A thermodynamic model for sulfur content at sulfide saturation (SCSS) in hydrous silicate melts: With implications for arc magma genesis and sulfur recycling

Abstract

Modeling the effect of H2O on the “sulfur content at sulfide saturation” (SCSS) in silicate melt is essential for the estimation of SCSS in both arc magmas and slab melts. Here we present a thermodynamic approach, in which, SCSS in hydrous silicate melt has been modeled as a combination of S dissolving as S2− and HS−/H2S, based on the sulfide capacity (CS2−) and the hydro-sulfide capacity (CHS), respectively. By adopting the thermodynamic framework of O’Neill and Mavrogenes (2002), S dissolution as HS−/H2S can be modeled in an analogous fashion to that for modeling S2− in anhydrous melt with the relation: lnHSSCSS=-ΔG°Fes-FeO/RT+lnCHS-lnaFeOmelt+lnaFeSsulfide. To obtain an expression for CHS, we compiled published experimental data on SCSS in hydrous silicate melts covering a PT range of 0.15–3 GPa and 785–1600 °C, and melt H2O contents of ∼1–13 wt%. While the contribution of S dissolving as S2− in basaltic and andesitic melts can be calculated based on the updated SCSS model for anhydrous basic melt from O’Neill (2021), S2− is considered negligible in rhyolitic and dacitic melts, i.e., when melt FeO content <∼5 wt%. We propose an expression for CHS as a function of temperature, mole fractions of cations for the normalized anhydrous melt composition, ln(XOH + XH2O), and a Na + K-Al term for alkaline-enriched rhyolitic melts. The coefficients for cation mole fractions are adopted from the expression for CS2− from O’Neill (2021); the coefficients for the 1/T and Na + K-Al terms are obtained from regression. The reproducibility of compiled SCSS values is ±50% for hydrous rhyolitic and dacitic melts, and ±10–30% for hydrous basaltic and andesitic melts, representing a significant improvement in comparison to previous models.Our model produces SCSS values for the primitive arc magmas compiled by Ruscitto et al. (2012), that are in most cases higher than the measured S contents, implying sulfide under-saturated conditions during mantle wedge melting. The contribution of H2S dissolution to the calculated SCSS values varies in a range of 70–1234 ppm, which increases with the increase of H2O content (0.3–6.2 wt%). H2S dissolution therefore contributes to the higher S content in arc basalt compared to MORB, an observation which is further corroborated by the positive correlation between H2O and S contents in primitive arc magmas. Applying our current model to experimentally produced sediment melts spanning a PT range of 690–1050 °C and 2.5–4.5 GPa, demonstrates that sediment melts, especially those of intermediate supercritical character with >25 wt% H2O and peralkaline in composition, can have high SCSS values as a result of H2S dissolution, and act as the transfer medium for S recycling between the slab and mantle wedge under reduced conditions.