Effects of Hydrogen on Fe-S Alloy System and their Implications for the Martian Core

Early Mars had a global magnetic field protecting the atmosphere from the solar wind, which was important for the potential habitability of the surface. The InSight mission found that the Martian core is large and less dense, suggesting the significance of light elements for the structure and dynamics of the region. Among light element candidates, data on the effects of hydrogen on the Fe-S alloy is sparse under the conditions expected for the Martian core. We conducted laser-heated diamond-anvil cell (LHDAC) experiments on iron sulfides (Fe3S and FeS) in a range of H2 concentrations in an Ar medium (3, 33, and 67%). Synchrotron X-ray diffraction data was measured at 30-40 GPa during and after laser heating up to 2100 K. Under high and intermediate (33-67%) H environments, Fe3S breaks down and converts to FeHx (fcc or dhcp), FeS (VI), and Fe2S at 1300-1900 K and 30-40 GPa. Under the same H-rich environments, FeS(III) converts to FeS (VI) and FeS2, and some FeHx (fcc or dhcp) also formed at 1300-1500 K and 30-40 GPa. At 40 GPa in a 67% H2 + Ar medium, we also observed the tetragonal FeSxHy phase reported previously by Piet et al. from Fe3S heated in a pure H medium, suggesting that the phase is stable only under a H rich environment. Under a very low H environment (3% H2), Fe3S is stable with heating at 30 GPa. At 40 GPa, Fe3S remains stable but small amounts of FeS(VI) and FeHx were observed. Under a low H environment, FeS(III) shows complex behaviors during heating but ultimately converts to FeS(VI), FeS2 and Fe2S at temperatures above 1900 K. Our results imply that: (1) Fe3S would remain as an important phase if the Martian core does not contain much H; (2) If the Martian core contains low S and high H, Fe3S would be no longer stable. Instead, separate H-rich and S-rich Fe alloys would exist together; (3) The tetragonal FeSxHy may exist only if the Martian core contains large amounts of H and S; and (4) If the Martian core has much S, even a small amount of H can stabilize FeS2. If the Martian core has low S, stability of FeS2 would be controlled by the H amount. Therefore, if a solid part exists in the present-day Martian core, mineralogy would depend strongly on the concentration of S and H.