shipboard Scientific Party (1992) are compositionally indistinguish-able from 98% of the lithologic units observed in the upper sheeteddikes, transition zone, and volcanics of Hole 504B. Based on Sr andNd isotopic composition (Barrett, 1983; Kusakabe et al., 1989;Shimizu et al., 1989), petrography (e.g., Natland et al., 1983; Kemp-ton et al., 1985; Shipboard Scientific Party, 1992), and compatible el-ement concentrations, Hole 504B Group D basalts and diabases aresimilar to moderately evolved, Type I MORB. On further inspection,however, Group D samples exhibit unusual levels of depletion in in-

compatible elements (Figs. 1-4, and 13) and are not related to normalType I compositions through simple fractionation. Both calculatedand experimentally determined 1-atm fractionation paths are incon-sistent with the notion that normal Type I compositions can be pro-duced through extensive fractionation of the depleted CRRZ magmas(Autio and Rhodes, 1983; Kempton et al., 1985; Autio et al., 1989,this study). The range of aphyric to sparsely (0%-2%) phyric compo-sitions observed at this location, from the most primitive to the mostevolved (Mg'-values = 0.70-0.56), requires approximately 35% frac-tionation to expiain, based on calculated levels of highly incompati-ble elements (Autio et al., 1989) and calculated liquid lines of descentfrom major element phase equilibria (e.g., Nielsen, 1985, 1990). Fur-ther differentiation will produce incompatible element levels that aretypical of Type I basalts, but only at the expense of reducing the com-patible elements to unacceptably low levels.



Mean

50.00.9415.0110.490.188.5212.791.930.0060.065

0.446.725.053.4<0.515.67193102280305

0.6436.831.80

Legs 137/140

1 std. dev.

0.60.120.630.680.010.470.340.160.0050.017

0.26.12.57.80.10.81517238636

0.0270.670.05

Min.

48.50.7114.178.990.157.7911.691.67

<0.0020.036

0.335.020.243.0<0.514.0425674127231

0.6005.201.64

Max.

50.81.14

16.5211.740.209.5713.272.280.0200.128

1.557.530.377.30.916.7109136155432374

0.7018.151.88

The CaO/Na2O ratios (5.2-9.6) for Group D basalts are larger thanwhat is commonly observed in normal MORB. These ratios are cal-culated to be in equilibrium with unusually anorthitic plagioclase (upto An92; Autio and Rhodes, 1983), and are in agreement with naturalplagioclase compositions from the volcanic section (up to An90; Nat-land et al., 1983) and the upper sheeted dikes (Kempton et al, 1985).Natland et al. (1983) have suggested that the highly depleted, Ca-richCRRZ basalts represent unusual primitive melt compositions, but thisnotion is inconsistent with observed concentrations of compatible el-ements such as Ni (102 ± 23 ppm in aphyric-sparsely phyric sam-ples), and the multisaturated nature of the CRRZ basalts. Otherworkers have argued that these compositions are the result of a mul-tistage melting of a normal MORB mantle source, followed by exten-sive crystallization (Autio and Rhodes, 1983; Autio, 1984; Kemptonetal., 1985; Autio etal., 1989; Kusakabeetal., 1989; Shimizu, 1989).Multistage melting produces melts that are extremely depleted in in-

compatible elements (Nb and Ba < 1 ppm), whereas compatible ele-ments, isotopic ratios (e.g., Sr and Nd), and incompatible elementratios with the more incompatible element in the denominator (e.g.,Zr/Nb) remain at normal, or slightly elevated levels (e.g., Duncan andGreen, 1980)—all features that are characteristic of Group D compo-sitions. Furthermore, multistage melting would raise CaO/Na2O ra-tios relative to normal MORB (Green et al., 1979; Duncan and Green,1980; Autio and Rhodes, 1983), and this increase would be reflectedin higher than normal anorthite content in equilibrium plagioclase.

