Stability of Fe2S and Fe12S7 to 125 GPa; implications for S-rich planetary cores

Abstract

The Fe-FeS phase relations were explored in the 22-25 wt. % S compositional range using single crystal X-ray diffraction in a laser heated diamond anvil cell. At pressures up to 125 GPa and at high temperatures, Fe2S and Fe12S7 were determined to cocrystallise. The novel Fe12S7 compound adopts the Co12P7 structure and Fe2S assumes the Fe2P-type structure. Applying these results to an Fe-FeS binary phase diagram exposes a complex series of FeS phase assemblages in the 16-25 wt. % S range, whereby minor changes in S content significantly affect the crystallisation sequence of Fe-S rich planetary cores. For core compositions S-rich of the Fe2SFe12S7 eutectic, the small density difference between solid Fe12S7 and Fe2S is likely to result in the formation of a core slush rather than a gravitationally stable inner core. Crystallisation of denser Fe2S at eutectic conditions could then result in gravitational settling of an Fe2S-rich inner core over time. As the Fe2P-type Fe2S has previously been identified forming at high temperatures to pressures as low as 22 GPa, the core crystallisation regimes determined here also elucidate that the Martian core sulfur composition must lie on the S-rich side of the Fe-Fe3S eutectic or even the S-rich side of the Fe3S-Fe2S eutectic to maintain a fully molten core.