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• The effects of Al and Fe3+on the grain growth rates of bridgmanite are investigated. • The FeFeO 3 , AlAlO 3 , and FeAlO 3 components enhance the growth rate of bridgmanite. • The MgAlO 2.5 and MgFeO 2.5 have minor effects on the growth rate of bridgmanite. • Al and Fe3+ have negligible effect on lower mantle rheology due to their small contents. Trivalent cations such as Al3+ and Fe3+ can be incorporated into the crystal structure of bridgmanite either by the charge-coupled mechanism forming the FeFeO 3 , AlAlO 3 , and FeAlO 3 components or by the oxygen vacancy mechanism forming the MgAlO 2.5 and MgFeO 2.5 components. They may affect the physical properties of bridgmanite and thus affect lower mantle dynamics. In this study, we investigated the effects of Al and Fe3+ on the grain growth kinetics of bridgmanite at a pressure of 27 GPa and temperatures of 2000 – 2300 K by multi anvil experiments. The experimental results indicate that the FeFeO 3 , AlAlO 3 , and FeAlO 3 components enhance the growth rate of bridgmanite, while the MgAlO 2.5 and MgFeO 2.5 components have negligible effects. However, due to the relatively low Fe3+ and Al3+ contents of bridgmanite in the lower mantle, none of them affect the lower mantle rheology significantly. In particular, the mid-mantle viscosity jump interpreted from geoid analysis is unlikely to be caused by the decreasing of MgAlO 2.5 and MgFeO 2.5 concentrations with increasing depth. [ABSTRACT FROM AUTHOR]

Ferric iron can be incorporated into the crystal structure of bridgmanite by either oxygen vacancy substitution (MgFeO2.5 component) or charge‐coupled substitution (FeFeO3 component) mechanisms. We investigated the concentrations of MgFeO2.5 and FeFeO3 in bridgmanite in the MgO‐SiO2‐Fe2O3 system at 27 GPa and 1700–2300 K using a multianvil apparatus. The FeFeO3 content increases from 1.6 to 7.6 mol.% and from 5.7 to 17.9 mol.% with and without coexistence of (Mg,Fe)O, respectively, with increasing temperature from 1700 to 2300 K. In contrast, the MgFeO2.5 content does not show clear temperature dependence, that is, ~2–3 and < 2 mol.% with and without the coexistence of (Mg,Fe)O, respectively. Therefore, the presence of (Mg,Fe)O enhances the oxygen vacancy substitution for Fe3+ in bridgmanite. It is predicted that Fe3+ is predominantly substituted following the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ is below ~0.025 pfu, whereas the charge‐coupled mechanism occurs when Fe3+ > 0.025 pfu. Plain Language Summary: Bridgmanite, the most abundant mineral of the Earth's lower mantle, can contain Fe3+ although the valance of iron is 2+ in general. An important question is how Fe3+ is substituted in the crystal structure of bridgmanite. It may form the MgFeO2.5 component, in which oxygen anions are partly missing. Or it may form the FeFeO3 component, which has no missing cations or anions. Since bridgmanite is present in the lower mantle together with (Mg,Fe)O, we investigated the MgFeO2.5 and FeFeO3 contents in Al‐free bridgmanite that coexists with and without (Mg,Fe)O under the topmost lower mantle conditions. The results show that the presence of (Mg,Fe)O enhances the formation of MgFeO2.5. The solubility of MgFeO2.5 component is about 2.5 mol.% in bridgmanite that coexists with (Mg,Fe)O, whereas it is nearly zero when (Mg,Fe)O is absent. Key Points: The presence of (Mg,Fe)O enhances the formation of the MgFeO2.5 component in Al‐free bridgmaniteFe3+ substitution predominantly follows the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ content is lowThe solubility of the MgFeO2.5 component in Al‐free bridgmanite is about 0.025 pfu, and relatively insensitive to temperature [ABSTRACT FROM AUTHOR]