The Fate of Sulfur During Fluid-Present Melting of Subducting Basaltic Crust at Variable Oxygen Fugacity

Abstract

To constrain the effect of redox state on sulfur transport from subducting crust to mantle wedge during fluid-present melting and the stability of sulfur-bearing phases in the downgoing ocean crust, here we report high-pressure phase equilibria experiments on a H2O-saturated mid-ocean ridge basalt with 1 wt % S at variable oxygen fugacity (⁠⁠). Double-capsule experiments were conducted at 2·0 and 3·0 GPa and 950–1050°C, using Co–CoO, Ni–NiO, NixPd1–x–NiO, and Fe2O3–Fe3O4 external buffers. Sulfur content at sulfide saturation (SCSS) or sulfur content at sulfate saturation (SCAS) of experimental hydrous partial melts was measured by electron microprobe. All experiments were fluid-saturated and produced either pyrrhotite- or anhydrite-saturated assemblages of silicate glass, clinopyroxene, garnet, and rutile or titanomagnetite, ± amphibole ± quartz ± orthopyroxene. The silicate partial melt composition evolves from rhyolitic at 950°C to trachydacitic and trachyandesitic at 1050°C with increasing ⁠. At pyrrhotite saturation, melt S contents range from ∼30 ppm S at < FMQ – 1 to ∼500 ppm S at FMQ < ≤ FMQ + 1·1, whereas at anhydrite saturation (⁠ ≥ FMQ + 2·5) melt S concentrations range from ∼700 ppm S to 0·3 wt % S. Mass-balance calculations suggest that the aqueous fluid phase at equilibrium may contain as much as ∼15 wt % S at 1050°C at pyrrhotite saturation (≤ FMQ + 1·1), in agreement with previous estimates, and up to 8 wt % S at anhydrite saturation. Our data also show that decreases markedly with increasing at pyrrhotite saturation, from several thousand at < FMQ – 1 to ∼ 200–400 at FMQ < ≤ FMQ + 1·1, owing to the increase of melt S content. At anhydrite saturation, is very low (<100) but increases with decreasing temperature, in an opposite way to previous observations at pyrrhotite saturation. As a consequence, at T ≤ 900°C, might be in the range 200 ± 100, irrespective of ⁠. The present study confirms that slab partial melts saturated with pyrrhotite are unable to efficiently transport S from slab to mantle wedge, and suggests that slab partial melts in equilibrium with anhydrite also have very limited power to enrich the mantle wedge in S. Importantly, slab-derived aqueous fluids appear to be efficient vectors for the transport of sulfur from slab to mantle wedge at all ⁠. Therefore, S transfer from ocean crust to wedge mantle is not dependent and could take place over a range of conditions, and oxidized slab conditions are not necessarily required to enrich the mantle wedge in S. Finally, depending on the initial amount of sulfur in the slab, the proportion of residual anhydrite and pyrrhotite in the dehydrated slab below the region of formation of arc magmas is likely to be significant and may efficiently be recycled into the deep mantle.