The redox state, FeO content, and origin of sulfur-rich magmas on Mercury

MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) orbital observations of Mercury have revealed elevated S abundances, Ca-S and Mg-S correlations, and a low upper limit for ferrous iron in surface silicates. These data indicate the presence of Ca and/or Mg sulfides in volcanic rocks and a low oxygen fugacity (fO₂) in their parental magmas. We have evaluated coupled fO₂ and fS₂ values and FeO contents in Mercury's magmas from silicate-sulfide equilibria and empirical models for silicate melts and metallurgical slags. The evaluated fO₂ at 1700-1800 K is 4.5 to 7.3 log₁₀ units below the iron-wüstite buffer. These values correspond to 0.028-0.79 wt % FeO, implying that Fe must be also present in sulfides and metal and are also consistent with the composition of the partial melt of an enstatite chondrite. This derived upper limit for FeO is substantially lower than the limits obtained from reflectance measurements of Mercury's surface materials. The low fO₂ and FeO values provide new constraints for igneous processes on Mercury as well as the formation, evolution, and internal structure of the innermost planet.