It is possible that multistage melting is more common than previ-ously thought. Many mid-ocean-ridge basalts contain plagioclasephenocrysts that are too calcic to be in equilibrium with their hostglasses (Bryan et al., 1976; Rhodes et al., 1979; Fisk et al., 1982). Amultistage melting scenario could explain their presence, if smallamounts of depleted second-stage melts were mixed with large vol-ume, early melts during the normal production of mid-ocean-ridgebasalts. The resulting mixture would erase the depleted signature ofthe second-stage melts, but highly calcic phenocrysts would remain(Autio etal., 1989).

The mechanism(s) that produced and erupted CRRZ melts with solittle Type I MORB component is still unclear. Autio (1984) has sug-gested that early-stage melts could have been produced and removedfrom a normal MORB source at the East Pacific Rise (EPR) or theGalapagos thermal anomaly, but the difficulties of explaining thetransport of the residual mantle source and the fate of the enriched,early melts are unresolved. Furthermore, little is known about thegeographic extent of these Group D basalts. Among the variousDSDP sites along the Cocos-Nazca Spreading Center, only basaltsfrom Sites 501 and 505 (200 and 120 km south of the CRRZ, respec-tively; e.g., Etoubleau et al., 1983), and Site 425 (62 km north of theGalapagos Rift; e.g., Joron et al., 1980; Mattey and Muir, 1980;Srivastava et al., 1980) exhibit Group D characteristics. Basalts fromDSDP Site 424, approximately 20 km south of the Galapagos Rift,are clearly normal Type I MORB and represent a significant compo-sitional shift within 82 km of Site 425.

200

150

3 100

N

50

Legs 69, 70, 83, and 111

Legs 137 and 140

CRRZ fractionation path

50 100 200 250 300150Ni (ppm)

Figure 2. Ni vs. Zr. Group D compositions from Hole 504B exhibit low to moderate levels of Ni, but Zr values that are atypical of most MORB. The liquid lineof descent calculated from 1 -atm melting experiments of CRRZ basalts (Autio, 1984) demonstrates that common multiphase fractionation of CRRZ liquids willnot produce normal levels of Zr without severely depleting Ni. MORB field constructed as in Figure 1. Data from Legs 137 and 140 exclude highly (>60%)altered samples. Analytical error is within the width of each plot symbol.

J.W. SPARKS

200

Legs 69, 70, 83, and 111

Legs 137 and 140

10

Nb (ppm)

Figure 3. Nb vs. Zr. All samples from this study plot in the D Group classifi-cation of Autio and Rhodes (1983), bringing the total number of samples inthis class to over 270. Although the Zr/Nb ratios are similar to Group M (30), or normal Type I MORB, the Group D compositions are extremelydepleted in Nb and Zr. Group E (Type II: Enriched) and Type T (Transitional)are the other basalt classes observed at Hole 504B. Data from Legs 137 and140 exclude highly (>60%) altered samples. Analytical error is approxi-mately the width of each plot symbol.

Variations in Ti, Zr, Y, and REEs in HydrothermallyAltered Samples

The localized zones of Ti, Zr, Y, REE, Cu, and S depletion occurin areas of enhanced fluid flow and porosity in the halos around veinsand vugs (Alt et al., 1992; this study). The nature of these vugs is un-clear. It has been suggested that they represent primary pore space,subsequently filled with secondary minerals (Scientific Shipboard

N

+ Legs 69, 70, 83, and 111

• Legs 137 and 140

10 20 50

Zr (ppm)

100 200

Figure 4. Zr vs. Zr/Y. The depleted samples of Hole 504B have Zr/Y ratiosand Zr concentrations that are lower than typical MORB values. MORB fieldplotted as in Figure 1. Data from Legs 137 and 140 exclude highly (>60%)altered samples. Analytical error is within the width of each plot symbol.

