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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 …