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Melting of Al‐Rich Phase D up to the Uppermost Lower Mantle and Transportation of H2O to the Deep Earth

We investigated the stability of the Al‐rich dense hydrous magnesium silicate Phase D (PhD) in a MgO‐Al2O3‐SiO2‐H2O system between 14 and 25 GPa at 900–1,500 °C. Al‐rich PhD has a very wide stability region from 900 °C and 14 GPa to at least 1,500 °C and 25 GPa, showing strong temperature stability with increasing pressure. Al‐rich PhD decomposes to Phase Egg at pressure of the mantle transition zone, whereas it decomposes to δ‐AlOOH phase with a temperature increase at pressure of the uppermost lower mantle. X‐ray diffraction and Raman spectroscopy measurements of Al‐rich PhD show that the unit‐cell volume is slightly larger, but the Raman spectra resemble that of Al‐free PhD. The wide stability region of Al‐bearing PhD would contribute an important storage site for water in the mantle transition zone, suggesting that it can deliver a certain amount of water into the lower mantle along hot subduction and even at the normal mantle geothermal P‐T condition. Furthermore, the dehydration of Al‐bearing PhD might be responsible for a series of observed seismic discontinuities from the transition zone to the uppermost lower mantle and even for deep earthquakes in some typical locations.