Sulfur in serpentinized oceanic peridotites: Serpentinization processes and microbial sulfate reduction

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 ƒO₂ 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-δ³⁴S 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 ƒO₂. Microbial reduction of seawater sulfate resulted in addition of low-δ³⁴S sulfide (∼15 to ∼43‰) and formation of higher-sulfur assemblages that include valleriite and pyrite. The high SO₄/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high δ³⁴S of total sulfur (mean ∼8‰). In contrast, Iberian Margin serpentinites gained large amounts of ³⁴S-poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in δ³⁴S 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 × 10¹² 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.