Phase Stability of Al-Bearing Dense Hydrous Magnesium Silicates at Topmost Lower Mantle Conditions: Implication for Water Transport in the Mantle

Abstract

In this study, we investigated the phase stability of Al-free and Al-bearing superhydrous phase B (shy-B) up to 55 GPa and 2500 K. In comparison with Al-free shy-B, the incorporation of 11.7 wt.% Al₂O₃ in shy-B expands the stability by ∼400–800 K at 20–30 GPa. The determined dehydration boundary for Al-bearing phase D indicates that it could be present even at normal mantle geotherm conditions at 30–40 GPa. Up to 23.8 mol.% Al₂O₃ can be dissolved into the structures of akimotoite and bridgmanite as a result of the decomposition reactions of Al-bearing shy-B and phase D between 20 and 40 GPa. Results of further experiments indicate that δ-AlOOH is the stable hydrous phase coexisting with Al-depleted bridgmanite at pressures above 52 GPa. This study shows that the incorporation of Al in dense hydrous magnesium silicates can have a profound impact on our picture of the water cycle in the deep Earth.