(Table 1) Lithium contents and isotopic composition in sediments from DSDP Holes 64-477 and 64-477A

Lithium isotopic compositions of hydrothermally altered sediments of Deep Sea Drilling Project (DSDP) site 477/477A, as well as high temperature vent fluids of the Guaymas Basin, have been determined to gain an understanding of lithium exchange during fluid-sediment interaction at this sediment-covered spreading center. Unaltered turbidite of the basin has a d6Li value of -10%, 5-7% heavier than fresh oceanic basalts. Contact metamorphism induced by a shallow sill intrusion results in a decrease of the lithium content of the adjacent sediments and a lighter isotopic value (-8%). Below the sill, sediments altered by a deep-seated hydrothermal system show strong depletions in lithium, while lithium isotopic compositions vary greatly, ranging from -11 to +1%. The shift to lighter composition is the result of preferential retention of the lighter isotope in recrystallized phases after destruction of the primary minerals. The complexity of the isotope profile is attributed to inhomogeneity in mineral composition, the tortuous pathway of fluids and the temperature effect on isotopic fractionation. The range of lithium concentration and d6Li values for the vent fluids sampled in 1982 and 1985 overlaps with that of the sediment-free mid-ocean ridge systems. The lack of a distinct expression of sediment input is explained in terms of a flow-through system with continuous water recharge. The observations on the natural system agree well with the results of laboratory hydrothermal experiments. The experimental study demonstrates the importance of temperature, pressure, water/rock ratio, substrate composition and reaction time on the lithium isotopic composition of the reacted fluid. High temperature authigenic phases do not seem to constitute an important sink for lithium and sediments of a hydrothermal system such as Guaymas are a source of lithium to the ocean. The ready mobility of lithium in the sediment under elevated temperature and pressure conditions also has important implications for lithium cycling in subduction zones.

Petrological and geochemical characteristics of DSDP Hole 37-334 gabbroic cumulates (Table 1)

The gabbronoritic cumulates drilled at DSDP Site 334 (Mid-Atlantic Ridge off the FAMOUS area) are neither crystallization products of the associated basalts, nor from any MORB composition documented along ocean ridges. Their parent melts are richer in SiO2 than MORB at a given MgO content, as attested by the crystallization sequence starting with an olivine+calcic and sub-calcic pyroxene assemblages. These melts are issued from a source highly depleted in incompatible elements, likely residual peridotite left after MORB extraction. To understand the role of water in the genesis of these lithologies whose occurrence in a mid-ocean ridge setting is rather puzzling, we performed a geochemical study on clinopyroxene separates following an analytical protocol able to remove the effects of water rock interactions post-dating their crystallization. Accordingly, the measured isotopic signatures can be used to trace magma sources. We find that Site 334 clinopyroxenes depart from the global mantle correlation: normal MORB values for the 143Nd/ 144Nd ratio (0.51307-0.51315) are associated to highly radiogenic 87Sr / 86Sr (0.7034-0.7067) ratios. This indicates that the parent melts of Site 334 cumulates are issued from a MORB source but that seawater contamination occurred at some stage of their genesis. The extent of contamination, traced by the Sr isotopic signature, is variable within all cumulates but more developed for gabbronorites sensus stricto, suggesting that seawater introduction was a continuous process during all the magmatic evolution of the system, from partial melting to fractional crystallization. Simple masse balance calculations are consistent with a contaminating agent having the characters of a highly hydrated (possibly water saturated) silica-rich melt depleted in almost all incompatible major, minor and trace elements relative to MORB. Mixing in various proportions of contaminated melts similar to the parent melts of Site 334 cumulates with MORB can account for part of the variability in the Sr isotopic signature of oceanic basalts, among other to the short wavelength isotopic "noise" superimposed on regional trends. We conclude that seawater introduction into residual peridotite at shallow depth beneath mid-ocean ridges can lead mantle rocks and their melts to follow complex P-T-fH2O paths that mimic petrogenetic contexts classically attributed to subduction zone environments, like the production of boninitic-andesitic magmas.

