First‐Principles Determination of the Dissociation Phase Boundary of Phase H MgSiO 4 H 2

Phase H (MgSiO 4 H 2 ) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H 2 O ice VII and MgSiO 3 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 H 2 O 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 MgSiO 3 bridgmanite and H 2 O 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.