5. PETROLOGY AND ISOTOPE CHARACTERISTICS (H, O, S, Sr, AND Nd) OF BASALTS FROM OCEAN DRILLING PROGRAM HOLE 504B, LEG 111, COSTA RICA RIFT 1

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1 Becker, K., Sakai, H., et al., 1989 Prceedings f the Ocean Drilling Prgram, Scientific Results, Vl. Ill 5. PETROLOGY AND ISOTOPE CHARACTERISTICS (H, O, S, Sr, AND Nd) OF BASALTS FROM OCEAN DRILLING PROGRAM HOLE 504B, LEG 111, COSTA RICA RIFT 1 Minru Kusakabe, 2 Tsugi Shibata, 3 Masahir Yamamt, 3 Shing Mayeda, 2 Hir Kagami, 2 Hirji Hnma, 2 Harue Masuda, 4,5 and Hitshi Sakai 4 ABSTRACT Petrgraphy and istpe gechemical characteristics f H, O, S, Sr, and Nd have been described fr basalts recvered frm Hle 504B during Leg 111 f the Ocean Drilling Prgram. The petrgraphic and chemical features f the recvered basalts are similar t thse btained previusly (DSDP Legs 9, 70, and 83); they can be divided int phyric (plagiclase-rich) and aphyric (Plagiclase- and clinpyrxene-rich) basalts and shw lw abundances f Ti0 2, Na 2 0, K 2 0, and Sr. This indicates that the basalts belng t Grup D, cmprising the majrity f the upper sectin f the Hle 504B. The dipside-rich nature f the clinpyrxene phencrysts and Ca-rich nature f the Plagiclase phencrysts are als cnsistent with the preceding statement. The Sr and Nd istpe systematics (average 87 Sr/ 8 Sr = ± 07 and average 143 Nd/ 144 Nd = ± 041) indicate that the magma surces are istpically hetergeneus, althugh the analyzed samples represent nly the lwermst 200-m sectin f Hle 504B. The rcks were subjected t mderate hydrthermal alteratin thrughut the sectin recvered during Leg 111. Alteratin is limited t interstices, micrfractures, and grain bundaries f the primary minerals, frming chlrite, actinlite, talc, smectite, quartz, sphene, and pyrite. In harmny with the mderate alteratin, the fllwing alteratin-sensitive parameters shw rather limited ranges f variatin: H 2 0 = 1.1 ± wt%, 5D = - 38% ± 4%, = 5.4% ± 0.3%, ttal S = 52 ± 181 ppm, and 34 S = 0.8% ± 0.3%. Based n these data, it was estimated that the hydrthermal fluids had 5D and 18 0 values nly slightly higher than thse f seawater, the water/rck ratis were as lw as 0.02-, and the temperature f alteratin was C. Sulfur exists predminantly as pyrite and in minr quantities as chalcpyrite. N primary mnsulfide was detected. This and the 5 34 S values f pyrite (5 34 S = 0.8%) suggest that primary pyrrhtite was almst cmpletely xidized t pyrite by reactin with hydrthermal fluids cntaining very little sulfate. INTRODUCTION The basalt samples recvered frm Hle 504B during Leg 111 f the Ocean Drilling Prgram (ODP) represent the deepest sectin f the ceanic crust ever drilled. Studies f these basalts will reveal the in-situ petrlgical structure and gechemical characteristics f rck units in the deep ceanic crust near the Galapags Spreading Center. Mid-ceanic ridge basalts (MORB) are currently believed t be derived frm picritic primary magmas, which are generated at pressures frm 10 t 15 kb and change their cmpsitins t ne alng the livine/plagiclase/clinpyrxene liquidus bundary at shallw depths during ascent (e.g., Shibata and Thmpsn, 198). The Hle 504B basalt samples recvered during Leg 111, tgether with thse frm the previus Deep Sea Drilling Prject (DSDP) Legs 9, 70, and 83, culd give further insight int the magmatic prcesses at a typical diverging plate bundary. Knwledge f the depth f seawater penetratin int the ceanic crust and the physicchemical cnditins under which the ceanic crust is altered by interactin with the penetrating seawater frms a framewrk fr ur understanding f the bulk gechemistry f the ceanic crust subducting at cnverging plate bundaries. The rcks recvered frm Hle 504B during DSDP 1 Becker, K., Sakai, H., et al., Prc. ODP, Sci. Results, 111: Cllege Statin, TX (Ocean Drilling Prgram). 2 Institute fr Study f the Earth's Interir, Okayama University, Misasa, Tttri-ken 82-02, Japan. 3 Department f Earth Sciences, Okayama University, Tsushima Naka, Okayama 700, Japan. 4 Ocean Research Institute, The University f Tky, Minamidai, Nakan-ku, Tky 14, Japan. 5 Present address: Department f Gesciences, Osaka City University, Sugimt-ch, Sumiyshi-ku, Osaka 558, Japan. Legs 9, 70, and 83, t a maximum depth f 1350 m belw seaflr (mbsf), are variusly altered by the circulating seawater at lw t hydrthermal temperatures (Alt et al., 198a, 198b; Barrett and Friedrichsen, 1982; Friedrichsen, 1985; Kawahata and Furuta, 1985; Kawahata et al., 1987). It is thus a matter f interest t knw t what depth and t what extent alteratin persists in the basalts drilled frm Hle 504B during Leg 111. Such alteratin is best understd by the mineralgical, chemical, and istpic variatins f the rcks. In the present paper, we reprt the preliminary results f ur study n the petrgraphy and whle-rck variatins f hydrgen, xygen, sulfur, strntium, and nedymium istpe ratis f the Leg 111 massive basalts that are free frm visible veins and fractures. ANALYTICAL METHODS Sample Selectin Sub-bttm depths f the Hle 504B rcks recvered during Leg 111 range frm t mbsf. Ten representative specimens were selected at an even distributin thrughut the cred interval. The samples are massive and devid f apparent veins r fractures in hand sample. Each f the 10 specimens was trimmed fr any dirt, washed with distilled water, and dried. A thin sectin f each specimen was prepared fr petrgraphic and electrn micrprbe analyses. The remaining prtin f each specimen was then pulverized fr chemical and istpic analyses, which were perfrmed n a whle-rck basis. Whle-Rck Chemistry The pwdered samples were decmpsed by a mixture f HN0 3, HC10 4, and HF in sealed tefln bmbs. After decmpsitin, Na and K cntents were measured by atmic absrptin spectrphtmetry. Ca, Mg, Al, Ti, Fe (as ttal Fe), Mn, and ther minr elements were determined with inductively cupled plasma emissin spectrphtmetry. Silica cntent was determined clrimetrically with the mlybdenum yellw methd after Na 2 C0 3 fusin n a separate aliqut f the sample. 47