Party, 1992), but the amount of primary porosity is likely to be van-ishingly small, given the depth and pressure of magma emplacement(e.g., Moore, 1979). An alternative explanation that the vugs are theresult of complete recrystallization of localized regions in glassy orfine-grained groundmass (Scientific Shipboard Party, 1992) is morelikely, as glass reacts more readily than the crystalline material be-tween 70° and 300°C (Seyfried and Bischoff, 1979, 1981). The deple-tion of these elements is not restricted to the lower sheeted dikecomplex. Alt and Emmermann (1985) reported TiO2 loss in hydro-thermally altered samples from Leg 83, which penetrated the lowervolcanic section, transition zone, and upper sheeted dike complex.Furthermore, data from lithologic Unit 60 (Emmermann, 1985) sug-

200

400

600

800

1000

S.1200

ΦQ

1400

1600

1800

2000

:+‰ <

i • i • i •

Volcanics

" jt Transition zone

•

P i

Sheeted dikecomplex

Nb error

i i i i i i

2 3

Nb (ppm)50 100

Zr (ppm)150

Figure 5. Depth vs. Nb (A), Zr (B), Zr/Y (C). All samples, aphyric to moderately phyric (<10%), with less than 60% alteration are plotted. Crosses = data fromLegs 69, 70, 83, and 111; solid circles = data from Legs 137 and 140. Except for Nb, analytical error is within the width of each plot symbol. The error bar forNb is given in Figure 5A.


200

400

600

800

1000

S.1200Φ

Q1400

1600

1800

2000

+

Volcanics

Transitionzone

+•Hftf»• + Sheeted dike _i4" - h ^ . complex

-

•

• i i i i i

13 14 15 16 17 18 19 20ALO, (wt%)2 3

8 9 10MgO (wt%)

50 100 150 200 250

Ni (ppm)

Figure 6. Depth vs. A12O3 (A), MgO (B), Ni (C). All samples, aphyric to moderately phyric ( 10%), with less than 60% alteration are plotted. Symbols and ana-lytical error are the same as in Figure 5.

200

400

600

800

1000 -

3.1200ΦQ

14001600

1800

2000

1 I ' I

+ 4

-

• i • i

TT

• I • I • ^

Volcanics

Transition •

L * +

+ Sheeted dike» * . complex

-

t

47 48 49 50 51 52 53 54SiO. (Wt%)

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8TiO. (Wt%)

8 9 10 11 12 13 14Fe,0 * (wt%)2 3

Figure 7. Depth vs. SiO2 (A), TiO2 (B), Fe2O3* (C). All samples, aphyric to moderately phyric (<10%), with less than 60% alteration are plotted. Symbols andanalytical error are the same as in Figure 5.

gests the loss of Zr (61-38 ppm), Y (27-17 ppm), Cu (118-15 ppm),and rare-earth elements as well.

Documenting the variability of Ti, Y, Zr, and REE is made easierby the fact that igneous trends involving these elements are reason-ably well defined for this location (Figs. 1 and 2). Comparing rela-tively fresh samples with the highly altered patches or halos, the mostnoticeable variation is a trend toward moderately strong depletion(30%-50%) that is echoed by many of the rare-earth elements andP2O5 (Figs. 12 and 13). The abundances of these elements in the al-teration patches are significantly lower than all other values reportedat this site and, as such, they are easily distinguishable from relatively

fresh material. Examples of alteration-induced enrichment, on theother hand, are more difficult to substantiate given that all samples liewithin the normal range of primary Ti, Y, and Zr variation. Most ofthe enriched samples, although moderately recrystallized, do not con-tain intense alteration patches, nor do they display significant differ-ences in secondary mineralogy (i.e., increased titanite or Fe-Tioxides). As a result, it is not currently known if the enrichments are aprimary signature, or an artifact of alteration.

