Mercury Silicate Phase Equilibria Changes Due to S Speciation

The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%) due to high S solubility in silicate melt under its very low oxygen fugacity (ƒO2). However, the speciation of that S remains poorly constrained. In this study, we evaluate the role of pressure, temperature, and melt composition on S solubility and speciation in reduced magmas relevant to Mercury. Sulfur speciation was determined by S K-edge XANES spectra collected in 60 experiments that span a range of pressure (0.1 to 5 GPa), temperature (1225 to 1850 °C), and ƒO2 (IW-0.8 to IW-8.6). Data were analysed using new relevant XANES standards and XANES spectral unmixing techniques. Stepwise forward regression was used to develop empirical equations for S species (MgS, CaS, and TiS). We found that ƒO2, P/T, and S content in the silicate melt at sulfide saturation (SCSS) exert the main controls on MgS content (wt.%) in the silicate melt, and that ƒO2 and MgS content in the silicate melt exert the main controls on SCSS.

[MgSliq wt.%] = a + bP⁄T + c log ⁡ƒO2 + d[S wt.%]SCSS

We find that as ƒO2 decreases from IW-2 to IW-7, S speciation in silicate melt goes through two major changes. Between IW-2 and IW-4, FeS and FeCr2S4 species are destabilized, and CaS becomes the dominant S species with minor TiS. Below IW-4, MgS is the dominant S species with minor CaS. At low ƒO2, S bonding with Fe, Mg, Ca, Ti, Na, and Mn affect the activities of SiO2, MgO, CaO, TiO, Na2O, and MnO in the silicate melt. This stabilizes enstatite at the expense of forsterite, destabilizes the Ca-bearing minerals plagioclase and clinopyroxene, and shifts plagioclase chemistry from the Ca-rich endmember anorthite to the Na-rich endmember albite as understood by reprojecting silicate ternary diagrams incorporating S speciation data. At the expense of MgS, CaS is more stable in the silicate melt at higher pressures at ƒO2 below IW-4, creating a pathway for CaS to be carried in the silicate melt from depth to the surface before oldhamite (CaS) crystallization. These S speciation changes have substantial impacts on physicochemical properties of silicate melt such as viscosity, melting temperature, and mineral stability, which led to the distinct evolution of Mercury and other reduced planetary interiors. This work is published in Anzures et al. (2025).