2 M. KUSAKABE ET AL. Mineral Analysis Mineral analysis was made using a three-channel JEOL X-ray micrprbe analyzer at an accelerating vltage f 15 kv and a sample current f fia. The crrectin prcedure was that f Bence and Albee (198) with the alpha factrs f Nakamura and Kushir (1970). Hydrgen, Oxygen, and Sulfur Istpe Analyses Hydrgen was quantitatively extracted as H 2 0 frm a sample by inductive heating t 1300 C in vacuum after absrbed water was remved at 200 C fr at least 2 hr (Suzuki and Epstein, 197). The extracted H 2 0 was quantitatively cnverted t H 2 ver heated uranium, which was manmetrically measured t give the H 2 0 cntent and analyzed with a mass spectrmeter t determine the D/H rati. Oxygen was extracted frm samples with the BrF 5 methd (Claytn and Mayeda, 193) and cnverted t C0 2 fr istpic measurement. Sulfur was extracted by decmpsing the samples in vacuum with a strng phsphric acid slutin cntaining Sn 2+ ins (Ueda and Sakai, 1983). With this methd, sulfur in sulfide and sulfate in the samples are recvered as H 2 S and S0 2, respectively. The H 2 S was cnverted t S0 2 with CuO. Sulfur cntents were wrked ut frm manmetric determinatin f the S0 2 gases. The sulfur istpe measurements were made nly fr the sulfide sulfur because f the small sulfate sulfur cntent f the samples. Istpic results are expressed in a cnventinal 5-ntatin with respect t the V-SMOW standard fr hydrgen and xygen and t the Canyn Diabl trilite fr sulfur. Analytical accuracies are ±% fr <5 18 0, ±% 0 fr <5 34 S, and ± 1.0% fr 5D. We btained = 9.5% fr the NBS-28 quartz reference sample and 5D = -58.8% fr the NBS-30 bitite reference sample. Strntium and Nedymium Istpe Analyses Rck pwders were disslved in a mixture f 40% HF, 5% HN0 3, and 30% HC1 in a sealed tefln bmb. Separatin chemistry and mass spectrmetry were fllwed by the prcedures described in Kagami et al. (1982, 1987). The 87 Sr/ 8 Sr and 143 Nd/ 144 Nd ratis were nrmalized using 8 Sr/ 88 Sr = 194 and 14 Nd/ 144 Nd = , respectively. The 87 Sr/ 8 Sr ratis f the reference samples NBS-987 and JB-1A are ± 017 (2 sigma, 5 analyses) and ± 011 (2 sigma, single analysis), respectively. The 143 Nd/ 144 Nd ratis f the reference samples BCR-1, JB-1A, and JB-1 are ± 005 (18 analyses), ± 020 (single analysis), and ± 015 (single analysis), respectively (Kagami et al., 1987). Mean relative experimental errrs at the 2 sigma level are 3% and 4% fr Sr and Nd istpic ratis, respectively. Values f e Sr and e Nd were reprted using the bulk Earth parameters 87 Sr/ 8 Sr present = and 143 Nd/ 144 Nd present = PETROGRAPHY AND MINERAL CHEMISTRY The Leg 111 Shipbard Scientific Party (1988) indicated that the lithlgic characters f basalts recvered frm Hle 504B are essentially the same as thse described frm the lwermst sectin f the hle cred during Leg 83, and hence, the basalts recvered represent the cntinuatin f the sheeted dike zne first recgnized during Leg 83 (Andersn et al., 1982). The Leg 111 basalts ccur predminantly as massive units. In additin, a small number f dikes were als identified by the presence f the chilled intrusive cntacts. The exact mde f emplacement is nt knwn fr the basalts ccurring as massive units, althugh several lines f evidence strngly supprt their rigin as the interirs f large dikes (Adamsn, 1985). The basalts f this study can be divided int tw petrgraphic grups: aphyric and phyric basalts. Fr cnsistency, we adpted the classificatin scheme fr basalts emplyed by the Leg 83 petrlgists in the fllwing petrgraphic descriptin (Kemptn et al., 1985). The aphyric basalts (Samples B-147R-1, cm [Piece 8C], and B-154R-1, cm [Piece A]) are medium grained (-2 mm), cntain less than 1% crystals that qualify as phencrysts, and are characterized by a subphitic t intersertal texture. Euhedral t subhedral Plagiclase and subhedral t anhedral, equant clinpyrxene are the main primary cnstituents, frming mre than 80% f these samples (Table 1). Slightly larger clinpyrxene crystals tend t enclse subhedral Plagiclase lathes subphitically at their marginal znes. In additin t these tw majr minerals, the samples cntain minr amunts f livine, Fe-Ti xides, and quartz as primary cnstituents. The livine is, hwever, cmpletely altered t ne r mre phyllsilicates and ccurs nly as pseudmrphs that preserve euhedral t subhedral livine mrphlgies. In general, quartz ccurs interstitially between Plagiclase lathes and clinpyrxene crystals, and it frmed in clse assciatin with r ccurs in vermicular intergrwth with sdic Plagiclase. The sdic Plagiclase and quartz are heavily charged with hairlike micrlites (prbably actinlite) and Fe-Ti xides. Textural relatins suggest that the quartz and sdic Plagiclase prbably frmed tgether with hairlike micrlites and Fe-Ti xides thrugh late-stage deuteric prcesses, but at present we cannt cmpletely rule ut the pssibility that they are the prducts f lw-temperature seawater alteratin. The rest f the basalt samples chsen fr ur study are sparsely r mderately phyric (<5% phencrysts) and cntain phencryst assemblages cmpsed f livine, Plagiclase, and/ r clinpyrxene, althugh the livine phencrysts are ttally altered t ne r mre phyllsilicates (see Table 1). A few equant, brwn spinel grains (0.05 mm in diameter) are nted as inclusins in Plagiclase phencrysts in Samples B-143R-1, 4-48 cm (Piece 9), and B-153R-1, 4- cm (Piece 2). Plagiclase is the mst abundant, ubiquitus phencryst phase in the physic basalts, frming up t 3% f the rck in this mde. The Plagiclase phencrysts r micrphencrysts are typically subhedral, with the lng axis ranging frm 1 t 3 mm. Large Plagiclase crystals rarely have cavities filled with cryptcrystalline brwnish material r devitrified melt. Clinpyrxene ccurs as a phencryst phase in all but tw f the eight phyric basalts examined. The clinpyrxene phencrysts frm up t 2% f the rck, with the grain size ranging frm less than 0.5 t 5 mm alng the c-axis. Sme f the clinpyrxene phencrysts subphitically embay Plagiclase lathes that are similar in size and mrphlgy t thse ccurring in the grundmass. In mst cases, these phencryst r micrphencryst phases, including pseudmrphs after livine, ccur as single crystals, but glmercrystic aggregates were cmmnly fund in samples with a cryptcrystalline r fine-grained grundmass. The glmercrystic clumps usually cnsist f several Plagiclase crystals cmmnly clustering tgether with livine and/r clinpyrxene. Grundmass textures fr phyric basalts with a mediumgrained grundmass (Samples B-145R-1, cm [Piece 8A], B-148R-1, cm [Piece A], B-153R-1, 4- cm [Piece 2], and B-159R-1, 4-48 cm [Piece 8]) are generally similar t thse described fr the aphyric basalts, and need nt be repeated here. Samples B-10R-1, 1-17 cm (Piece 3), and B-15R-1, 2-27 