Aluminum Components in Bridgmanite Coexisting With Corundum and the CF‐Phase With Temperature

Abstract

Abstract Phase relations in the MgSiO₃–MgAl₂O₄–Al₂O₃ system are investigated at 27 GPa and 2000–2600 K using a multi‐anvil apparatus. The AlAlO₃ content in the bridgmanite increases from 8.6 to 20.0 mol% with increasing temperature from 2000 to 2600 K, while the MgAlO_2.5 content remains nearly constant, that is, 3–4 mol% at these temperatures. Therefore, the presence of an additional Al‐bearing phase suppresses the oxygen vacancy substitution for Al³⁺ in bridgmanite. Conversely, significant amounts of the Al_8/3O₄ component can be dissolved into the calcium‐ferrite type MgAl₂O₄–Mg₂SiO₄–Al_8/3O₄ phase (CF‐phase), implying that the CF‐phase likely contains some amount of vacancies. Therefore, the CF‐phase could also be a candidate mineral for transporting volatiles into the lower mantle. Our results, combined with previous studies on Al‐bearing bridgmanite, indicate that, once the Al per formula unit exceeds 0.12 in bridgmanite, the MgAlO_2.5 content remains nearly constant and the AlAlO₃ component becomes dominant.

Plain Language Summary: Bridgmanite, the most abundant mineral in the Earth's mantle, can contain Al³⁺ in the forms of MgAlO_2.5 and AlAlO₃. To constrain the substitution mechanism of Al³⁺ in bridgmanite, we investigate the MgAlO_2.5 and AlAlO₃ contents in bridgmanite coexisting with a calcium‐ferrite type phase and corundum at different temperatures. Our results demonstrate that the MgAlO_2.5 content reaches saturation (3–4 mol%) when an additional Al‐bearing phase exists in the system.

Key Points:

The MgAlO_2.5 content in bridgmanite remains constant with temperature when bridgmanite coexists with corundum and the CF‐phase.

The MgAlO_2.5 content in bridgmanite reaches a solubility of 3–4 mol% when bridgmanite coexists with an additional Al‐bearing phase.

A significant amount of the Al_8/3O₄ component can be dissolved into the CF‐phase.