The overall effect of the alteration zones is to significantly in-crease the compositional scatter of moderate to highly incompatibleelements (Fig. 14). Employing an alteration "filter" such as H2O

+

J.W. SPARKS

1600

1800

200020r÷ . àEVV• ,

Volcanics

Transition zone

Sheeted dikecomplex

40 60 80Zn (ppm)

100 120 0.08 0.12 0.16 0.2 0.24 0.28MnO (wt%)

50 100 150Cu (ppm)

200

Figure 8. Depth vs. Zn (A), MnO (B), Cu (C). All samples, regardless of the degree of alteration, are plotted. Open boxes = data from Legs 69, 70, 83, and 111;solid circles = data from Legs 137 and 140 with less than 50% alteration; crosses = data from Legs 137 and 140 with 50%-60% alteration; open triangles = datafrom Legs 137 and 140 with greater than 60% alteration. Analytical error is within the width of each plot symbol.

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

o, -

3%-49% alteration

50%-60% alteration

61%-95% alteration

50 100Cu (ppm)

150 200

Figure 9. Cu vs. TiO2. Most Leg 137/140 samples with up to 60% alterationfall within the normal range of Hole 504B variation, as defined by analysesof slightly altered samples in the upper levels of the hole. Samples with over60% alteration display abnormally low levels of TiO2. In the absence of pet-rographic data, highly mobile Cu could possibly be used as an alteration indi-cator. The amount of error in estimating alteration is roughly ±10%.

(e.g., Shipboard Scientific Party, 1992) or Cu content to separate theaffected samples is useful, but inexact. The arbitrary definition of <60% secondary minerals as being "unaltered" in terms of Ti, Zr, andY has been used in this study, but this method is inexact as well, andmay be too permissive if future work reveals the enriched samples tobe the result of alteration.

Rare-earth elements in hydrothermal fluids from mid-ocean-ridgevents are enriched 10 to 10,000 times relative to seawater (Michardet al., 1983; Campbell et al., 1988), and it is intriguing to think thatthe intense alteration zones could be one of the main sources of Y and

80

70

60

50E& 40

N30 -

20 -

10 -

_ ΛΔ •& + ^

+ T *• ù + < β k ^

+, ^^^^

f

* •• -ft-•

Φ

? -(0

COTj

omToE *-δ

50 100

Cu (ppm)

150 200

Figure 10. Cu vs. Zr. As in Figure 9, the Zr concentrations in most Leg 137/140 samples up to 60% alteration are within the typical Hole 504B range. Atmore intense levels of alteration, Zr is commonly lower.

REE enrichment. This notion is consistent with the typical depletionof Ti, Zr, Y, and the REE in alteration halos, and the fact that thereare no obvious regions of enrichment in the 504B crustal section.However, the REE pattern of a typical hydrothermal vent fluid has apronounced light-rare-earth enrichment, and a large (10-20), positiveEu/Eu* anomaly (Michard et al., 1983; Campbell et al., 1988). Thispattern could not be produced from the one example given here.

Similar suggestions for Ti and Zr in hydrothermal fluids are diffi-cult to make at this time because it is common practice to use titaniumcollection bottles or syringes when sampling high-temperature hy-drothermal vents (e.g., Von Damm et al., 1985; Michard and Al-

;


>-

N

50 100Cu (ppm)

150 200

Figure 11. Cu vs. Zr/Y. The Zr/Y ratio decreases slightly at high degrees ofalteration, but it is less affected than either Zr or Y, individually. There is nostrong decoupling of these two elements with increased alteration. The sam-ples with 50%-95% alteration and Zr/Y values between 2.2 and 2.6 arewithin the range described by relatively fresh samples from higher levels inthe hole. There is no significant evidence of Ti, Zr, or Y enrichment resultingfrom alteration.

barède, 1986). The level of Ti and Zr contamination precludesaccurate trace element analysis of these elements.