cm (Piece 3B), are characterized by a fine-grained grundmass with an intergranular t intersertal texture. In these tw samples, the grundmass clinpyrxene ccurs as anhedral, equant crystals in intergranular r very rarely subphitic relatinships with interlcking subhedral Plagiclase lathes. We als cmmnly bserved interstitial pckets cmpsed f vermicular sdic Plagiclase and quartz bth heavily charged with tiny paque grains and hairlike micrlites. Neither fresh livines nr livine pseudmrphs were bserved in the grundmass. Grundmass textures fr Samples B- 143R-1, 4-48 cm (Piece 9), and B-19R-1, cm (Piece 22), are partly subvarilitic with fine-grained, sheaf r plumse clinpyrxenes and slender Plagiclase prisms. Skeletal crystals that typically develp in the glassy, chilled selvage f abyssal basalt pillws were nt seen in these samples. Mdal analyses indicate that the Leg 111 basalt samples are mderately t extensively altered, with mdal prprtins f secndary minerals ranging frm 7% t 3% f the rck (Table 1). 48

3 PETROLOGY AND ISOTOPE CHARACTERISTICS OF BASALTS, HOLE 504B Table 1. Mdal cmpsitins f basalts frm Hle 504B, Leg 111. Depth (mbsf) Sample n. 143R-1, 4-48 cm (Piece 9) R-1, cm (Piece 8A) R-1, cm (Piece 8C) R-1, cm (Piece A) R-1, 4- cm (Piece 2) R-1, cm (Piece A) R-1, 4-48 cm (Piece 8) R-1, 1-17 cm (Piece 3) R-1, 2-27 cm (Piece 3B) R-1, cm (Piece 22) a Rck type OP O A P A O O Clinpyrxene Plagiclase Quartz Spinel Fe-Ti xide tr tr _ Ttal fresh Clays Chlrite Talc Actinlite Quartz Sphene Sulfides Ttal altered Ttal Nte: tr = trace. a OP = livine-plagiclase-phyric basalt; O = livine-plagiclase-clinpyrxene-phyric basalt; = plagiclase-clinpyrxene-phyric basalt; P = plagiclase-phyric basalt; A = aphyric basalt. The secndary minerals bserved under the micrscpe include chlrite, actinlite, talc, smectite, quartz, sphene, and sulfide minerals (mstly pyrite). Neither zelite grup minerals nr albite were psitively identified in ur examinatin f the samples. Epidte and prehnite are nt present, either. In mst cases, alteratin apparently tk place in parts f the interstitial finegrained r cryptcrystalline materials and alng micrfractures, cleavages, and grain bundaries f primary minerals. In the samples with a medium-grained grundmass, the secndary minerals are thus generally cnfined t brwnish r dark green clts, with irregular utlines and up t 0.5 mm in diameter in spradic distributin thrughut the samples. On the ther hand, alteratin is generally slight in the remaining part f the samples, where primary minerals are cnspicuusly fresh. The brwnish green clts are usually cmpsed f brwnish green smectite and chlrite with lesser amunt f actinlite. It is ntewrthy that the clinpyrxene appears t be mre susceptible t alteratin prcesses than the Plagiclase; thus, clinpyrxene crystals cmmnly are partly replaced by sheaflike bundles f actinlite, but in cntrast, Plagiclase alteratin is restricted nly t the frmatin f chlrite and/r smectite alng micrfractures and cracks. As nted previusly, livine is cmpletely altered t ne r mre phyllsilicates. The pseudmrphs after livine are generally cmpsed f either (1) brwn smectite, (2) brwn smectite + talc + magnetite, r (3) talc + magnetite. Sme f the pseudmrphs shw znal structures with brwn smectite at the center and talc plus magnetite in the marginal zne, and vice versa. Representative micrprbe analyses f clinpyrxenes are listed in Table 2, and all the pyrxene analyses btained are pltted in the cnventinal pyrxene quadrilateral (Fig. 1). N Capr pyrxene was bserved in any f the samples analyzed. The pyrxenes are mstly augites, and there is an verall, wide variatin in cmpsitin. The cres f pyrxene phencrysts, hwever, shw a rather limited range f cmpsitin and are pltted in the regin f endipside in the pyrxene quadrilateral (Fig. 1A), which prbably implies that the samples were derived frm magmas with similar, if nt the same, FeO/MgO ratis. These phencrystic pyrxenes are strngly zned; e.g., a single pyrxene phencryst shws an increase in FeO frm 5.03 wt% at the cre t wt% at the rim, accmpanied by decreases in MgO and CaO. This irn increase in the cmpsitinal znatin is als accmpanied by a slight enrichment in titanium and depletin in aluminum. Such trends in the cmpsitinal znatin were als, thugh less extensively, bserved in the grundmass pyrxenes. They d nt exhibit a nticeable increase in Ti plus Al abundance, hwever, in cmparisn with the phencrystic pyrxenes, which is a feature knwn frm many ther grundmass clinpyrxenes frm abyssal basalts (e.g., Dungan and Rhdes, 1978). The Ti plus Al abundances bserved in the Hle 504B phencrystic, as well as the grundmass, pyrxenes fall in the range f thse fr gabbrs dredged frm the cean flr (e.g., Shibata et al., 1979; Elthn, 1987). The chemical characteristics f the Leg 111 clinpyrxenes are generally similar t thse f the clinpyrxenes btained during Leg 83 frm the same hle (Kemptn et al., 1985). Representative micrprbe analyses f Plagiclase are listed in Table 3. The anrthite cntents f Plagiclase bserved as phencrysts, micrphencrysts, and grundmass grains f the Leg 111 samples frm Hle 504B vary greatly, ranging frm An 90 t An 49 (Fig. 2). Plagiclase phencrysts cmmnly have a relatively hmgeneus, brad cre f An 90 _ 80, with an scillatry zned, mre sdic rim in which we bserved a few thin znes with cmpsitins as calcic as r mre calcic than that f the cre. Grundmass plagiclases are generally nrmally zned. Kemptn et al. (1985) nted that Plagiclase cmpsitins frm the Leg 83 recvery f Hle 504B are distinctive in their relatively high An cntent (up t 90 ral%). Natland et al. (1983) reprted anrthite cntents f as high as An 94 fr Plagiclase frm the upper prtin f Hle 504B. The Ca-rich nature f plagiclases frm the Hle 504B basalts cred during the previus DSDP legs apparently persists in the Leg 111 plagiclases. Figure 2 shws the relatinship between the An cntent and the Mg/(Mg + Fe) rati in Leg 111 phencryst and grundmass plagiclases. There is a regular, psitive crrelatin between An 49

