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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 MgAlO2.5 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 Al8/3O4 component can be dissolved into the calcium‐ferrite type MgAl2O4–Mg2SiO4–Al8/3O4 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 MgAlO2.5 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 MgAlO2.5 and AlAlO3. To constrain the substitution mechanism of Al3+ in bridgmanite, we investigate the MgAlO2.5 and AlAlO3 contents in bridgmanite coexisting with a calcium‐ferrite type phase and corundum at different temperatures. Our results demonstrate that the MgAlO2.5 content reaches saturation (3–4 mol%) when an additional Al‐bearing phase exists in the system. Key Points: The MgAlO2.5 content in bridgmanite remains constant with temperature when bridgmanite coexists with corundum and the CF‐phaseThe MgAlO2.5 content in bridgmanite reaches a solubility of 3–4 mol% when bridgmanite coexists with an additional Al‐bearing phaseA significant amount of the Al8/3O4 component can be dissolved into the CF‐phase [ABSTRACT FROM AUTHOR]

Bridgmanite may contain a large proportion of ferric iron in its crystal structure in the forms of FeFeO3 and MgFeO2.5 components. We investigated the pressure dependence of FeFeO3 and MgFeO2.5 contents in bridgmanite coexisting with MgFe2O4‐phase and with or without ferropericlase in the MgO‐SiO2‐Fe2O3 ternary system at 2,300 K, 33 and 40 GPa. Together with the experiments at 27 GPa reported in Fei et al. (2020, https://doi.org/10.1029/2019GL086296), our results show that the FeFeO3 and MgFeO2.5 contents in bridgmanite decrease from 7.6 to 5.3 mol % and from 2 to 3 mol % to nearly zero, respectively, with increasing pressure from 27 to 40 GPa. Accordingly, the total Fe3+ decreases from 0.18 to 0.11 pfu. The formation of oxygen vacancies (MgFeO2.5 component) in bridgmanite is therefore dramatically suppressed by pressure. Oxygen vacancies can be produced by ferric iron in Fe3+‐rich bridgmanite under the topmost lower mantle conditions, but the concentration should decrease rapidly with increasing pressure. The variation of oxygen‐vacancy content with depth may potentially affect the physical properties of bridgmanite and thus affect mantle dynamics. Plain Language Summary: Bridgmanite is the most abundant mineral in the Earth's lower mantle. Although its basic chemical formula is MgSiO3, large amounts of Fe3+ can be added in the following two ways: (1) Two Fe3+ replace Mg2+ and Si4+ and form the FeFeO3 component. This is called charge‐coupled substitution because the overall charge does not change. (2) One Fe3+ replaces one Si4+ and forms the MgFeO2.5 component. Here the loss of positive charge is compensated by a loss of oxygen and is therefore called oxygen‐vacancy substitution. In this study, we measured the effect of pressure on the abundance of these two components of bridgmanite. We found that the MgFeO2.5 content decreases greatly with increasing pressure. Some oxygen sites may therefore be vacant in bridgmanite at the top of lower mantle, but the concentration of oxygen vacancies should decrease rapidly in deeper regions. The decrease of oxygen‐vacancy concentration in bridgmanite will change the nature of the lower mantle, for example, rocks will become harder, and electrical conductivity will decrease with increasing depth. Key Points: MgFeO2.5, FeFeO3, and total Fe3+ contents in bridgmanite decrease with increasing pressureFe3+‐linked oxygen vacancies in bridgmanite are destabilized by increasing pressureMgFeO2.5 can be formed in Fe3+‐rich bridgmanite under the topmost lower mantle conditions [ABSTRACT FROM AUTHOR]

(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. Plain Language Summary: Earth′s lower mantle, spanning from approximately 660 to 2,890 km depth, is mainly composed of bridgmanite, (Mg, Fe, Al)(Si, Al)O3 with a distorted perovskite‐type structure. In the topmost 500 km of the lower mantle, experiments showed that an MgAlO2.5 oxygen vacancy component may be present in bridgmanite. However, the effect of oxygen vacancies on the thermoelastic properties of bridgmanite is still not well constrained. These parameters are of crucial importance not only to model the thermodynamic stability of bridgmanite components at high‐pressure and high‐temperature but also to calculate the physical properties of lower mantle rocks. Here, we have determined the high‐pressure and high‐pressure‐temperature equations of state of three bridgmanite samples having different concentrations of Al and oxygen vacancies using X‐ray diffraction in diamond anvil cells. These data are complemented by the analysis of the bridgmanite crystal structure as a function of pressure, which revealed that oxygen vacancies have a minor effect on the elastic properties of bridgmanite at concentrations relevant for the lower mantle. Combining our results with experimental data on the stability of bridgmanite in the MgO–AlO1.5–SiO2 system, we have modeled the thermodynamic stability of the MgAlO2.5 component as a function of pressure and bulk Al content. Key Points: The thermal equation of state of Al‐bearing bridgmanite was determined for the first time using single crystalsThe bulk compressibility of Al‐bearing bridgmanite is mostly controlled by Al‐Si substitution in the octahedral siteUsing the obtained equation of state parameters, we modeled the pressure dependency of oxygen vacancies in bridgmanite in MgO–AlO1.5–SiO2 [ABSTRACT FROM AUTHOR]