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[SIT20-16] Alumina partitioning between silica minerals and bridgmanite at top lower mantle conditions: Implication for water cycling by basaltic crust
Keywords:mantle , water, slab, basaltic crust, subduction zone
Water is subducted into the deep mantle by subducting plates. Although discovery of hydrous minerals stable in mantle transition zone conditions suggests deep water reservoir at least down to these depths, water distribution in the lower mantle is not clear yet. Previous experimental studies reported high water concentration in aluminous silica minerals, which are stable in subducting basaltic crusts, compared with other lower mantle minerals by the Al-H coupling substitution. These results suggest subducting basaltic crusts can be a potential candidate to transport water into the deep lower mantle. One open question is water partitioning among minerals in a basaltic crust. Because the alumina content in silica minerals links with its water content, the alumina partitioning among minerals is crucial to more precisely understand water transport into the deep lower mantle. Aluminous bridgmanite is a major mineral and an alumina host in a lower-mantle basaltic crust. In this study, alumina partitioning between bridgmanite and silica minerals in a simplified basaltic system were investigated at top-lower-mantle conditions by means of Kawai-type multi-anvil experiments to better understand water transport and distribution in the lower mantle.
The present study showed Al-poor bridgmanite and stishovite coexist with Al-rich hydrous phases and alumina is strongly partitioned into the hydrous phases at a relatively low temperature of 1500 K. With increasing temperature, alumina contents in bridgmanite and stishovite increase. At 1900 K, CaCl2-type aluminous silica coexists with aluminous bridgmanite. These results indicate that alumina is strongly partitioned into silica minerals even under the coexistence of bridgmanite compared with the dry system, and water can be mainly retained in aluminous silica minerals. Thus, aluminous silica minerals are a main water carrier in the lower mantle.
The present study showed Al-poor bridgmanite and stishovite coexist with Al-rich hydrous phases and alumina is strongly partitioned into the hydrous phases at a relatively low temperature of 1500 K. With increasing temperature, alumina contents in bridgmanite and stishovite increase. At 1900 K, CaCl2-type aluminous silica coexists with aluminous bridgmanite. These results indicate that alumina is strongly partitioned into silica minerals even under the coexistence of bridgmanite compared with the dry system, and water can be mainly retained in aluminous silica minerals. Thus, aluminous silica minerals are a main water carrier in the lower mantle.