Abstract
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−, S0 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+/∑S=0.6–1) although sulfate micro-phases, such as barite and anhydrite, are absent. Indeed, μ-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 X-ray fluorescence mapping revealing large areas (1600 μm2) 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.
Highlights
Sulfur is a redox sensitive and fluid mobile element that is extensively involved in chemical exchange between the Earth’s surface and deep interior
The X-ray absorption near-edge structure (XANES) spectra of serpentinite powders correspond to mixtures between oxidized (S6+) and reduced (S2− or S−) sulfur which demonstrates the incorporation of both S-species during oceanic serpentinization
No sulfate grains were identified at micro or nano-scales in this study or in published works. This is in good agreement with the XANES spectra of serpentine textures that are similar to those of the sulfate-bearing haüyne suggesting that S is present as S6+ within the silicate serpentine minerals (Fig. 4)
Summary
Sulfur is a redox sensitive and fluid mobile element that is extensively involved in chemical exchange between the Earth’s surface and deep interior. A number of studies have documented variations in sulfur concentrations and isotope compositions in bulk rock serpentinites in various environments (e.g. Alt et al, 2007, 2013; Alt and Shanks, 1998, 2003; Delacour et al, 2008a, 2008b; Schwarzenbach et al, 2012). We complement these observations by providing new data on sulfur distribution and redox state in serpentine minerals
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