Al partitioning between phase D and bridgmanite at the uppermost lower mantle pressure

Phase D is proposed to be the most important hydrous phase at the upper part of the lower mantle, and it has been shown to coexist with bridgmanite (Brg), the most abundant mineral and main host for Al2O3 in the lower mantle. The concentration of Al in Phase D could significantly increase the thermal stability field of Phase D, therefore, partitioning of Al between Brg and Phase D is of particular importance to constrain water distribution in the deep mantle. Here, we performed high P–T experiments in MgO–Al2O3–SiO2–H2O system to investigate the partitioning of Al between Brg and Phase D up to 32 GPa and 1350 °C. Our results indicated that Al distributes strongly into Phase D relative to Brg and the partition coefficient slightly decreases with increasing temperature. Al-bearing Phase D exhibits a very high thermal stability region, but it completely decomposed around 28 GPa and 1350 °C, at which point Brg coexisted with a large amount of melt. The depth ~ 850 km (28 GPa) is thus proposed to be the second choke point for hydrous minerals. This may shed new lights on several important geophysical observations in subduction zones.