4 M. KUSAKABE ET AL. Table 2. Micrprbe analyses f clinpyrxene frm Hle 504B, Leg 111. Analysis n. a 143R-1, 4-48 cm (Piece;9) , cm (Piece 8A) R-1, 4- cm (Piece 2) 154R-1, cm (Piece A) R-1, 1-17 cm (Piece 3) 9 19R-1, cm (Piece 22) 10 Si0 2 Ti0 2 A Cr b FeO* MnO NiO MgO CaO Na 2 0 Ttal Catins calculated n the basis f i axygens Si Al Ti Cr Mg Ni Fe 2 + Mn Ca Na Ttal Mg Fe Ca a 1 = cre f grundmass grain; 2 = rim f grain in analysis n. 1; 3 = cre f phencryst grain with subphitic rim; 4 = rim f grain in analysis n. 3; 5 = cre f subphitic grundmass grain; = rim f grain in analysis n. 5; 7 = cre f subphitic grain; 8 = rim f grain in analysis n. 7; 9 = cre f grundmass grain; 10 = cre f phencryst grain. b Ttal irn as FeO. Figure 1. Pyrxene quadrilateral shwing plts f the mlecular prprtins f clinpyrxenes frm the Hle 504B basalts recvered during Leg 111. A. Clinpyrxene phencryst cmpsitins. Slid triangles indicate cmpsitins f the phencryst cre; slid circles indicate thse f the rim. Analyses btained frm the same grain are cnnected by a tie line. B. Cmpsitins f phencryst, micr phencryst, and grundmass clinpyrxenes. cntent and Mg/(Mg + Fe) fr plagiclases with less than An 75. A similar relatinship was bserved fr the Leg 83 plagiclases (Kemptn et al., 1985). WHOLE-ROCK CHEMISTRY The basalts btained frm Hle 504B during the previus DSDP Legs 9, 70, and 83 were classified int three distinct chemical grups: Grups M, D, and T (Auti and Rhdes, 1983; Kemptn et al., 1985). The Grup M basalts, including thse f Kemptn et al.'s (1985) Grup M', are similar in majr and trace element abundances t thse called Type I by Bryan et al. (197) and are cmparatively rare. They exhibit mderate FeO*/ MgO ratis (1.10) and mderate abundances f Ti0 2 ( wt%), Na 2 0 ( wt%), Y (30-31 ppm), Zr ( ppm), and Nb (2-3 ppm). Auti and Rhdes (1983) and Kemptn et al. (1985) indicated that the Grup M basalts are multiply saturated and that substantial fractinatin, invlving livine, Plagiclase, and spinel, must have taken place. The majrity f the Hle 504B basalts studied previusly belngs t Grup D (Table 4). These basalts, including thse f Kemptn et al.'s (1985) 50

5 PETROLOGY AND ISOTOPE CHARACTERISTICS OF BASALTS, HOLE 504B Table 3. Micrprbe analyses f Plagiclase frm Hle 504B, Leg 111. Analysis n. a 143R-1, 4-48 cm (Piece 9) R-1, cm (Piece 8A) 3 147R-1, cm (Piece 8C) R-1, cm (Piece A) 153R-1, 4- cm (Piece 2) 7 154R-1, cm (Piece A) R-1, 2-27 cm (Piece 3B) Si0 2 Ti0 2 AI2O3 b FeO* MnO MgO CaO Na z O K 2 0 Ttal Catins calculated n the basis f 8 xygens Si Al Ti Mg Fe 2 + Mn Ca Na K Ttal Ca Na K Mg/(Mg + Fe) = cre f euhedral phencryst; 2 = rim f grain in analysis n. 1; 3 = cre f grundmass Plagiclase; 4 = cre f subhedral phencryst; 5 = rim f grain in analysis n. 4; = cre f grundmass Plagiclase; 7 = cre f a glmercryst; 8 = cre f Plagiclase lath frm aphyric basalt; 9 = rim f grain in analysis n. 8; 10 = cre f grundmass Plagiclase; 11 = rim f grundmass Plagiclase. b Ttal irn as FeO. Grup D', have lw abundances f Ti0 2 ( wt%), Na 2 0 ( wt%), and K 2 0 (<-0.5 wt%), with extremely lw abundances f Zr (34-0 ppm), Sr (47-94 ppm), Y (1-28 ppm), and Nb (< ppm). The Grup T basalts, thugh minr in amunts, are similar t the Grup D basalts in terms f abundances f mderately incmpatible elements (e.g., Ti0 2, Na 2 0, Y, and Zr), but are enriched in highly incmpatible elements such as La and Nb relative t the ther magmaphile elements. Majr and trace element abundances f the Hle 504B basalts recvered during Leg 111 (Table 4) exhibit chemical characteristics similar t thse f the Grup D basalts described in the preceding, althugh these data alne d nt permit us t distinguish between Grup D and Grup T unambiguusly. Trace element data btained by the Leg 111 shipbard scientists, hwever, indicate that 2 analyzed Leg 111 samples are depleted in Nb (<2 ppm) and Zr (<0 ppm) and, hence, belng t Grup D (Shipbard Scientific Party, 1988). Thus, mst f the petrlgic inferences made fr the Grup D basalts by the previus investigatrs wuld als be applicable t ur samples. Nne f the samples listed in Table 4 are cnsidered primitive. These samples are mderately evlved, with FeO*/MgO ratis f 0.90 t 1.3, and shw a slightly scattered, limited range f distributin that defines a pssible fractinatin trend in varius variatin diagrams (e.g., as FeOVMgO increases Ti0 2 and Na 2 0 cntents crrespndingly increase whereas A and CaO cntents decrease). Majr and trace element chemistry f these samples is n dubt mdified thrugh lw-temperature alteratin prcesses. OXYGEN AND HYDROGEN ISOTOPIC VARIATIONS OF THE LEG 111 BASALTS The Hle 504B basalts have been divided int three znes accrding t the mde f alteratin (Alt et al., 198a; Kawahata and Furuta, 1985): (1) xic submarine weathering characterized by iddingsite, sapnite, and irn xide (upper pillw alteratin zne r zne I, mbsf); (2) subxic t anxic alteratin at slightly warmer temperatures (lwer pillw alteratin zne r zne II, mbsf); and (3) high-temperature hydrthermal metamrphism (transitin zne and dike sectins, mbsf). Kawahata et al. (1987) further subdivided the lwermst part int tw znes, emphasizing the transitin zne f hydrthermal alteratin under greenschist facies cnditins with extensive veining as zne III ( mbsf) and a zne f hightemperature hydrthermal alteratin in the deeper sheeted dike prtin as zne IV (> 1050 mbsf). The basalt samples analyzed in this investigatin represent the cntinuatin f zne IV. The results f xygen and hydrgen istpic analyses fr the Leg 111 basalts, tgether with their water cntent, are given in Table 5 and shwn as a functin f depth belw seaflr in Figure 3, which als shws the results given by Kawahata et al. (1987) fr the Hle 504B samples recvered during previus DSDP legs. Fresh MORB samples have very lw water cntents (H ) f t 0.4 wt%, with SD values f - 70% t - 80% (Craig and Luptn, 197; Kyser and O'Neil, 1984). The rcks frm znes I and II cntain wt% H 2 0 +, with SD values ranging frm -70% t -40%, reflecting frmatin f lw- 51

