The role of sulfur on the liquidus temperature and olivine-orthopyroxene equilibria in highly reduced magmas

The geochemical data provided by the NASA MESSENGER spacecraft unveiled the geochemical heterogeneity of the volcanic crust of Mercury. Surprisingly, a high amount of sulfur was detected which combined with a low iron content, imply highly reduced conditions of parental magmas. Several variables (temperature, pressure, oxygen fugacity fO₂, and to a lesser extent, melt composition) affect the solubility of sulfur in silicate melts. In reduced silicate melts, sulfur has an oxidation state of S²⁻ and replaces anionic oxygen to form MgS and CaS complexes. Experimental studies have shown the high S solubility in silicate melts at low fO₂. As observed with other volatile elements, high S contents in silicate melts can deeply affect their properties such as (1) lowering the liquidus as compared to S-free compositions and (2) changing solid-liquid phase equilibria. In this study, we performed high temperature (1500–1950 °C) and high pressure (1.5–3 GPa) piston-cylinder experiments on Fe-poor compositions relevant to the petrogenesis of Mercury's volcanic crust with the aim of quantifying the effect of sulfur on depressing their liquidus temperature and understanding its role on phase equilibria. Several compositions were prepared to track the stability fields of olivine (high melt Mg/Si ratio) and orthopyroxene (low melt Mg/Si ratio) in both S-saturated melts and S-free melts. A range of reduced conditions were obtained by using different Si/SiO₂ ratios in the mixes. S-saturated experiments show increasing S abundances in the silicate melts (∼ 1–9 wt%) as fO₂ decreases (from IW -2.9 to IW -6.2, IW representing the iron-wüstite thermodynamic equilibrium). Parameterizing our experimental results gives the liquidus depression as a function of the sulfur content in the melt (mol. fraction):$∆T_{\mathrm{liq}}\left(°C\right)=-65208.22{\left(S\right)}_{\mathrm{melt}}^3+16595.32{\left(S\right)}_{\mathrm{melt}}^2+532.31{\left(S\right)}_{\mathrm{melt}}$

(MSWD = 3.24; SEE = 35 °C)

The range of sulfur concentration in our experimental melts would cause a liquidus depression of ca. 20–190 °C. Moreover, our experiments illustrate the role of sulfur in promoting the stability field of orthopyroxene over that of olivine which has major implications for the crystallization of the Mercurian magma ocean and the primordial mineralogical stratification of the mantle. In addition, the presence of sulfur lowers the pressure and temperature conditions of the olivine-orthopyroxene cotectic.