Zinc Depletion With Depth

The leaching of Zn and Cu, coupled with the presence of massivesulfide deposits, is a common feature in ophiolite complexes (e.g.,Troodos [Richardson et al., 1987] and Semail [Koski et al., 1987]).At the Troodos ophiolite, Zn and Cu depletion is strongly linked to

the presence of epidosite zones (quartz + epidote rocks) in the lowersections of the sheeted dike complex. The epidosites represent fossilconduits for hydrothermal vents and their associated massive sulfidedeposits. Within these zones, Zn and Cu are thought to be liberatedfrom the epidosite and surrounding diabase dikes by reactions withhydrothermal fluids at temperatures of 350°-^00°C (Richardson etal., 1987; Schiffman and Smith, 1988). Much of this hydrothermalactivity probably occurred as the crustal section moved off-axis andwas reheated by the emplacement of hypabyssal stocks (Schiffmanand Smith, 1988).

In contrast, at Hole 504B a stockwork-like sulfide-mineralizationzone (910-928 mbsf) occurs in the transition from sheeted dikes tothe overlying pillow lavas, but large-scale epidosite veins have yet tobe identified. Sulfide precipitation is thought to have resulted fromhydrothermal fluids (up to 38O°C) upwelling through low porositysheeted dikes and reacting with cooler, down-flowing seawater fromthe highly porous volcanic section (Alt, 1984; Altet al., 1985, 1986a;Honnorez et al., 1985). A likely source of hydrothermal Zn (and Cu)for sulfide mineralization zones and ridge-crest black smokers is theslightly to highly altered diabase of the (lower?) sheeted dikes. Typ-ical primary Zn concentrations at Hole 504B are approximately 70 ±10 ppm (Fig. 8A), but between 1700 and 2000 mbsf, Zn is progres-sively depleted in both "fresh" and highly altered samples. A compa-rable pattern of Zn depletion with depth has been observed in thelower section of sheeted diabase dikes of the Josephine ophiolite(Harper et al., 1988) where, as at Troodos, epidosites are thought tobe remnants of a circulation system that allowed transport of hydro-thermal fluids to ridge-crest black smokers and to sulfide mineraliza-tion zones. The lack of extensive epidosite veins at Hole 504Bsuggests that the hydrothermal circulation system in this section ofoceanic crust is less well developed than either the Troodos or Jose-phine ophiolite. Upward transport of metal-enriched hydrothermalfluids must be accomplished through open, partially mineralizedfractures that have yet to develop into large-scale epidosite zones.

High-temperature leaching experiments (Seyfried and Janecky,1985) help place constraints on the reaction temperatures that arenecessary for Zn and Cu depletion. These experiments indicate thatthe concentration of Zn and Cu in leachate solutions reaches a maxi-mum between 375° and 400°C and then decreases considerably at

10

oü

0)

03

Q.

C/D

193R-1, 28-31 cm (80% alteration)

193R-1, 60-63 cm (95% alteration)

0.1

Figure 12. Comparison of "fresh diabase" vs. alteration patches, Unit 220. XRF and ICP-MS analyses of alteration patches are normalized against the averageof three samples (140-504B-193R-1, 17-20 cm; -193R-1, 4 4 ^ 6 cm; and -194R-1, 5-9 cm) representing slight to moderate alteration. Four of the five Unit 220samples were taken within a 45-cm interval in one core section (193R-1); the remaining sample (140-504B-194R-1, 5-9 cm) was located within 10 cm of thetop of the next core.

J.W. SPARKS

10

CD

Ioü

Q.E 1CO

0.1

193R-1, 17-20 cm (50% alteration)

193R-1, 44-46 cm (56% alteration)

194R-1, 5-9 cm (15% alteration)

193R-1, 28-31 cm (80% alteration)

193R-1, 60-63 cm (95% alteration)

La CΘ Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 13. Chondπte-normalized rare-earth element patterns for Unit 220. Chondrite values from Haskin et al. (1968) and Boynton (1989).