6 M. KUSAKABE ET AL PHENOCRYST cre rim.»4 \ a ml / An Figure 2. An vs. Mg/(Mg + Fe) in Plagiclase frm Leg 111 basalt samples. A. Plagiclase phencrysts. B. Phencryst, micrphencryst, and grundmass Plagiclase. temperature minerals like iddingsite, sapnite, smectite, and irn xide (Kawahata et al., 1987). In zne III, a cnspicuus peak in the H cntent f up t 5.3 wt% at abut 1000 mbsf is assciated with a cluster f high 5D values ranging frm - 34% t - 25% (average - 32%). The large variatin f the H cntent and high SD values reflect variable and extensive alteratin f pillw lavas and sheeted dikes with high permeability in zne III. The 5D values are in gd agreement with thse reprted fr submarine greenstnes (Satake and Matsuda, 1979; Stakes and O'Neil, 1982). In zne IV, where massive sheeted dikes are dminant, the water cntent decreases dwn t 1350 mbsf, but belw this depth it becmes rather cnstant (1.1 ± wt%). The 5D values tend t becme lwer tward the bttm f the hle, but still shw <5D values typical f minerals hydrthermally altered under submarine cnditins. The mst abundant hydrgen-bearing minerals in zne IV (Leg 111) are brwnish green smectite, chlrite, talc, and actmlite, all develping as livine pseudmrphs r alng micrfractures and bundaries f primary minerals. Smectite and chlrite cntain mre than 10 wt% water, whereas talc and actinlite cntain less than 5 wt%. Taking the relative abundances f these minerals int accunt (Table 1), we thus infer that the whle-rck water cntent and hydrgen istpic cmpsitin are mstly determined by thse values f smectite and chlrite. Figure 4 is a plt f the whle-rck 5D values f zne IV as a functin f a reciprcal f the water cntent. The slid line in Figure 4 is the SD vs. 1/H 2 0 relatinship when fresh MORB with <5D = -70% and H = wt% is prgressively hydrated with frmatin f chlrite having 5D = -34% and H = 13 wt%. The value f 5D chl = - 34% 0 was taken frm a vein chlrite fund in zne IV at mbsf (Sample 504B , cm) (Kawahata et al., 1987). The tw dashed lines limit a range f pssible 5D values f chlrite in equilibrium with seawater at 300 C, where the equilibrium hydrgen istpe fractinatin factr (1000 lna) between chlrite and water was taken t be -35 ± 10 (Taylr, 1974; Marum et al., 1980). Mst f the data pints are abve the "chlritizatin line." This is prbably because either the hydrthermal fluids that reacted with the rcks had 5D values slightly higher than that f seawater r alteratin minerals such as chlrite and smectite were frmed at temperatures higher than 300 C, althugh hydrgen istpe fractinatin factrs fr the chlrite-water and smectite-water at hydrthermal temperatures have nt been well defined yet. It is suggested frm Figure 4 that the 5D values f the alteratin assemblages, extraplated t H > 5 wt%, are within - 30% ± 5%, almst the same range as that bserved fr the highly altered rcks in zne III (see Fig. 3). This leads t the inference that the SD values f the hydrthermal fluids respnsible fr alteratin in zne IV f the Leg 111 basalts are nt significantly different frm thse values in znes III and IV fr the Leg 83 basalts (i.e., 5D nuid = 3% ± 13%), althugh the temperatures f the hydrthermal fluids may have been smewhat higher than thse estimated frm fluid inclusin measurements f vein quartz (323 C at mbsf, 315 C at mbsf, and 287 C at 1200 mbsf; Kawahata et al., 1987). The whle-rck values thrughut Hle 504B (Fig. 3) shw that the rcks in znes I and II are mre enriched in ls O than typical fresh MORB ( = 5.8% ± 0.3%; Taylr, 198; Muehlenbachs and Claytn, 1972) and that the Leg 83 rcks in znes III and IV are hydrthermally highly altered with variable < values (4.4% 0-7.0%). The values f the zne IV rcks recvered during Leg 111 range frm 4.9% t 5.7% and are less variable in cmparisn with thse frm zne III, with an average (5.4%) nly slightly lwer than the value fr the fresh 52

7 Table 4. Chemical cmpsitin f basalt frm Hle 504B, Leg 111, cmpared with Legs 9, 70, 83 and the Kane Fracture Zne. Majr elements (wt%) Legs 9 143R-1, 145R-1, 147R-1, 148R-1, 153R-1, 154R-1, 159R-1, 10R-1, 15R-1, 19R-1, and 70: Leg 83: Kane Fracture 4-48 cm cm cm cm 4- cm cm 4-48 cm 1-17 cm 2-27 cm cm JB-lA a JB-lA b Leg 111 Grup D Grup D' Zne, Mid-Atlantic (Piece 9) (Piece 8A) (Piece 8C) (Piece A) (Piece 2) (Piece A) (Piece 8) (Piece 3) (Piece 3B) (Piece 22) (R) (GSJ) basalts c basalts' 1 basalts e Ridge f Si Ti A g FeO* MnO MgO CaO Na K Ttal Trace elements (ppm) Sr Cr C Ni Cu h (80) 8 FeOVMgO Nte: Dash indicates n analysis. a GSJ standard analyzed tgether with the Leg 111 samples. Recmmended values fr the GSJ standards JB-1 and JB-1 A. c Average values f 10 samples f Leg 111 basalts (this study). Average values f 1 samples (Auti and Rhdes, 1983). e Average values f 5 samples (Kemptn et al., 1985). Average values f 52 samples (Bryan et al., 1981). 8 Ttal irn as FeO. h Average value withut Sample B-19R-1, cm (Piece 22). (JB-1)

8 M. KUSAKABE ET AL. Table 5. Hydrgen, xygen, and sulfur istpe analyses f basalts frm Hle 504B, Leg 111. Sample n. Cre, sectin, interval (cm), piece Rck type 3 Degree f alteratin (%) H (wt%) SD (%) (%) Sulfide as sulfur (ppm) Sulfate as sulfur (ppm) a 34 s f sulfide sulfur (%) Average 1 sigma 143R-1, 4-48, 9 145R-1, 50-52, 8A 147R-1, 73-77, 8C 148R-1, 43-45, A 153R-1, 4-, 2 154R-1, , A 159R-1, 4-48, 8 10R-1, 1-17, 3 15R , 3B 19R-1, , 22 OP O A P A O O OP = livine-plagiclase-phyric basalt; O = livine-plagiclase-clinpyrxene-phyric basalt; = plagiclase-clinpyrxene-phyric basalt; P = plagiclase-phyric basalt; A = aphyric basalt. O O Zne 1 -- O -- - O Zne II -.Q E a 0) Q 1200 " " -fc O - r, & u,s- O O O Zne III Zne IV - - " 9j > < i i i I w % * H (%) 8 D (%) 18 0(%) Figure 3. Water cntent, SD, and S 18 0 values f Leg 111 basalts (slid circles) pltted as a functin f depth. The data fr the upper rcks (pen circles) are frm Kawahata et al. (19?7). Zne I = xic weathering; zne II = subxic t anxic alteratin; zne III = extensive hydrthermal alteratin in the pillw/dike transitin; and zne IV = hydrthermal alteratin f sheeted dikes (Kawahata et al., 1987). MORB. The values in Table 5 and a general istpic trend depicted in Figure 3 are in gd agreement with thse given by Alt et al. (198b), Friedrichsen (1985), and Barrett and Friedrichsen (1982) fr the Hle 504B basalts btained in the previus legs. Figure 5 shws the variatin in f zne IV Leg 111 samples as a functin f percentage alteratin and the rck type. The decrease in f the whle rcks is apparently related t the extent f alteratin, but nt t the textural rck types (Table 5). Assuming that the alteratin minerals were in xygen istpe equilibrium with hydrthermal fluids at 300 C and that the unaltered part retained the riginal value f < MORB = 5.8%, we can calculate the whle-rck values in terms f the relative prprtins f alteratin minerals t the primary minerals and mineral-water fractinatin factrs. The fractinatin factrs are nt well knwn fr the minerals fund in the alteratin assemblage, s that nly a rugh apprximatin is made thrugh regressin equatins cmpiled by Cle et al. (1987) fr smectite, chlrite, talc, actinlite, and sphene. The calculated whle-rck < values are in reasnable agreement with the measured values when the <5 18 O nuid is taken t be between 0 and 3%. The relatively lw 18 0 values f Samples B-143R-1, 4-48 cm (Piece 9) (5.0%), B-10R-1, 1-17 cm (Piece 3) (5.0% 0 ), and B-19R-1, cm (Piece 22) (4.9%) may indicate the presence f hydrthermal fluids with temperatures as high as 400 C. The values f the altered prtin f the samples can be estimated frm a material-balance calculatin based n the assumptins that the fresh prtin retains the riginal value f = 5.8% and that the percentage f alteratin determined 54

