In-situ Iron isotope measurements and mineral compositions from ODP Hole 206-1256D and IODP Hole 335-U1256D

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (> 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of < 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

Chemical and isotopic compositions of ocean authigenic carbonates

Oceanic authigenic carbonates are classified according to origin of the carbonate carbon source using a complex methodology that includes methods of sedimentary petrography, mineralogy, isotope geochemistry, and microbiology. Mg-calcite (protodolomite) and aragonite predominate among the authigenic carbonates. All authigenic carbonates are depleted in 13C and enriched in 18O (in PDB system) that indicates biological fractionation of isotopes during carbonate formation. Obtained results show that authigenic carbonate formation is a biogeochemical (microbial) process, which involves carbon from ancient sedimentary rocks, abiogenic methane, and bicarbonate-ion of hydrothermal fluids into the modern carbon cycle.

(Table 1) Geochemistry of feldspars at DSDP Holes 62-464 and 62-465A

Labradorite is the principal feldspar in tholeiitic basalt from Deep Sea Drilling Project Site 464, on northern Hess Rise. Andesine and oligoclase constitute most primary feldspar in the reportedly trachytic rocks from southern Hess Rise at Site 465. Secondary sanidine(?) has replaced the primary phases at Site 465. The secondary potassium feldspar probably resulted from reaction of trachyte with potassium-bearing hydrothermal fluids or sea water.

Composition of phyllosilicates from hydrothermally altered basalts of DSDP Hole 83-504B, Costa Rica Rift, Pacific Ocean

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or olivine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from < 100 A to a few hundred A. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystallinity of saponite. By contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of A thick. The Si/(Si + A1) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe + Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+A1) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite ± mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.

Composition of ferromanganese crusts from the Atlantic, Pacific and Indian Oceans

A collection of layered ferromanganese ores (27 samples) from the Atlantic and Pacific oceans was studied. Trace element and PGE contents were determined layer-by-layer (up to 10 microlayers) in 13 of these samples. The trace, rare earth, and platinum group element distributions, including their layer-to-layer variations, were compared in hydrogenic and hydrothermal crusts from different regions. It was found that the main PGE variations (by a factor of 10-50) are related to their layer-to-layer variations within a given ore field. The distributions of PGE and trace elements are strongly heterogeneous, which is related, first, to different contents of the elements in the layers of different age in ferromanganese crusts (FMC) and, second, to the observed regional heterogeneity and influence of hydrothermal fluids. Geochemical data indicate that CFC formation was mainly caused by the hydrochemical precipitation of material from seawater. This process was accompanied by diagenetic phenomena, water-rock interaction, and influence of volcanic and hydrothermal sources.

Chemical composition of clay minerals from ODP Leg 168 sites

The exchangeable cation compositions of organic-poor terrigenous sediments containing smectite as primary ion exchanger from a series of holes along ODP Leg 168 transect on the eastern flank of the Juan de Fuca Ridge have been examined as a function of distance from the ridge axis and burial depth. The total cation exchange capacity (CEC) values of the sediments ranged from 2 to 59 meq/100 g, increasing with increases in the wt.% smectite. At the seafloor, the exchangeable cation compositions involving Na, K, Mg, and Ca, expressed in terms of equivalent fraction, are nearly constant regardless of the different transect sites: XNa = 0.21 ± 0.04, XK = 0.08 ± 0.01, XMg = 0.33 ± 0.09, and XCa = 0.38 ± 0.09. The calculated selectivity coefficients of the corresponding quaternary exchange reactions, calculated using porewater data, are in log units -5.45 ± 0.39 for Na, 1.97 ± 0.49 for K, 0.42 ± 0.41 for Mg, and 3.06 ± 0.69 for Ca. The exchangeable cation compositions below the seafloor change systematically with distance from the ridge crest and burial depth, conforming to the trends of the same cations in the porewaters. The selectivities for Na and Mg are roughly constant at temperatures from 2 to 66°C, indicating that the equivalent fractions of these two cations are independent of sediment alteration taking place on the ridge flank. Unlike Na and Mg, the temperature influence is significant for K and Ca, with Ca-selectivity decreases being coupled with increases in K-selectivity. Although potentially related to diagenetic and/or hydrothermal mineral precipitation or recrystallization, no evidence of such alteration was detected by XRD and TEM. In sites where upwelling of hydrothermal fluids from basement is occurring, the K-selectivity of the sediment is appreciably higher than at the other sites and corresponds to the formation of (Fe, Mg) rich smectite and zeolites. Our study indicates that local increases in K-selectivity at hydrothermal sites are caused by the formation of these authigenic minerals.

