First‐Principles Study of FeO 2 H x Solid and Melt System at High Pressures: Implications for Ultralow‐Velocity Zones

Pyrite‐type FeO 2 H x (P phase) has recently been suggested as a possible alternative to explain ultralow‐velocity zones due to its low seismic velocity and high density. Here we report the results on the congruent melting temperature and melt properties of P phase at high pressures from first‐principles molecular dynamics simulations. The results show that P phase would likely be melted near the core–mantle boundary. Liquid FeO 2 H x has smaller density and smaller bulk sound velocity compared to the isochemical P phase. As such, relatively small amounts of liquid FeO 2 H x could account for the observed seismic anomaly of ultralow‐velocity zones. However, to maintain the liquid FeO 2 H x within the ultralow‐velocity zones against compaction requires special physical conditions, such as relatively high viscosity of the solid matrix and/or vigorous convection of the overlying mantle.