Sulfur concentrations and isotopic composition of serpentinized oceanic peridotites@en

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（oceanic serpentinites）的矿物学特征、硫含量及其硫同位素组成，可反映温度与流体通量的变化。赫斯深渊（Hess Deep）处年龄小于1 Ma的橄榄岩（peridotites）发生的蛇纹岩化作用，形成于高温（200℃~400℃）与低水岩比环境中。亚铁（ferrous iron）氧化为磁铁矿（magnetite）的过程，维持了低氧逸度（ƒO₂），并形成了一套还原型、低硫的矿物组合，其中包含镍铁合金（NiFe alloy）。随着体系冷却，嗜热微生物（thermophilic microbes）介导的硫酸盐还原作用少量发生，生成了δ³⁴S（delta34S）值偏低的硫化物，其δ³⁴S范围为1.5‰~-23.7‰。与之相反，伊比利亚边缘（Iberian Margin）橄榄岩的蛇纹岩化作用发生于低温（~20℃~200℃）与高水岩比环境中。橄榄岩完全蛇纹岩化以及亚铁被消耗，使得体系氧逸度得以升高。海水硫酸盐的微生物还原作用，带来了δ³⁴S值偏低的硫化物，其δ³⁴S范围为~15‰~~43‰，并形成了包含硫铁镍矿（valleriite）与黄铁矿（pyrite）的高硫矿物组合。赫斯深渊蛇纹岩的硫酸盐/总硫比值较高（0.89），导致其总硫含量升高，且总硫的δ³⁴S值偏高（平均值约为8‰）。与之相对，伊比利亚边缘蛇纹岩富集了大量δ³⁴S值偏低的硫化物，其总硫平均含量为3800 ppm，且硫化物/总硫比值较高（0.61），使得总硫的δ³⁴S值整体降低（平均值约为-5‰）。因此，蛇纹岩化作用是海水硫的净汇，但其固定的硫总量及硫同位素组成存在显著差异。蛇纹岩化作用每年可从海洋中吸收0.4×10¹²~14×10¹²克硫，这一规模与海底热液系统基性岩中的同位素交换过程相当，且接近全球河流硫酸盐输入与沉积硫化物输出的通量水平。