Geochemistry of the lower sheeted dike complex of ODP Holes 137-504B and 140-504B

Sixty-three samples representing 379 m of sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program Site 504B have been analyzed for major and selected trace elements by X-ray fluorescence. The samples range from microcrystalline aphyric 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 moderate to slightly elevated compatible elements (MgÆ-value = 0.65% ± 0.03%; Al2O3 = 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 < 0.5 ppm; Ba ~ 1 ppm; P2O5 = 0.07% ± 0.02%). These compositions are consistent with a multistage melting of a normal ocean ridge 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 from similarly phyric samples at higher levels in the hole. An examination of the entire crustal section, from the overlying volcanics through the sheeted dikes observed in Leg 140, reveals no significant trends indicating the enrichment or depletion of Costa Rica Rift Zone source magmas over time. Similarly, significant trends toward increased or decreased differentiation cannot be identified, 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, the result of high-temperature hydrothermal leaching. This zone of depletion is thought to be a significant source of Zn for the hydrothermal fluids depositing metal sulfides at ridge-crest hydrothermal vents and the sulfide-mineralization zone, located in the 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 lithologic units. Within these zones, normally immobile elements Ti, Zr, Y, and rare-earth elements are strongly depleted compared with "fresher" samples centimeters away. The extent of compositional variability of these elements tends to obscure primary igneous trends if the highly altered samples are not identified or removed. At levels up to 40% (or possibly 60%) recrystallization, Ti, Zr, and Y retain their primary signatures. Although the mechanisms are unclear, it is possible that these intense alteration zones are a source of Y and rare-earth elements for the typically rare-earth-element-enriched hydrothermal vent fluids of mid-ocean ridges.

Late Quaternary calcareous nannofossils in ODP Leg 139 holes

Upper Quaternary calcareous nannofossils contained in drill cores taken in the heavily sedimented Middle Valley of the northern Juan de Fuca Ridge in the northeast Pacific Ocean (Ocean Drilling Program Leg 139) are investigated. The host sediments have been subjected at depth to high temperatures and hot hydrothermal fluids that have altered or destroyed in part or in toto the nannofossil assemblages, thereby raising at several sites the level of the first (deepest) stratigraphic occurrence of nannofossils or of the important Emiliania huxleyi datum. The degree of alteration of the nannofossil assemblages is dependent on the intensity of the hydrothermal activity, which is indicated by paleotemperatures derived independently from studies of color alteration of palynomorphs and by vitrinite reflectance (Mao et al., this volume). State of preservation and the downhole level at which assemblages have been destroyed correlate well with the inferred paleotemperature estimates. Destruction of the assemblages appears to be species selective and follows in general the dissolution rankings determined independently by others for Recent nannofossils of the Pacific basin. More systematic correlation of these phenomena is hampered, however, by the fact that nannofossil preservation is already quite variable at the time of deposition because of the predominance of turbidite activity in the study area.

Major oxides, trace elements and rare earth elements of selected basalt samples at DSDP Hole 83-504B

DSDP Hole 504B is the deepest section drilled into oceanic basement, penetrating through a 571.5-m lava pile and a 209-m transition zone of lavas and dikes into 295 m of a sheeted dike complex. To define the basement composition 194 samples of least altered basalts, representing all lithologic units, were analyzed for their major and 26 trace elements. As is evident from the alteration-sensitive indicators H2O+, CO2, S, K, Mn, Zn, Cu, and the iron oxidation ratio, all rocks recovered are chemically altered to some extent. Downhole variation in these parameters enables us to distinguish five depth-related alteration zones that closely correlate with changes in alteration mineralogy. Alteration in the uppermost basement portion is characterized by pronounced K-uptake, sulfur loss, and iron oxidation and clearly demonstrates low-temperature seawater interaction. A very spectacular type of alteration is confined to the depth range from 910 to 1059 m below seafloor (BSF). Rocks from this basement portion exhibit the lowest iron oxidation, the highest H2O+ contents, and a considerable enrichment in Mn, S, Zn, and Cu. At the top of this zone a stockwork-like sulfide mineralization occurs. The chemical data suggest that this basement portion was at one time within a hydrothermal upflow zone. The steep gradient in alteration chemistry above this zone and the ore precipitation are interpreted as the result of mixing of the upflowing hydrothermal fluids with lower-temperature solutions circulating in the lava pile. Despite the chemical alteration the primary composition and variation of the rocks can be reliably established. All data demonstrate that the pillow lavas and the dikes are remarkably uniform and display almost the same range of variation. A general characteristic of the rocks that classify as olivine tholeiites is their high MgO contents (up to 10.5 wt.%) and their low K abundances (-200 ppm). According to their mg-values, which range from 0.60 to 0.74, most basalts appear to have undergone some high-level crystal fractionation. Despite the overall similarity in composition, there are two major basalt groups that have significantly different abundances and ratios of incompatible elements at similar mg-values. The majority of the basalts from the pillow lava and dike sections are chemically closely related, and most probably represent differentiation products of a common parental magma. They are low in Na2O, TiO2, and P2O5, and very low in the more hygromagmaphile elements. Interdigitated with this basalt group is a very rarely occurring basalt that is higher in Na2O, TiO2, P2O5, much less depleted in hygromagmaphile elements, and similar to normal mid-ocean ridge basalt (MORB). The latter is restricted to Lithologic Units 5 and 36 of the pillow lava section and Lithologic Unit 83 of the dike section. The two basalt groups cannot be related by differentiation processes but have to be regarded as products of two different parental magmas. The compositional uniformity of the majority of the basalts suggests that the magma chamber beneath the Costa Rica Rift reached nearly steady-state conditions. However, the presence of lavas and dikes that crystallized from a different parental magma requires the existence of a separate conduit-magma chamber system for these melts. Occasionally mixing between the two magma types appears to have occurred. The chemical characteristics of the two magma types imply some heterogeneity in the mantle source underlying the Costa Rica Rift. The predominant magma type represents an extremely depleted source, whereas the rare magma type presumably originated from regions of less depleted mantle material (relict or affected by metasomatism).

