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The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of <1 Ma peridotites at Hess Deep occurred at high temperatures (200°–400°C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low ƒO 2 and produced a reduced, low‐sulfur assemblage including NiFe alloy. Small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled, producing low‐δ 34 S sulfide (1.5‰ to −23.7‰). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures(∼20°–200°C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher ƒO 2 . Microbial reduction of seawater sulfate resulted in addition of low‐δ 34 S sulfide (∼15 to ∼43‰) and formation of higher‐sulfur assemblages that include valleriite and pyrite. The high SO 4 /total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high δ 34 S of total sulfur (mean ∼8‰). In contrast, Iberian Margin serpentinites gained large amounts of 34 S‐poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in δ 34 S of total sulfur (mean ≈ −5‰). Thus serpentinization is a net sink for seawater sulfur, but the amount fixed and its isotopic composition vary significantly. Serpentinization may result in uptake of 0.4–14 × 10 12 g S yr −1 from the oceans, comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output.
Alt et al. (Sun,) studied this question.