First‐Principles Determination of the Dissociation Phase Boundary of Phase H MgSiO4H2.

Phase H (MgSiO4H2) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H2O ice VII and MgSiO3 bridgmanite under pressure. However, the dissociation phase boundary under the mantle pressure and temperature conditions has not been determined thus far. In this work, the dissociation phase boundary of phase H is determined by the calculation of Gibbs free energy of H2O ice VII. The stability field of phase H is found to be significantly extended from 52 to 62 GPa by the inclusion of zero‐point vibrational energy. Phase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPa (at ∼1000 K). This result indicates that the transportation of water by dense hydrous magnesium silicates may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle in a pure Mg‐endmember composition. Key Points: The dissociation phase boundary of phase H MgSiO4H2 is determined by calculation of Gibbs free energy of ice VIIPhase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPaThe transportation of water by DHMSs may be terminated at a depth of approximately 1,500‐km depth in a pure Mg‐endmember composition [ABSTRACT FROM AUTHOR]