Stability of hydrous phases in the transition zone and the upper most part of the lower mantle

Phase relation for Mg2SiO4+20.4 wt.%H2O composition was studied in the pressure range from 11 GPa to 22.5 GPa. The sequence of the phase transitions in the temperature range from 800°C to 1000°C with increasing pressure was as follows; Forsterite + Water → Phase A + Clinoenstatite + Water → Phase E + Water → Superhydrous phase B + Phase F + Water → Brucite + Phase F (+Water). Coexistence of brucite and phase F was observed at pressures above 20 GPa. These phases are stable at least up to 22.5 GPa and around 1000°C, and are the candidates for the hydrous phase in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle.