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.

Low-pressure polymorphs of AlOOH and FeOOH are common natural oxyhydroxides at the Earth's surface, which may transport hydrogen to the deep mantle via subduction. At elevated pressures, the low-pressure polymorphs transform into δ-AlOOH and ε-FeOOH with CaCl₂-type structure, which form a solid solution above 18 GPa. Nevertheless, few studies have examined the solid solution behavior of this binary system in detail. In this study, we ascertain the phase relations in the AlOOH–FeOOH binary system at 15–25 GPa and 700–1200 °C. X-ray diffraction (XRD) measurements of quenched samples show that δ-AlOOH and ε-FeOOH partly form solid solutions over wide pressure and temperature ranges. Our results demonstrate that a binary eutectic diagram is formed without dehydration or melting below 1200 °C at 20 GPa. We also observe that the maximum solubilities of Al and Fe in the solid solutions are more strongly influenced by temperature than pressure. Our results suggest that the CaCl₂-type hydroxides subducted into the deep mantle form a solid solution over a wide composition range. As AlOOH and FeOOH are present in hydrous crust, these phases may be subducted into the deep interior, transporting a significant amount of hydrogen to deeper regions. Therefore, a better understanding of this binary system may help elucidate the model geodynamic processes associated with the deep water cycling in the Earth.

Sound velocities of Al‐bearing phase D were investigated up to 22 GPa and 1300 K using in situ synchrotron X‐ray techniques combined with ultrasonic measurements in a multianvil apparatus. The isothermal bulk modulus of Al‐bearing phase D was found to be ~16.7% lower than that of Mg‐endmember, suggesting a strong effect of Al incorporation on the bulk modulus of phase D. Al‐bearing phase D has higher P‐ (V_P) and S‐ (V_S) wave velocities compared to other mantle transition zone minerals, up to 4.3% for V_P and up to 9.6% for V_S compared with hydrous iron‐bearing ringwoodite, which might engender reduction of velocity contrast between anhydrous mantle and hydrous slab components. The accumulation of hydrated slab components carrying Al‐phase D in the uppermost lower mantle can explain some local negative shear velocity perturbations (ΔV_S) up to −1.5%, although ΔV_P (−0.5%) is expected to remain below the detection limit of seismological techniques.