9 PETROLOGY AND ISOTOPE CHARACTERISTICS OF BASALTS, HOLE 504B Q CO /H 2 0 Figure 4. The whle-rck SD values f zne IV rcks (this study; Kawahata et al., 1987) pltted against the reciprcal f water cntent. A straight line represents the relatinship f fresh MORB (5D = - 70% and H 2 0 = wt%) prgressively hydrated with the frmatin f chlrite (5D = -34% and H 2 0 = 13 wt%; see text). The tw dashed lines limit a range f pssible 5D values f chlrite in equilibrium with seawater at 300 C. n a micrscpic scale apprximates that actually ccurring n a bulk sample scale. The calculated values range frm 0.7% t 5.0% 0, with an average f 3.0% 0. The S 18 O alt values tend t be higher with increasing predminance f chlrite and talc. This is smewhat strange, because their mineral-water xygen istpe fractinatin factrs are rather small in the temperature interval between 300 and 400 C, which indicates that the fluid was mre enriched in ls O during alteratin in the zne IV Leg 111 rcks than estimated either lcally r reginally. This suggestin is cnsistent with the high < values (5%-7%) f vein minerals (chlrite and mixed-layer clays) frm the same zne (H. Masuda, unpubl. data). The xygen istpe cmpsitin f the rcks that have interacted with hydrthermal fluids can be related t water/rck ratis, the 18 0 values f the interacting fluids, and the xygen istpe fractinatin factrs between the rcks and fluids (Taylr, 1977). On the assumptin that the values f fluids and unaltered basalts are 0 and + 5.8%, respectively, and that a basalt-water fractinatin is apprximated by that given in Cle et al. (1987), the maximum <5 18 O alt value f 5% may be attained when the water/rck rati is higher than (in weight) at 300 C. Water/rck ratis as great as 2 and temperatures as high as 420 C are required fr the lwest 5 18 O alt value f 1.5% (excluding an exceptinally lw value f 0.7%). Kawahata et al. (1987) estimated a water/rck rati f 1. fr zne III. This value, hwever, shuld be taken as the maximum, because the rcks frm zne III exhibit very high degrees f alteratin. The water/ rck ratis described here refer t the weight ratis f fluids t "cmpletely reacted" rcks. The extent t which the rcks examined here are altered is abut 15% n the average. Therefre, the "actual" water/rck ratis shuld be smaller than thse previusly stated by a factr f up t 10, namely in the range f 0.02 t. SULFUR CONCENTRATIONS AND ISOTOPIC VARIATIONS Micrscpic bservatins f re minerals reveal that the sulfide mineralgy is essentially the same fr all f the samples. Sulfide minerals are mstly pyrite with trace amunts f chalcpyrite. Pyrite ccurs as irregular grains up t 0.3 mm in size, dispersed in the mre extensively altered part f the basalts. The pyrite grains are usually assciated with small particles f chalcpyrite (<0.02 mm in size) at r near the rims f the grains. Chalcpyrite ccurs als as discrete tiny grains (< mm in size) in silicate matrices. Only a single ccurrence f pyrrhtite was fund as a tiny inclusin in a pyrite grain frm Sample 111- O P %A A* #O O OP % Alteratin Figure 5. The whle-rck vs. percentage f alteratin as determined by mdal analysis fr Leg 111 basalts (see Table 1 fr rck type abbreviatins). 55

10 M. KUSAKABE ET AL. 504B-143R-1, 4-48 cm (Piece 9). N ther sulfide minerals were detected. The measured cncentratins and istpe cmpsitins f sulfur are given in Table 5. The cncentratins f sulfide sulfur range frm 258 t 900 ppm. The cncentratins f sulfate sulfur are very lw, ranging frm 1 t 14 ppm, frming nly 0.4% t 1.7% f the ttal sulfur. The S 34 S values f sulfide sulfur exhibit a very narrw spread, ranging frm + % t + 1.4%, with an average f +0.7%. The sulfur cntent f the basalts frm the anxic alteratin zne (zne II) is highly variable (20 t 2490 ppm) (Belyi et al., 1983; Hubberten, 1983). In cmparisn with the basalts in zne II, the sulfur cncentratin f the Leg 111 basalts is nt as variable. The average fr the Leg 111 samples is 52 ppm, clse t the average value f 50 ppm calculated by Belyi et al. (1983) fr the unaltered basalts frm the anxic alteratin zne. Althugh the sulfur cntents f the Leg 111 samples are similar t thse f the zne II samples, there is a significant difference between them in terms f the sulfur species present. In ur samples, sulfur cmes almst exclusively frm pyrite, whereas chemical analyses f shallw samples indicated the presence f sulfur as mnsulfide, pyrite, and sulfate as well (Belyi et al., 1983; Hubberten, 1983). Distler et al. (1983) described the ccurrence f primary sulfides frm zne II as glbules cnsisting f single-phase sulfide slid slutins (irn-nickel and irn-cpper) and as multiphase sulfide aggregates cmmnly cnsisting f pyrrhtite, pentlandite, chalcpyrite, cubanite, and magnetite. The ccurrence f anhydrite in veins was als reprted frm varius hrizns f Hle 504B (Alt et al., 1983, 1985; H. Masuda, pers. cmm., 1988). Bth the mde f ccurrence and sulfur istpe ratis f the anhydrite suggest that it is secndary in rigin (Alt et al., 1983, 1985). A cnsiderable fractin (>5%) f sulfur exists as sulfate in mst fresh cean flr basalts (Sakai et al., 1984). Thus, the almst cmplete absence f sulfate sulfur as well as f mnsulfide sulfur in the Leg 111 samples indicates that the primary sulfur species in the basalts were almst cmpletely lst during the hydrthermal alteratin. Figure shws a plt f the ttal sulfur cncentratin as a functin f H cntent. Althugh there is a large scatter f data in Figure, the ttal sulfur cncentratin tends t decrease as alteratin prceeds, and the ttal sulfur cncentratin extraplated t H = 0.4 wt% is > 1000 ppm, which is typical fr submarine basalts frm the Galapags Ridge (Sakai et al., 1984). Sulfate sulfur may have either been disslved away int the hydrthermal fluids r riginally been very