We report the stability field of a new high-pressure hydrous phase, phase H MgSiO4H2, and its implications for water transport into the deep lower mantle. We observed the existence of hydrous phase H at pressures around 50 GPa, and this phase was stable up to 60 GPa. Our results, together with those of previous works, indicate that pure phase H MgSiO4H2 has a very narrow stability field in the pressure range 35 < P < 60 GPa, equivalent to the uppermost part of the lower mantle. The stability field expands significantly toward higher pressures and temperatures on dissolution of the hydrous AlOOH component. The hydrous phase H-phase δ solid solution (aluminous phase H), (MgSi,Al2)O4H2, is potentially the most important hydrous phase present under the deep lower mantle conditions.
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