Assessing sulfur redox state and distribution in abyssal serpentinites using XANES spectroscopy
Assessing sulfur redox state and distribution in abyssal serpentinites using XANES spectroscopy
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DOI：
10.1016/j.epsl.2017.02.029
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发表时间：
2017-05-15
影响因子：
5.3
通讯作者：
Williams, Helen
中科院分区：
文献类型：
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作者：
Debret, Baptiste;Andreani, Muriel;Williams, Helen
硫是参与地球表面和地幔深处化学转移的主要氧化还原敏感元素和挥发性元素之一。在大洋中脊，硫循环深受蛇纹岩形成的影响，蛇纹岩形成在各种氧化态（S2-、S-、S-0 和 S6+）下充当硫汇。蛇纹岩中的硫封存通常归因于次生矿物的结晶，例如硫化物（例如黄铁矿、磁黄铁矿）或硫酸盐（例如硬石膏）。然而，蛇纹石矿物作为潜在硫载体的作用并不受限制。我们结合原位光谱（X 射线吸收近边缘结构：XANES）和地球化学（SIMS）测量，研究了 SWIR（西南印度洋中脊）、Rainbow 和 MARK（大西洋中脊、凯恩断裂带）地区深海蛇纹岩中硫的微尺度分布和氧化还原态。这些蛇纹岩是在不同的构造变质环境中形成的，为了解海底蛇纹石化过程中硫的命运提供了有意义的数据库。蛇纹石粉末的 XANES 光谱表明，尽管不存在重晶石和硬石膏等硫酸盐微相，但研究样品的硫收支主要是氧化硫 (S6+ /Sigma S = 0.6-1)。事实上，对薄片中的网状、氟碳铁矿和叶蛇纹石脉以及蛇纹石颗粒的 mu-XANES 分析表明，蛇纹石矿物中存在 S6+ 离子。 X射线荧光图显示蛇纹石大面积（1600μm(2)）的S均匀分布，也支持了S在蛇纹石矿物中的结构掺入。我们的观察表明，蛇纹石矿物含有高浓度的硫，从 140 到 1350 ppm，这可以占深海蛇纹岩硫预算的 60% 到 100%。因此，蛇纹石矿物在洋中脊水圈和地幔之间的硫交换中发挥着重要作用，并可能参与俯冲带的硫回收。 (C) 2017 年作者。由 Elsevier B.V. 出版
Sulfur is one of the main redox sensitive and volatile elements involved in chemical transfers between earth surface and the deep mantle. At mid-oceanic ridges, sulfur cycle is highly influenced by serpentinite formation which acts as a sink of sulfur under various oxidation states (S2-, S-, S-0 and S6+). Sulfur sequestration in serpentinites is usually attributed to the crystallization of secondary minerals, such as sulfides (e.g. pyrite, pyrrhotite) or sulfates (e.g. anhydrite). However, the role of serpentine minerals as potential sulfur carriers is not constrained. We investigate the distribution and redox state of sulfur at micro-scale combining in situ spectroscopic (X-ray absorption near-edge structure: XANES) and geochemical (SIMS) measurements in abyssal serpentinites from the SWIR (South West Indian Ridge), the Rainbow and the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) areas. These serpentinites are formed in different tectono-metamorphic settings and provide a meaningful database to understand the fate of sulfur during seafloor serpentinization. XANES spectra of serpentinite powders show that the sulfur budget of the studied samples is dominated by oxidized sulfur (S6+ /Sigma S = 0.6-1) although sulfate micro phases, such as barite and anhydrite, are absent. Indeed, mu-XANES analyses of mesh, bastite and antigorite veins in thin sections and of serpentine grains rather suggest the presence of S6+ ions incorporated into serpentine minerals. The structural incorporation of S in serpentine minerals is also supported by Xray fluorescence mapping revealing large areas (1600 mu m(2)) of serpentinite where S is homogeneously distributed. Our observations show that serpentine minerals can incorporate high S concentrations, from 140 to 1350 ppm, and that this can account for 60 to 100% of the sulfur budget of abyssal serpentinites. Serpentine minerals thus play an important role in S exchanges between the hydrosphere and the mantle at mid-oceanic ridges and may participate to S recycling in subduction zones. (C) 2017 The Authors. Published by Elsevier B.V.