Oxygen vacancy substitution mechanism linked to ferric iron in bridgmanite with pressure
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.
Figure:
Substitution mechanisms in bridgmanite and MgFe2O4-phase. (a) Fe3+ substitution in bridgmanite under MgO-rich conditions. The maximum MgFeO2.5 content reported by Hummer and Fei (2012) at 25 GPa, 1,970–2,070 K is also shown for comparison. (b) Fe3+ substitution in bridgmanite under Fe2O3-rich conditions. (c). Substitution mechanisms in the MgFe2O4-phase. The data points with blue symbols at 27 GPa are from Fei et al. (2020). The error bars represent one standard deviation of the analyzed points by electron microprobe as shown in Table 1.
Fei, H., Liu, Z., Huang, R., Kamada, S., Hirao, N., Kawaguchi, S., McCammon, C. A., Katsura, T., Pressure destabilizes oxygen vacancies in bridgmanite, J. Geophys. Res.: Solid Earth, 126, e2021JB022437, 2021.
