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[1] Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15°20 0 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) Hightemperature hydrothermal alteration ($250-350°C) at Sites 1268 and 1271 is characterized by 18 O depletion (2.6-4.4%), elevated sulfide-S, and high d 34 S (up to $2 wt% and 4.4-10.8%). Fluids were derived from high-temperature (>350°C) reaction of seawater with gabbro at depth. These cores contain gabbroic rocks, suggesting that associated heat may influence serpentinization. (2) Low-temperature (<150°C) serpentinization at Sites 1272 and 1274 is characterized by elevated d18 O (up to 8.1%), high sulfide-S (up to $3000 ppm), and negative d 34 S (to À32.1%) 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 d 34 S of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized lowd 34 S 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.

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