Mineralogy and isotopic composition of sulfur in ODP Hole 118-735B gabbros

Sulfide mineralogy, sulfur contents, and sulfur isotopic compositions were determined for samples from the 500-m gabbroic section of Ocean Drilling Program Hole 735B in the southwest Indian Ocean. Igneous sulfides (pyrrhotite, chalcopyrite, pentlandite, and troilite) formed by accumulation of immiscible sulfide droplets and crystallization from intercumulus liquids. Primary sulfur contents average around 600 ppm, with a mean sulfide d34S value near 0 per mil, similar to the isotopic composition of sulfur in mid-ocean ridge basalt glass. Rocks from a 48-m interval of oxide gabbros have much higher sulfur contents (1090-2530 ppm S) due to the increased solubility of sulfur in Fe-rich melts. Rocks that were locally affected by early dynamothermal metamorphism (e.g., the upper 40 m of the core) have lost sulfur, averaging only 90 ppm S. Samples from the upper 200 m of the core, which underwent subsequent hydrothermal alteration, also lost sulfur and contain an average of 300 ppm S. Monosulfide minerals in some of the latter have elevated d34S values (up to +6.9 per mil), suggesting local incorporation of seawater-derived sulfur. Secondary sulfides (pyrrhotite, chalcopyrite, pentlandite, troilite, and pyrite) are ubiquitous in trace amounts throughout the core, particularly in altered olivine and in green amphibole. Pyrite also locally replaces igneous pyrrhotite. Rocks containing secondary pyrite associated with late low-temperature smectitic alteration have low d34S values for pyrite sulfur (to - 16.6 per mil). These low values are attributed to isotopic fractionation produced during partial oxidation of igneous sulfides by cold seawater. The rocks contain small amounts of soluble sulfate (6% of total S), which is composed of variable proportions of seawater sulfate and oxidized igneous sulfur. The ultimate effect of secondary processes on layer 3 gabbros is a loss of sulfur to hydrothermal fluids, with little or no net change in d34S.

Strontium and Oxygen isotope compositions from DSDP Holes 54-424A, 54-424B and 70-509B

Strontium and O isotope compositions of green clay minerals from sediment cores of three boreholes drilled into (sites 424A and 509B) and close to a hydrothermal mound (site 424B) near the Galapagos Spreading Center (DSDP Legs 54 and 70) were determined. The green clays consist mostly of a transition from Fesmectite (nontronite) to glauconite. 87Sr/86Sr ratios were measured on clay size-fractions after gentle acid leaching and on the recovered leachates from different samples. The 87Sr/86Sr ratios of the clay residues from both the 424A and B sites are well below the modern seawater value, which points consistently to precipitation from hydrothermal fluids that contained variable amounts of seawater, even away from mound. However, most of the clay residues from mound site 509B have 87Sr/86Sr ratios significantly above the seawater value, suggesting the occurrence of a detrital component together with the new authigenic particles. The clay minerals of the hydrothermal mound are mixed with detrital components, and that of the sample taken outside but near the mound as a reference for the surrounding oceanic environment, yields a hydrothermal signature. Crystallization temperatures of the clays range from 32 to 63 °C assuming a d18O value of +2.2 per mil for the mineralizing fluids. Hydrothermal fluids generated in the underlying oceanic crust, mixed in varied proportions with ambient seawater and migrated into beds of the mound in a sequence of recurrent processes that ultimately resulted in the formation of the observed clay minerals. No significant temperature differences were detected for crystallization of the K-rich glauconite and K-depleted nontronite. The 87Sr/86Sr ratios of the Sr leached off the clay particles are near the value of modern seawater, inferring a progressive replacement of the hydrothermal fluids by seawater in the pore space of the mound sediments.

