First-Principles Study of FeO₂H_x Solid and Melt System at High Pressures: Implications for Ultralow-Velocity Zones

Pyrite-type FeO₂H_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 FeO₂H_x has smaller density and smaller bulk sound velocity compared to the isochemical P phase. As such, relatively small amounts of liquid FeO₂H_x could account for the observed seismic anomaly of ultralow-velocity zones. However, to maintain the liquid FeO₂H_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.