small in cncentratin. Mnsulfide sulfur wuld have been xidized t frm pyrite r partially xidized t frm sulfate ins. The istpic cmpsitins f sulfide sulfur studied in this investigatin shw a very narrw variatin. The average value f + 0.7% fr the Leg 111 samples is very clse t the 5 34 S values f fresh basalt glasses frm the nearby Galapags Ridge (+ 0.4% and + 0.5%) and ther fresh cean flr basalt glasses ( + 0.3% ± 0.5%) (Sakai et al., 1984). In cntrast, the sulfides frm zne II have highly variable 5 34 S values ranging frm % t % 0 (Belyi et al., 1983; Hubberten, 1983). T explain the large variatin bserved, decmpsitin by xidatin f primary sulfides t frm sulfite r thisulfate ins was pstulated. The sulfite and/r thisulfate ins thus frmed were then disprprtinated t pyrite and sulfate with a large istpe fractinatin (Field et al., 197; Belyi et al., 1983). Hnnrez et al. (1985) btained 5 34 S values f +2.9% t + 5.0% fr pyrite frm the stckwrk sulfide veins drilled at mbsf in Hle 504B n Leg 83. They interpreted the increase in 5 34 S values relative t cean flr basalts as incrpratin f sulfide derived frm the reductin f seawater sulfate. The 5 34 S values f pyrites studied herein apparently indicate that such a disprprtinatin accmpanied by a large istpe fractinatin is nt applicable t the Leg 111 samples. The clse similarity in <5 34 S values t thse f fresh cean flr basalts, especially when the ttal sulfur cncentratins are high (Fig. 7), suggests that the primary sulfur was cnverted t the secndary pyrite withut a large istpe fractinatin. The frmatin f pyrite frm the primary mnsulfides (mainly pyrrhtite) is an xidatin reactin. The pssible reactin that is respnsible fr the pyrrhtite-pyrite cnversin may be 3FeS + Fe H 2 0 = FeS 2 + Fe H 2 S + 2H 2 + 2H +. Lss f H 2 S wuld lwer the sulfur cntent f the altered basalts withut giving rise t a large change in the istpic cmpsitins (Ohmt and Rye, 1979). The afrementined reactin wuld prduce a mineral assemblage f pyrite-magnetite-quartz, which was a typical run prduct f the experimental fayaliteseawater interactin (Shanks et al., 1981). The fllwing pyritepyrrhtite cnversin reactin has been prpsed by Shanks H 2 0 (wt.%) Figure. Relatinship between the ttal sulfur and water cntents f Leg 111 basalts. The line is the least-squares fit Ttal S (ppm) Figure 7. Sulfur istpic ratis pltted as a functin f ttal sulfur cncentratin fr Leg 111 basalts. The line is the least-squares regressin f the data. 5

11 and Seyfried (1987), assuming that seawater-derived sulfate is the required xidizing agent: PETROLOGY AND ISOTOPE CHARACTERISTICS OF BASALTS, HOLE 504B 7FeS + 8H+ + SO^ = 4FeS 2 + 4H Fe 2+. This reactin successfully explains the slight but distinct enrichment in 34 S f the stckwrk pyrite frm Hle 504B (Hnnrez et al., 1985) as well as sulfides frm cean flr massive sulfide depsits (e.g., the East Pacific Rise at 21 NStyrt et al., 1981; Juan de Fuca RidgeShanks and Sey fried, 1987; Galapags Rift at 8 W Skirrw and Cleman, 1982). The latter reactin might be applicable als t the Leg 111 case, if the rati f sulfide-sulfur t sulfate-sulfur is clse t r larger than 7 in basaltic magmas and if bth sulfur frms are rembilized with hydrthermal fluids that are mre xidative than in the magmas. VARIATIONS IN STRONTIUM AND NEODYMIUM ISOTOPE RATIOS The strntium and nedymium istpic analyses f the Leg 111 basalts frm Hle 504B are given in Table. The variatin in the 87 Sr/ 8 Sr ratis fr the Leg 111 sectin is shwn in Figure 8 alng with previusly reprted data (Kawahata et al., 1987). In cntrast t the variatin in zne III and the uppermst part f zne IV, the 87 Sr/ 8 Sr ratis f the deeper part f zne IV (Leg 111) are unifrm and very clse t thse reprted fr fresh MORB (i.e., ± 20; Hart, 197). These data indicate that the basalts are little affected by hydrthermal alteratin, in agreement with bservatins f mineralgy, H cntent, 5D, and variatins discussed in the previus sectins. The Sr and Nd istpic ratis f basalts frm Hle 504B are pltted in Figure 9 and cmpared t thse frm the East Pacific Rise, East Pacific Rise small seamunts, and Galapags Islands (Zindler et al., 1984). The data frm the Galapags Spreading Center reprted by Verma et al. (1983) are als shwn in Figure 9. Zindler et al. (1984) ascribed the scatter f the Sr and Nd istpic ratis f rcks frm the East Pacific Rise small seamunts t mantle hetergeneity. White and Hfmann (1978) suggested that the larger scatter fr the Galapags Islands was due t mixing f tw magmas derived frm tw different mantle surces with istpically distinct features. Subsequently, White (1979) indicated that much f the scatter resulted frm weathering effects n Sr istpic ratis. Verma et al. (1983) explained the scatter f the istpic data frm the Galapags Spreading Center by mixing f the MORB surce and Galapags plume. The basalts frm Hle 504B are quite similar t thse frm the East Pacific Rise but are significantly different frm thse frm the Galapags Islands with respect t Sr and Nd istpic cmpsitins. The systematic variatins f the 87 Sr/ 8 Sr and i43 Nd/ i44 Nd ratis f the Hle 504B basalts may be interpreted t represent either hydrthermal alteratin after emplacement Table. Strntium and nedymium istpic analyses f basalts frm Hle 504B, Leg 111. Sample n Cre, sectin, interval (cm), piece 143R-1, 4-48, 9 145R-1, 50-52, 8A 147R-1, 73-77, 8C 148R-1, 43-45, A 153R-1, 4-, 2 154R-1, , A 159R-1, 4-48, 8 10R-1, 1-17, 3 15R-1, 2-27, 3B 19R-1, , Sr/ 8 Sr (2) (2) (2) (2) (2) (2) (3) (5) (2) (2) e Sr Nd/ 144 Nd (19) (27) (22) (27) (29) (21) (24) (27) (27) (18) e Nd Nte: Numbers in parentheses represent an analytical reprducibilty in terms f 2 sigma, referring t the last digits. CA Q. Q Zne I Zne II Zne III Zne IV Sr/ bb 8, Sr Figure 8. The 87 Sr/ 8 Sr ratis (slid circles) f the Leg 111 basalts pltted against depth. Open circles are data frm Kawahata et al. (1987) shwn fr cmparisn. r existence f tw magmas generated at distinct surces with different istpic ratis, that is, hetergeneity in the mantle surce. We examine these pssibilities in the fllwing. Varying degrees f hydrthermal alteratin may be invked t explain the systematic istpic variatin, because Sample B-19R-1, cm (Piece 22), which has the highest 87 Sr/ 8 Sr rati and the lwest 143 Nd/ 144 Nd rati, was subjected t intense alteratin (Table 1). Basalt-seawater interactin is knwn t raise the 87 