Pressure Destabilizes Oxygen Vacancies in Bridgmanite

Abstract

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.

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

“…Another essential strategy in our series of investigations is to uniquely control the bridgmanite chemistry via the coexisting phases based on the phase rule. Using these strategies, the phase relations of the MgO–Al 2 O 3 –SiO 2 , MgO–Fe 2 O 3 –SiO 2 , and MgO–FeAlO 3 –SiO 2 systems with specific coexisting phases as a function of pressure and temperature have been investigated in recent studies (Fei et al., 2020, 2021; Liu, Akaogi & Katsura, 2019; Liu, Ballaran, et al., 2019; Liu, Ishii, & Katsura, 2017; Liu et al., 2020).…”

Ferric Iron Substitution Mechanism in Bridgmanite under SiO2-Saturated Conditions at 27 GPa

“…In general, vacancies enhance atomic diffusion, which may affect diffusion-controlled physical and chemical processes and thus affect mantle dynamics. Although cation vacancies should reduce bridgmanite viscosity, − Fei et al proposed that the oxygen vacancies can also reduce bridgmanite viscosity. Furthermore, Fei et al argued that the presence of the MgFeO 2.5 component should increase the ionic conductivity of bridgmanite.…”

“…Although cation vacancies should reduce bridgmanite viscosity, − Fei et al proposed that the oxygen vacancies can also reduce bridgmanite viscosity. Furthermore, Fei et al argued that the presence of the MgFeO 2.5 component should increase the ionic conductivity of bridgmanite. Without this component, the viscosity of bridgmanite should be high, and the ionic conductivity should be low.…”

“…22 However, the Fe 3+ solubility and the MgFe 3+ O 2.5 fraction decrease with increasing pressure, leading to a pure chargecoupled substitution mechanism at 40 GPa. 23 The above results are vital for understanding the structure and properties of the bulk lower mantle because bridgmanite is saturated with Mg and Fe due to the coexistence with ferropericlase. 24−26 However, bridgmanite should coexist with stishovite instead of ferropericlase in the basaltic layer of subducted slabs.…”

“…However, their bridgmanite samples were not saturated with Fe because the phases coexisting with bridgmanite were periclase and stishovite. This system was reinvestigated under MgO- and Fe 2 O 3 -saturated conditions at constant pressure and different temperatures by Fei et al and at constant temperature and different pressures by Fei et al They concluded that Fe 3+ in bridgmanite is predominantly incorporated by the oxygen vacancy mechanism when Fe 3+ is below ∼0.025 atoms pfu, whereas the charge-coupled mechanism dominates when Fe 3+ > 0.025 atoms pfu . However, the Fe 3+ solubility and the MgFe 3+ O 2.5 fraction decrease with increasing pressure, leading to a pure charge-coupled substitution mechanism at 40 GPa …”