(Table 1) Physical properties of ODP Site 129-801

Compressional wave velocities and densities were measured for 6 basalt samples from ODP Hole 801B and 16 samples from ODP Hole 801C, a site that represents the first drilling of Jurassic-age crustal rocks in the Pacific basin. Incremental measurements, taken to a total pressure of 200 MPa, show a systematic decrease in velocity with increasing porosity and a related increase with increasing wet-bulk density. A comparison of the plot of porosity vs. compressional wave velocity with the theoretical equation from Wyllie et al. (1958) suggests this equation is inappropriate for oceanic basalts because of mineral alteration in high porosity samples. Also of interest is the dramatic change in velocity across a hydrothermal boundary. Basalts below this hydrothermal layer have a mean velocity of 6.05 km/s at 60 MPa while those above show a mean velocity of 4.55 km/s at 60 MPa. The low velocity values of the basalts above the hydrothermal deposit may be attributed to the higher porosity and composition observed in these rocks; the higher porosity is possibly the result of increased exposure to circulating seawater.

(Table 1) Summary of basalt unit thickness, vesicularity, number of flow units, and average porosity at ODP Site 135-839

An inflatable drill-string packer was used at Site 839 to measure the bulk in-situ permeability within basalts cored in Hole 839B. The packer was inflated at two depths, 398.2 and 326.9 mbsf; all on-board information indicated that the packer mechanically closed off the borehole, although apparently the packer hydraulically sealed the borehole only at 398.2 mbsf. Two pulse tests were run at each depth, two constant-rate injection tests were run at the first set, and four were run at the second. Of these, only the constant-rate injection tests at the first set yielded a permeability, calculated as ranging from 1 to 5 * 10**-12 m**2. Pulse tests and constant-rate injection tests for the second set did not yield valid data. The measured permeability is an upper limit; if the packer leaked during the experiments, the basalt would be less permeable. In comparison, permeabilities measured at other Deep Sea Drilling Project and Ocean Drilling Program sites in pillow basalts and flows similar to those measured in Hole 839B are mainly about 10**-13 to 10**-14 m**2. Thus, if our results are valid, the basalts at Site 839 are more permeable than ocean-floor basalts investigated elsewhere. Based on other supporting evidence, we consider these results to be a valid measure of the permeability of the basalts. Temperature data and the geochemical and geotechnical properties of the drilled sediments all indicate that the site is strongly affected by fluid flow. The heat flow is very much less than expected in young oceanic basalts, probably a result of rapid fluid circulation through the crust. The geochemistry of pore fluids is similar to that of seawater, indicating seawater flow through the sediments, and sediments are uniformly underconsolidated for their burial depth, again indicating probable fluid flow. The basalts are highly vesicular. However, the vesicularity can only account for part of the average porosity measured on the neutron porosity well log; the remainder of the measured porosity is likely present as voids and fractures within and between thin-bedded basalts. Core samples, together with porosity, density, and resistivity well-log data show locations where the basalt section is thin bedded and probably has from 15% to 35% void and fracture porosity. Thus, the measured permeability seems reasonable with respect to the high measured porosity. Much of the fluid flow at Site 839 could be directed through highly porous and permeable zones within and between the basalt flows and in the sediment layer just above the basalt. Thus, the permeability measurements give an indication of where and how fluid flow may occur within the oceanic crust of the Lau Basin.

(Table 1) Sulfur and oxygen isotopes from two anhydrite samples recovered from DSDP Hole 70-504B

