Pressure Destabilizes Oxygen Vacancies in Bridgmanite.

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]