Assessing sulfur redox state and distribution in abyssal serpentinites using XANES spectroscopy

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 (S 2 − , S − , S 0 and S 6 + ). 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 (S 6 + / 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 S 6 + 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 μm 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.NERC Deep Volatiles Consortium Grant NE/M000303/