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 degrees-400 degrees C) and low water/rock ratios. Oxidation of ferrous iron to magnetite maintained low fO(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-delta(34)S sulfide (1.5 parts per thousand to -23.7 parts per thousand). In contrast, serpentinization of Iberian Margin peridotites occurred at low temperatures (similar to 20 degrees-200 degrees C) and high water/rock ratios. Complete serpentinization and consumption of ferrous iron allowed evolution to higher fO(2). Microbial reduction of seawater sulfate resulted in addition of low-delta(34)S sulfide (-15 to -43 parts per thousand) 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 delta(34)S Of total sulfur (mean approximate to 8 parts per thousand). In contrast, Iberian Margin serpentinites gained large amounts of S-34-poor sulfide (mean total S = 3800 ppm), and the high sulfide/total S ratio (0.61) results in a net decrease in delta(34)S of total sulfur (mean = -5 parts per thousand). 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 x 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.