Wayne C. Shanks III J.C. Alt 1998 <p><span>The mineralogy, contents, and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes. Serpentinization of &lt;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</span>₂<span>&nbsp;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</span>³⁴<span>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</span>₂<span>. Microbial reduction of seawater sulfate resulted in addition of low-δ</span>³⁴<span>S sulfide (∼15 to ∼43‰) and formation of higher-sulfur assemblages that include valleriite and pyrite. The high SO</span>₄<span>/total S ratio of Hess Deep serpentinites (0.89) results in an increase of total sulfur and high δ</span>³⁴<span>S of total sulfur (mean ∼8‰). In contrast, Iberian Margin serpentinites gained large amounts of&nbsp;</span>³⁴<span>S-poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in δ</span>³⁴<span>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</span>¹²<span>&nbsp;g S yr</span>⁻¹<span>&nbsp;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.</span></p> application/pdf 10.1029/98JB00576 en American Geophysical Union Sulfur in serpentinized oceanic peridotites: Serpentinization processes and microbial sulfate reduction article