Mantle melting and magma ocean dynamics on Mercury impacted by sulfur in reduced mafic magmas

Chemical data from the MESSENGER spacecraft reveal that Mercury’s lavas are unusually sulfur-rich, suggesting highly reduced conditions during their formation. As a major volatile, sulfur profoundly affects the physical and chemical properties of silicate melts, potentially impacting key processes such as magma ocean crystallization and mantle melting. Here we conducted near-liquidus experiments (1650–2000 °C and 3–5 GPa) to quantify the effect of sulfur on phase relations in olivine- and orthopyroxene-saturated mafic compositions representative of Mercury’s mantle. Our results show that elevated sulfur contents (up to 6 wt.% S) can depress the liquidus of Mercurian mantle by up to 200 °C. The liquidus depression of silicate melt is positively correlated with sulfur concentrations, negatively correlated with pressure, and compositionally-sensitive with a larger S-effect on higher Mg/Si melt. Using a newly developed parameterization for sulfur-bearing melting, we show that the mantle potential temperatures required to produce the volcanic provinces are lower than previously estimated. Furthermore, modeling of magma ocean cooling and crystallization dynamics indicates that sulfur-induced liquidus depression can extend solidification timescales by tens to hundreds to thousands of years, especially beneath an insulating graphite crust (>100 m). Using viscosity models, we find that sulfur also reduces the critical crystal size for settling, thus promoting fractional crystallization and formation of a chemically stratified mantle. These findings provide critical constraints on Mercury’s interior structure and show that sulfur lowers the solidus and liquidus, enhances melt production, and helps to explain Mercury’s fertile mantle, extensive crustal formation, and diverse surface lavas.