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

Abstract

Whole rock sulfur and oxygen isotope compositions of altered peridotites and gabbros from near the 15°20′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 (∼250–350°C) at Sites 1268 and 1271 is characterized by 18O depletion (2.6–4.4‰), elevated sulfide‐S, and high δ34S (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 δ18O (up to 8.1‰), high sulfide‐S (up to ∼3000 ppm), and negative δ34S (to −32.1‰) 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 δ34S of sulfate may result from kinetic isotope fractionation during oxidation or may indicate readily oxidized low‐δ34S 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.