The first anhydrite reported from oceanic basalts occurs in altered basalts drilled during DSDP Leg 70 from Hole 504B. Anhydrite has been identified in several samples, two of which were studied in detail. Anhydrite in Sample 504B-40-3 (130-135 cm), which was acquired at 310 meters sub-basement, occurs in a dolerite at the center of a vug rimmed by saponite and calcite. Red iron-hydroxide-rich alteration halos occur from 0 to 310 meters sub-basement; primary sulfides in these halos are oxidized, and the rocks have lost large amounts of sulfur. The anhydrite in this sample has a d34S value of 18.5 per mil, and it is interpreted to have formed from a fluid containing a mixture of seawater sulfate (20.9 per mil) and basaltic sulfur (0 per mil) released during the oxidation of primary sulfides. Anhydrite in Sample 504B-48-3 (14-18 cm), which was found at 376 meters sub-basement, occurs intergrown with gyrolite at the center of a 1-cm-wide vein that is rimmed by saponite and quartz. At sub-basement depths below 310 meters to the bottom of the Leg 70 section (562 m sub-basement), the rocks exhibit the effects of anoxic alteration with common secondary pyrite. Anhydrite in Sample 504B-48-3 (14-18 cm) has a d34S value of 36.7 per mil, and it is interpreted to have formed from seawater-derived fluids enriched in 34S through sulfate reduction. Temperatures of alteration calculated from oxygen isotope data range from 60 to 100°C. Sulfate reduction may have occurred in situ, or elsewhere at higher temperature, possibly deeper in the crust. The secondary mineral paragenetic sequence indicates a progressive decrease in Mg and increase in Ca in the circulating fluids. This eventually led to anhydrite formation late in the alteration process.

Geochemistry of oceanic flood basalts from the Caribbean plate

The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar-39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ~90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau i.e., large volume, rapidly erupted, regionally extensive volcanism.. The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galápagos hotspot at its inception. The trace-element and isotopic compositions of the ~90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curacao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, andror tectonic emplacement of the plateau between North and South America. Still younger volcanics are found in the Dominican Republic (69 Ma) and the Quepos Peninsula of Costa Rica (63 Ma). The latter occurrence conceivably formed over the Galápagos hotspot and subsequently accreted to the western edge of the plateau during subduction of the Farallon plate.

Geochemistry, and Nd- and Pb-isotopic composition of basalts from the upper oceanic crust of the Costa Rica Rift

The DSDP/ODP Hole 504B, drilled in the 5.9 Ma southern flank of the Costa Rica Rift, represents the deepest section through modern ocean floor basaltic basement. The hole penetrates a 570 m thick volcanic zone, a 210 m thick transition zone of volcanic rocks and dykes, and 1056 m of dykes. A representative selection of these basalt types has been investigated with respect to Nd and Pb isotopes. The epsilonNd of the basalts varies from 7.62 to 11.16. This range in the Nd-isotope composition represents about 67% of the total range reported for Pacific MORB. The Pb-isotope composition also shows significant variation, with 206Pb/204Pb varying from 17.90 to 18.82. The isotopic data show that a small volume of enriched mantle existed in the source. The large ranges in isotopic composition in a single drill hole demonstrate the importance of small-scale mantle heterogeneities in the petrogenesis of MORB. Fractional melting and extraction of small magma batches by channelled flow, and small, short-lived crustal magma reservoirs, with limited potential for mixing of the mantle derived magmas, are favored by these isotopic data.

Isotope ratios of basalts from ODP Leg 115 holes (Table 1)

Strontium, neodymium, and lead isotope ratios are reported for 13 Leg 115 basalts as well as 3 basalts from Texaco drill hole SM-1 on the Mascarene Plateau. The 87Sr/86Sr ratios and eNd range from 0.70330 to 0.70439 and 5.5 to 7.4, respectively, although 87Sr/86Sr ratios higher than 0.70383 are found only in SM-1 basalts. The high 87Sr/86Sr values are thought to reflect seawater Sr in secondary phases, although all samples were strongly leached in HC1 before analysis. 206Pb/204Pb ratios range from 18.53 to 18.80, and sho high 207Pb/204Pb and 208Pb/204Pb ratios relative to 206Pb/204Pb ratios, typical of Indian Ocean mid-ocean ridge (MORB) and oceanic-island basalts (OIB). Isotopic compositions of Leg 115 basalts generally fall between fields for MORB and Reunion Island basalts, consistent with the conclusion drawn from geochronological studies that Deccan flood basalt volcanism, the Chagos-Laccadive Ridge, and the Mascarene Plateau are all products of the Reunion mantle plume. Isotopic compositions of magmas produced by this plume have varied systematically with time in the direction of less "depleted," less MORB-like isotopic signatures. This compositional change has been accompanied by a decrease in eruption rate. We interpret Deccan volcanism as the voluminous beginning of the plume. Reduced entrainment of asthenosphere following melting of the plume head resulted in less MORB-like isotope ratios in magmas and a decrease in eruptive activity with time.