Sulfur concentrations and isotopic composition of serpentinized oceanic peridotites

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-delta34S 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-delta34S 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 delta34S 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 delta34S 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 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.大洋蛇纹岩（serpentinites）的矿物学特征、元素含量及硫同位素组成，反映了温度与流体通量的变化差异。蛇纹石化作用（serpentinization）在赫斯深渊（Hess Deep）处年龄小于1 Ma的橄榄岩中发生于高温（200℃~400℃）、低水岩比环境。亚铁被氧化为磁铁矿（magnetite）的过程维持了低氧逸度（ƒO₂），并形成了还原型低硫矿物组合，其中包含镍铁合金。随着体系降温，嗜热微生物（thermophilic microbes）介导的少量硫酸盐还原作用发生，生成了δ³⁴S值为1.5‰~-23.7‰的低硫同位素硫化物。与之形成对比的是，伊比利亚边缘（Iberian Margin）的橄榄岩蛇纹石化作用发生于低温（约20℃~200℃）、高水岩比环境。完全蛇纹石化及亚铁的消耗使得体系氧逸度升高。海水中硫酸盐的微生物还原作用新增了δ³⁴S值约15‰~43‰的低硫同位素硫化物，并形成了包含纤硫镍铜矿（valleriite）与黄铁矿（pyrite）的高硫矿物组合。赫斯深渊蛇纹岩的SO₄²⁻/总硫比值高达0.89，这导致总硫含量升高且总硫的δ³⁴S值偏高（平均值约8‰）。与之相反，伊比利亚边缘蛇纹岩富集大量δ³⁴S值偏低的硫化物（总硫平均含量为3800 ppm），且高硫化物/总硫比值（0.61）使得总硫的δ³⁴S值整体降低（平均值约-5‰）。综上，蛇纹石化作用整体是海水硫的净汇，但固定的硫量及其同位素组成存在显著差异。蛇纹石化作用每年可从海洋中吸收0.4×10¹² g~14×10¹² g的硫，这一规模与海底热液系统（seafloor hydrothermal systems）中的基性岩（mafic rocks）同位素交换过程相当，且接近河流硫酸盐输入（riverine sulfate input）与沉积硫化物输出（sedimentary sulfide output）的全球通量。