Sr/ 8 Sr ratis f basalt, as seen in Figure 8, but the 143 Nd/ 144 Nd ratis usually remain almst unchanged because f very lw cncentratin f Nd in seawater (namely 2-3 pg/g; Piepgras and Wasserburg, 1980). Interactin with hydrthermal fluids that have reacted with pelagic clays with 87 Sr/ 8 Sr = and 143 Nd/ 144 Nd = (Gldstein and O'Nins, 1981) r thse reacted with a mantle plume at Galapags with 87 Sr/ 8 Sr = and 143 Nd/ 144 Nd = (Verma et al., 1983) might be a pssible cause fr the istpic variatins bserved in the Hle 504B basalts if the fluids had a mderately high Nd cncentratin. Hwever, Nd cncentratins ranging frm 20 t 33 pg/g with e Nd = -3 (r i43 Nd/ i44 Nd = ) were reprted fr hydrthermal fluids frm 21 N n the East Pacific Rise (Piepgras and Wasserburg, 1985). The preceding Nd cncentratins are -100 times larger than that f seawater, but still much lwer than that f basalts frm the East Pacific Rise and the East Pacific Rise small seamunts (i.e., -33 ppm; Zindler et al., 1984). If a maximum 57

12 M. KUSAKABE ET AL. 87 Sr/ 8 Sr i i I I "" \ (*^>^ \\+^C> East Pacific Rise W \ O0 0 V ^ %\ \ O /. \ v - ~~v-\ P'; v Galapags XN~ N,/ N \ Islands East Pacific Rise/ ~ ^ \ Small Seamunts "~"~-^ i ~"^^_/ J Nd i i Figure 9. Sr-Nd istpic crrelatin fr the Leg 111 basalts (slid circles) and related rcks. The regins fr the East Pacific Rise, East Pacific Rise small seamunts, and Galapags Islands are frm Zindler et al. (1984). Open circles represent basalts frm the Galapags Spreading Center (Verma et al., 1983). 'Sr +2 slubility f Nd in hydrthermal fluids is assumed t be 300 pg/g and 143 Nd/ 144 Nd is equal t , very high water/rck ratis f abut 4000 are required t change the 143 Nd/ 144 Nd rati f basalt frm (Sample B-159R-1, 4-48 cm [Piece 8]) t (Sample B-19R-1, cm [Piece 22]). This value cntradicts sharply with the lw water/rck ratis f based n the xygen istpic balance. Therefre, hydrthermal alteratin is highly unlikely as a cause f the bserved Nd and Sr istpic variatins in the Hle 504B basalts. The Nd istpic data f the Leg 111 basalts frm Hle 504B indicate that the 143 Nd/ 144 Nd ratis can be divided int tw grups, with grup averages f and , respectively (Fig. 9). There is n intermediate grup. The ratis f the first grup are clse t the highest nes f the Galapags Spreading Center basalts, whereas thse f the latter grup agree with the highest values f rcks frm the East Pacific Rise and the East Pacific Rise small seamunts. This suggests that each grup represents a distinct magma derived frm a different mantle surce withut mixing. The variatins in 87 Sr/ 8 Sr ratis within each grup and thse between the tw grups may als indicate a small-scale mantle hetergeneity. SUMMARY AND CONCLUSIONS The basalts recvered frm Hle 504B during Leg 111 ccur predminantly as the massive units that prbably represent the interirs f large dikes. The basalt samples chsen fr ur study can be divided int aphyric and phyric basalts. In the aphyric basalts Plagiclase and clinpyrxene are the main primary cnstituents (>80 vl%), and livine, Fe-Ti xide, and quartz are minr cnstituents. Phencrysts f the phyric basalts cnsist f abundant Plagiclase with lesser amunts f clinpyrxene and livine, althugh the livine is cmpletely altered t phyllsilicates. Plagiclases and clinpyrxenes remain rather fresh. The chemical features are essentially similar t thse f the Grup D basalts that cmprise the majrity f rcks recvered during the previus DSDP Legs 9, 70, and 83. The Leg 111 basalts are characterized by lw abundances f Ti0 2, Na 2 0, K 2 0, and sme minr elements such as Zr, Sr, Y, and Nb. The phencrysts clinpyrxenes are strngly zned and shw a wide range f cmpsitinal znatin, but the cre cmpsitins are limited in the range f endipside, implying that the Leg 111 basalts were derived frm magmas with similar FeOVMgO ratis. Plagiclase crystals als shw a wide chemical variatin frm An 90 t An 49, but the cres, in general, are rich in the An cmpnent. The Ca-rich nature f the plagiclases is in cmmn with that bserved in the previusly studied Hle 504B plagiclases. The Leg 111 basalts are mderately altered, typically in parts f interstitial fine-grained materials and alng micr fractures and grain bundaries f primary minerals. The secndary minerals are chlrite, actinlite, talc, smectite, quartz, sphene, and pyrite. The extent f alteratin n a whle-rck basis is rather unifrm thrughut the entire Leg 111 sectin, as shwn by the limited ranges f water cntent, 5D, , S 34 S, and 87 Sr/ 8 Sr values. The alteratin ccurred at temperatures f C under very lw water/rck ratis (<). The SD and < values f the hydrthermal fluids respnsible fr the alteratin are clse t r slightly greater than thse f seawater. During alteratin, primary sulfide (pyrrhtite) was exclusively rembilized t frm the secndary pyrite in ne part and t disslve int fluids in the ther. The 5 34 S values f pyrite are very clse t thse f fresh submarine basalt glasses, indicating that almst n istpic fractinatin nr incrpratin f seawater sulfate tk place during the pyritizatin prcess. The lw-sulfate nature f the hydrthermal fluids is cnsistent with the idea that the deeper sheeted dike prtin f the Hle 504B served as a feeder zne fr the fluids in which sulfate ins had been lst during passage thrugh the recharge zne. The Sr and Nd istpic cmpsitins f the studied basalts are indistinguishable frm thse f typical fresh MORB, again implying lw seawater/rck ratis during alteratin. Hwever, 58

13 PETROLOGY AND ISOTOPE CHARACTERISTICS OF BASALTS, HOLE 504B the Nd istpic cmpsitins are split int tw grups, suggesting istpic hetergeneity in their magma surces. ACKNOWLEDGMENTS We thank K. Kase fr help in re micrscpic examinatin. Technical assistance by H. Asada and S. Takami is appreciated. REFERENCES Adamsn, A. C, Basement lithstratigraphy, Deep Sea Drilling Prject Hle 504B. In Andersn, R. N., Hnnrez, J., Becker, K., et al., Init. Repts. DSDP, 83: Washingtn (U.S. Gvt. Printing Office), Alt, J. C, Hnnrez, J., Hubberten, H.-W., and Saltzman, E., Occurrence and rigin f anhydrite frm Deep Sea Drilling Prject Leg 70, Hle 504B, Csta Rica Rift. In Cann, J. R., Langseth, M. G., Hnnrez, J., Vn Herzen, R. P., White, S. M., et al., Init. Repts. DSDP, 9: Washingtn (U.S. Gvt. Printing Office), Alt, J. C, Hnnrez, J., Laverne, C, and Emmermann, R., 198a. 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