Hydrothermal alteration and microbial sulfate reduction in peridotite and gabbro exposed by detachment faulting at the Mid-Atlantic Ridge, 15°20′N (ODP Leg 209):: A sulfur and oxygen isotope study

[1] Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15 degrees 200'N Fracture Zone on the Mid-Atlantic Ridge were analyzed to investigate hydrothermal alteration processes and test for a subsurface biosphere in oceanic basement. Three processes are identified. (1) High-temperature hydrothermal alteration (similar to 250-350 degrees C) at Sites 1268 and 1271 is characterized by O-18 depletion (2.6-4.4 parts per thousand), elevated sulfide-S, and high delta S-34 (up to similar to 2 wt% and 4.4-10.8 parts per thousand). Fluids were derived from high-temperature (> 350 degrees C) reaction of seawater with gabbro at depth. These cores contain gabbroic rocks, suggesting that associated heat may influence serpentinization. (2) Low-temperature (< 150 degrees C) serpentinization at Sites 1272 and 1274 is characterized by elevated delta O-18 (up to 8.1 parts per thousand), high sulfide-S (up to similar to 3000 ppm), and negative delta S-34 (to -32.1 parts per thousand) that reflect microbial reduction of seawater sulfate. These holes penetrate faults at depth, suggesting links between faulting and temperatures of serpentinization. (3) Late low-temperature oxidation of sulfide minerals caused loss of sulfur from rocks close to the seafloor. Sulfate at all sites contains a component of oxidized sulfide minerals. Low delta S-34 of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low-delta S-34 sulfide derived from microbial sulfate reduction. Results show that peridotite alteration may be commonly affected by fluids +/- heat derived from mafic intrusions and that microbial sulfate reduction is widespread in mantle exposed at the seafloor.