Strontium chemistry of anhydrite from DSDP/ODP Hole 504B

A unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in ODP Hole 504B. Chemical and isotopic analyses 87Sr/86Sr, delta18O, delta34S of anhydrite constrain the changing composition of fluids due to reaction with basalt. There is a general trend of decreasing 87Sr/86Sr of anhydrite, corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks. 87Sr/86Sr ratios decrease rapidly with depth in the dikes to values identical to host basalt (0.7029). Sr/Ca ratios (<0.1 mmol/mol) suggest that recharge fluids have very low Sr concentrations and fluids evolve by first precipitating Sr-bearing phases before extensive exchange of Sr with the host basalt. There is a background trend of decreasing sulfate delta18O with depth from +12-13? in the lower volcanics to +7? in the lower sheeted dikes recording an increase in recharge fluid temperature from c. 150° to c. 250°C, and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures. From the amount of anhydrite recovered from Hole 504B and the amount of seawater sulfur that has been reduced to sulfide, a minimum seawater recharge flux can be calculated. This value is 4-25 times lower than estimates of high-temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater-derived sulfur in the oceanic crust. Only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker-type fluids. A significant proportion of the axial heat loss must be advected at 200-250°C by sulfate-bearing hydrothermal solutions that egress diffusely from the crust. These fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation, which would halt hydrothermal circulation for any reasonable fluid flux. The heating of the major proportion of hydrothermal fluids to only moderate temperatures (c. 250°C) reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets. The flux of hydrothermal sulfate would be of a magnitude similar to the riverine input, and oxygen-isotopic exchange at 200-250°C between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed.

Mineralogy and chemistry of basalts and secondary minerals from DSDP Site 417, Atlantic Ocean

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H20, increase of Fe 3 +/FeT and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H20, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3 b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe 3 +/FeT and H20 occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H20 , and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

Magnetic properties and heavy metals at DSDP Hole 83-504B

We report measurements of magnetic intensity, inclination, initial susceptibility, Koenigsberger's ratio, saturation magnetization, and Curie temperatures of 68 basalt samples from the Leg 83 section of Hole 504B. As in the upper part of the hole, reversely magnetized units predominate. Intensities of natural remanent magnetization vary widely, but the range of variation is an order of magnitude less than in the upper part of the hole. This and the other properties measured indicate that the magnetic characteristics of basalts from Hole 504B have been strongly affected by hydrothermal alteration, particularly in the deeper, Leg 83 section. The alteration states of the magnetic samples were studies using Xray diffraction, electron microprobe, X-ray fluorescence, and ion coupled plasma. Our results suggest three alteration zones in Hole 504B: a low-temperature zone (274.5-890 m) and two high-temperature zones (890-1050 m and 1050- 1350 m), differing in the number of veins observed in the samples and presumably differing in the volumes of hydrothermal fluids which reacted with the basalts.

Geochemistry at DSDP Site 60-456

Basalts in two holes spaced 200 meters apart at DSDP Site 456 in the Mariana Trough both show a downward sequence of nonoxidative and oxidative zones of alteration, each 10 to 15 meters thick, overlying fresh basalts. Basalts in the nonoxidative zone have been extensively chloritized and have vein and vesicle fillings of quartz, opal, chlorite, calcite, and pyrite. Minor sulfides are chalcopyrite and digenite. Basalts in the oxidative zone have abundant smectites and iron hydroxides and are variably enriched in K, Rb, and Ba, unlike the nonoxidative basalts above them. We propose that the oxidative zone was a zone of mixing between high-temperature, reduced hydrothermal fluids moving horizontally beneath impermeable sediments at the top of the pillowed basement lavas and cold, oxygenated seawater in interpillow voids deeper in the basement. Recrystallized vitric tuffs immediately above the basalts containing authigenic quartz and wairakite, as well as occurrence of chlorite, epidote, and chalcopyrite in the basalts, suggest temperatures of alteration in excess of 200°C.