Experimental constraints on Mercuryʹs core composition

The recent discovery of high S concentrations on the surface of Mercury by spacecraft measurements from the MESSENGER mission provides the potential to place new constraints on the composition of Mercuryʹs large metallic core. In this work, we conducted a set of systematic equilibrium metal–silicate experiments that determined the effect of different metallic compositions in the Fe–S–Si system on the S concentration in the coexisting silicate melt. We find that metallic melts with a range of S and Si combinations can be in equilibrium with silicate melts with S contents consistent with Mercuryʹs surface, but that such silicate melts contain Fe contents lower than measured for Mercuryʹs surface. If Mercuryʹs surface S abundance is representative of the planetʹs bulk silicate composition and if the planet experienced metal–silicate equilibrium during planetary core formation, then these results place boundaries on the range of possible combinations of Si and S that could be present as the light elements in Mercuryʹs core and suggest that Mercuryʹs core likely contains Si. Except for core compositions with extreme abundances of Si, bulk Mercury compositions calculated by using the newly determined range of potential S and Si core compositions do not resemble primitive meteorite compositions.