3.5 KATSURA Tomoo

The MgAlO2.5 contents in bridgmanite as a function of pressure and bulk composition based on defect chemistry and its applications to volatile storage capacity in the lower mantle

Tomoo Katsura

Bayerisches Geoinstitut, University of Bayreuth

Bridgmanite is the most abundant mineral in the Earth. Therefore, knowledge regarding its volatile storage capacity is vital to understand chemical interaction between the surface and interior of the Earth. Bridgmanite incorporates alumina in the form of MgAlO2.5 in addition to Al2O3, and the MgAlO2.5 component is expected to accommodate volatiles such as water and argon because of the oxygen vacancy. For this reason, we investigated the MgAlO2.5 content in bridgmanite as a function of pressure to find that the MgAlO2.5 content decreases with increasing pressure and becomes virtually zero at pressures higher than 40 GPa. We therefore concluded that bridgmanite will not incorporate volatiles at depths greater than 1000 km in the Earth’s mantle. We also investigated the MgAlO2.5 content as a function of bulk composition to change its coexisting phases. We found that the MgAlO2.5 content decreases with decreasing bulk Mg/Si ratio, and is virtually zero when bridgmanite coexists with stishovite. Hence, we concluded that volatiles will not be transported into the lower mantle by being accommodated in bridgmanite in basaltic layers of subducted slabs.