ALBERT

Description: Chemical exchange between oceanic lithosphere and seawater is important in setting the chemical composition of the oceans. In the past, budgets for chemical flux in the flanks of mid-ocean ridges have only considered exchange between basalt and seawater. Recent studies have shown that lower crustal and upper mantle lithologies make up a significant fraction of sea floor produced at the global mid-ocean ridge system. Moreover, the rugged topography of slow spread crust exposing lower crust and upper mantle facilitates prolonged fluid circulation, whereas volcanic ridge flanks are more rapidly isolated from the ocean by a sediment seal. Hence, elemental fluxes during lower crust-seawater reactions must be assessed to determine their role in global geochemical budgets. ODP Hole 735B penetrates more than 1500 m into lower ocean crust that was generated at the very slow spreading Southwest Indian Ridge and later formed the 5-km-high Atlantis Bank on the inside corner high of the Atlantis II Fracture Zone. The gabbroic rocks recovered from Hole 735B preserve a complex record of plastic and brittle deformation and hydrothermal alteration. High-temperature alteration is rare below 600 m below seafloor (mbsf), but the lowermost section of the hole (500-1500 mbsf) has been affected by a complex and multistage low-temperature (〈250°C) alteration history probably related to the tectonic uplift of the basement. This low-T alteration is localized and typically confined to fractured regions where intense alteration of the host rocks can be observed adjacent to veins/veinlets filled with smectite, smectite-chlorite mixed layer minerals, or chlorite +/- calcite +/- zeolite +/- sulfide +/- Fe-oxyhydroxide. We have determined the bulk chemistry and O and Sr isotope compositions of fresh/altered rock pairs to estimate the chemical fluxes associated with low-temperature interaction between the uplifted and fractured gabbroic crust and circulating seawater. The locally abundant low-temperature alteration in crust at Site 735 has significantly changed the overall chemical composition of the basement. The direction of these changes is similar to that defined for volcanic ridge flanks, with low-temperature alteration of gabbroic crust acting as a sink for the alkalis, H2O, C, U, P, 18O, and 87Sr. The magnitudes of element fluxes are similar to volcanic ridge flanks for some components (C, P, Na) but are one or two orders of magnitude lower for others. The flux calculations suggest that low-temperature fluid circulation in gabbro massifs can result in S uptake (3% of riverine sulfate input) in contrast to the S losses deduced for volcanic ridge flanks.

Description: The mineralogy and stable (O and C) and Sr isotopic compositions of low-temperature alteration phases were determined in Hole 735B gabbroic rocks in order to understand the processes of low-temperature alteration in this uplifted block of lower oceanic crust. Phyllosilicates include smectite (saponite, Mg montmorillonite, and nontronite), chlorite/smectite, chlorite, talc, and serpentine. Other phases include prehnite, albite, K-feldspar, analcite, natrolite, thompsonite, pyrite, and titanite. The low-grade mineral assemblages mainly represent zeolite facies and lower-temperature "seafloor weathering" processes. Phyllosilicates formed over a range of temperatures but may also reflect variable reaction progress. Alteration temperatures were probably somewhat greater below 1300 meters below seafloor. Mineralogy and isotopic data indicate that conditions were mostly reducing and that seawater solutions were rock dominated. Carbonates formed late from cold and generally oxidizing seawater solution, however, as seawater penetrated downward as the result of fracturing and faulting in the uppermost portion of the uplifted crustal block.

Description: During ODP Leg 111 Hole 504B was extended 212 m deeper into the sheeted dikes of oceanic Layer 2, for a total penetration of 1288 m within basement. Study of the mineralogy, chemistry, and stable isotopic compositions of the rocks recovered on Leg 111 has confirmed and extended the previous model for hydrothermal alteration at the site: axial greenschist hydrothermal metamorphism was followed by seawater recharge and subsequent off-axis alteration. The dikes are depleted in 18O (mean delta18O = +5.1 per mil +/- 0.6 per mil ) relative to fresh mid-ocean ridge basalt. Oxygen isotopic data on whole rocks and isolated secondary minerals indicate temperatures during axial metamorphism of 250°-350°C and water/rock ratios about one. Increasing amounts of actinolite with depth in the dike section, however, suggest that temperatures increased downward in the dikes. Pyrite + pyrrhotite + chalcopyrite + magnetite was the stable sulfide + oxide mineral assemblage during axial alteration, but these minerals partly re-equilibrated later at temperatures less than 200°C. The dikes sampled on Leg 111 contain an average of 500 ppm sulfur, slightly lower than igneous values. The delta34S values of sulfide average 0 per mil , which indicates the presence of basaltic sulfide and incorporation of little or no seawater-derived sulfide into the rocks. These data are consistent with models for the presence of rock-dominated sulfur in deep hydrothermal fluids. The presence of anhydrite at 1176 m within basement indicates that unaltered seawater can penetrate to significant depths in the crust during recharge.