Chemical Reaction Between Metallic Iron and a Limited Water Supply Under Pressure: Implications for Water Behavior at the Core‐Mantle Boundary

Water transportation to the deep lower mantle via plate subduction may induce a reaction between water and iron at the core‐mantle boundary. Recent experimental studies suggest that such a reaction may generate FeO2Hx‐rich domains, which can explain the seismic structures of the ultralow velocity zone in this region. In this study, the chemical reaction between metallic iron and a limited water supply at ~120 GPa was investigated using time‐resolved in situ synchrotron X‐ray diffraction measurements in combination with the laser‐heated diamond anvil cell technique. Contrary to the results of earlier studies, the formation of FeO instead of FeO2Hx without intermediate phases was observed. Considering the unlimited availability of iron in the core and the limited water supply resulting from mantle downflow, the FeO‐rich layers consisted of Fe‐bearing ferropericlase and postperovskite, which must have locally cumulated at the bottom of the mantle simultaneously with hydrogen incorporation into the core. Plain Language Summary Water strongly influences the structure, dynamics, and evolution of the deep Earth. Recent experimental studies suggest that hydrous phases play an important role as carriers of surface water to the deep mantle via the subduction of oceanic plates. Such deep‐water subduction processes may allow the surface water to reach the bottom of the mantle, where the mantle minerals are in direct contact with the iron at the core. Thus, the purpose of this study was to provide understanding regarding the behavior of water when it meets iron at the core‐mantle boundary. To investigate the reaction between water and iron at high pressures and temperatures, experiments were performed using in situ X‐ray diffraction measurements in combination with the diamond‐anvil cell technique. The results obtained confirmed the formation of FeO during the reaction. Thus, the deep water cycle may produce FeO‐rich layers at the core‐mantle boundary, which may explain the seismic characteristics of the bottom of the mantle and at the top of the outer core. Key Points The reaction between iron and a limited supply of water at ~120 GPa was studied via in situ X‐ray diffraction measurements The deep water cycle may produce local FeO‐rich layers at the bottom of the mantle Excess FeO contents may explain the seismic characteristics at the core‐mantle boundary