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

The chemistry of bridgmanite is a crucial parameter for understanding the structure and dynamics of the Earth's lower mantle. Incorporating Al, Fe2+, and Fe3+ into MgSiO3 bridgmanite can significantly affect its physical properties. We investigated the substitution mechanisms of Fe3+ in bridgmanite under SiO2- and Fe2O3-saturated conditions at 27 GPa and 1700-2000 K using a multianvil apparatus. The fraction of Fe3+ in bridgmanite increases from 0.085 to 0.202 atoms pfu with increasing temperature. We observed that only the charge-coupled mechanism takes place in bridgmanite under the studied conditions by forming the FeFeO3 component, which increases from 4.0 +/- 0.6 mol % at 1700 K to 9.9 +/- 0.7 mol % at 2000 K. The absence of vacancies in bridgmanite under SiO2-saturated conditions implies that bridgmanite in mid-ocean ridge basalt layers of subducted slabs in the lower mantle should have higher viscosity and lower electrical conductivity than that in the surrounding mantle. These differences in bridgmanite properties should affect lower-mantle dynamics, for example, enhance slab penetration more deeply into the lower mantle due to viscosity difference between mid-ocean ridge basalt and surrounding bridgmanite.