Chemical Reactions Between Fe and H₂O up to Megabar Pressures and Implications for Water Storage in the Earth's Mantle and Core

We investigated the phase relations of the Fe-H₂O system at high pressures based on in situ X-ray diffraction experiments and first-principles calculations and demonstrate that FeHx and FeO are present at pressures less than ~78 GPa. A recently reported pyrite-structured FeO₂ was identified in the Fe-H₂O system at pressures greater than ~78 GPa after laser heating. The phase observed in this study has a unit cell volume 8%–11% larger than that of FeO₂, produced in the Fe-O binary system reported previously, suggesting that hydrogen might be retained in a FeO₂H_x crystal structure. Our observations indicate that H₂O is likely introduced into the deep Earth through reaction between iron and water during the accretion and separation of the metallic core. Additionally, reaction between Fe and H₂O would occur at the core-mantle boundary, given water released from hydrous subducting slabs that intersect with the metallic core. Accumulation of volatile-bearing iron compounds may provide new insights into the enigmatic seismic structures observed at the base of the lower mantle.