Aluminum and hydrogen partitioning between bridgmanite and high-pressure hydrous phases: Implications for water storage in the lower mantle

Abstract

We clarified the phase relations of MgSiO3-Al2O3-H2O system under the uppermost lower-mantle conditions and the partitioning of aluminum and hydrogen between bridgmanite and hydrous minerals of hydrous phase δ-H solid solution and aluminous hydrous phase D. Bridgmanite coexists with hydrous phase D and δ-H at 25-28 GPa and 1000-1100°C. Hydrous phase D becomes unstable above 1200°C, while hydrous phase δ-H remains up to 1400°C in the pressure range. Aluminum is strongly partitioned to both aluminous phases D and δ-H resulting in alumina depletion in bridgmanite. Fourier transform infrared spectroscopy indicates that bridgmanite contains undetectable water when coexisting with these hydrous phases, showing strong hydrogen partitioning into hydrous phases, such as phases D and δ-H. The depletion of alumina in bridgmanite modified the phase relations significantly in hydrated slabs descending into the lower mantle, i.e., the pressures of the garnet-bridgmanite and post-perovskite transformations are lowered under the wet conditions where these hydrous phases coexist. The dry nature of bridgmanite coexisting with hydrous phases suggests that the major water carriers in the lower mantle are hydrous phases. Bridgmanite cannot be the water reservoir at least in the upper part of the lower mantle and could provide dry rheology of the wet slabs in the lower mantle.