N

• 3%-49% alteration

+ 50%-60% alteration

Δ 61 %-95% alteration

10 20 50

Zr (ppm)

100 200

Figure 14. Zr vs. Zr/Y. Highly altered samples add a considerable amount ofscatter to a relatively well defined Leg 137/140 field. The higher Zr/Y sam-ples are still within normal Hole 504B range, and it is likely that they repre-sent primary igneous signatures. MORB field constructed as in Figure 1.

425 °C. This temperature range is in broad agreement with estimatesfrom the dike section of Hole 504B. Alt et al. (1989b) estimated atemperature range of 250°-350°C in the Leg 111 section of the dikesbased on secondary mineralogy and oxygen isotope data. At greaterdepths, general increases in the proportions of actinolite replacingclinopyroxene, and the increasing stability of calcic plagioclase overalbite reflect rising temperatures with depth that approach upper-greenschist facies conditions in the Leg 140 section (Shipboard Sci-entific Party, 1992; Laverne et al., this volume).

The behavior of Cu (Fig. 8C) in the altered diabase is more com-plex than Zn, and may be related to Cu being a major component inonly one phase (chalcopyrite), whereas Zn is present in minoramounts in clinopyroxene, chlorite, actinolite, Ti-magnetite as wellas sulfides. According to Seyfried and Janecky (1985), Mn and Fe arestrongly leached at 425°C, depending on the pH of the hydrothermal

solution. With this in mind, similar Mn and Fe depletion zones maybecome apparent with further penetration of Hole 504B.

CONCLUSIONS

1. Disregarding highly mobile elements that are affected by sub-solidus alteration, the compositions of basalt and diabase samples re-covered on Leg 140 are virtually identical to those of the volcanicsection, transition zone, and the upper sheeted dike complex previ-ously sampled by Legs 69, 70, 83, and 111. All samples analyzed inthis study are classified as Group D compositions (e.g., Autio andRhodes, 1983; Kempton et al., 1985), with compatible element sig-natures that are similar to moderately evolved MORB, but with un-usual depletion in incompatible elements, and elevated levels ofCaO/Na2O ratios. These features are consistent with the multistagemelting of a normal MORB mantle source (e.g., Duncan and Green,1980), followed by extensive crystal fractionation (Autio andRhodes, 1983; Autio, 1984; Kempton et al., 1985).

2. Considering all of Hole 504B, no significant trends indicatingincompatible element enrichment or depletion of source magmasover time are apparent. Patterns of phenocryst addition and/or influxof slightly more primitive magma are identifiable at various levels inthe sheeted dikes, but broad trends signifying increased or decreaseddifferentiation are absent.

3. A broad zone of Zn depletion from about 1700 to 2000 mbsf re-sulting from high-temperature (375°-400°C) leaching is a likelysource of hydrothermal Zn for sulfide mineralization zones near thesheeted dike/transition zone interface, and ridge-crest black smokers.Manganese and iron may begin to exhibit similar depletion trends atgreater depths as temperatures increase (> 425°C). In contrast, Cu canbe virtually absent locally (down to 7 ppm), but there is no systematicvariation with depth.

4. Areas of intense alteration exhibit significant depletions in Ti,Zr, Y, and REE. These localized (centimeter to meter scale) alterationpatches can create significant compositional scatter in igneous trendsfeaturing these elements, if not accounted for. The patches are gener-ally associated with regions near fluid flow, such as veins or voidspace, and could be a significant source of Y and REE to the typicallyenriched hydrothermal fluids at ridge-crest vents.

94


ACKNOWLEDGMENTS

I thank L.K Autio and J.M. Rhodes for numerous discussions anddebates concerning the petrogenesis of CRRZ basalts. Their input hasbeen invaluable. I also thank J.A. Pearce and J.L. Karsten for promptreviews that added clarity and depth to the manuscript. This studywas supported by a grant from JOI/USSAC to J.W. Sparks and G.A.Mahood.

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Date of initial receipt: 20 April 1993Date of acceptance: 13 January 1994Ms 137/140SR-021

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