Thermal Equation of State and Structural Evolution of Al‐Bearing Bridgmanite

Abstract

Abstract(Mg, Fe, Al)(Si, Al)O3 bridgmanite is the most abundant mineral of Earth′s lower mantle. Al is incorporated in the crystal structure of bridgmanite through the Fe3+AlO3 and AlAlO3 charge coupled (CC) mechanisms, and the MgAlO2.5 oxygen vacancy (OV) mechanism. Oxygen vacancies are believed to cause a substantial decrease of the bulk modulus of aluminous bridgmanite based on first‐principles calculations on the MgAlO2.5 end‐member. However, there is no conclusive experimental evidence supporting this hypothesis due to the uncertainties on the chemical composition, crystal chemistry, and/or high‐pressure behavior of samples analyzed in previous studies. Here, we synthesized high‐quality single crystals of bridgmanite in the MgO–AlO1.5–SiO2 system with different bulk Al contents and degrees of CC and OV substitutions. Suitable crystals with different compositions were loaded in resistively heated diamond anvil cells and analyzed by synchrotron X‐ray diffraction at pressures up to approximately 80 GPa at room temperature and 35 GPa at temperatures up to 1,000 K. Single‐crystal structural refinements at high pressure show that the compressibility of bridgmanite is mainly controlled by Al–Si substitution in the octahedral site and that oxygen vacancies in bridgmanite have no detectable effect on the bulk modulus in the compositional range investigated here, which is that relevant to a pyrolytic lower mantle. The proportion of oxygen vacancies in Al‐bearing bridgmanite has been calculated using a thermodynamic model constrained using experimental data at 27 GPa and 2,000 K for an Fe‐free system and extrapolated to pressures equivalent to 1,250 km depth using the thermoelastic parameters of Al‐bearing bridgmanite determined in this study.