Origin of Fe³⁺ in Fe-containing, Al-free mantle silicate perovskite

We have studied the ferrous (Fe²⁺) and ferric (Fe³⁺) iron concentrations in Al-free Fe containing Mg-silicate perovskite (Mg-Pv) at pressure (P), temperature (T), and oxygen fugacity (fO₂) conditions related to the lower mantle using a thermodynamic model based on ab initio calculations. We consider the oxidation reaction and the charge disproportionation reaction, both of which can produce Fe³⁺ in Mg-Pv. The model shows qualitatively good agreement with available experimental data on Fe³⁺/σFe (σFe = total Fe in system), spin transitions, and equations of state. We predict that under lower-mantle conditions Fe³⁺/σFe determined by the charge disproportionation is estimated to be 0.01-0.07 in Al-free Mg-Pv, suggesting that low Al Mg-Pv in the uppermost pyrolitic mantle (where majoritic garnet contains most of the Al) and in the harzburgitic heterogeneities throughout the lower mantle contains very little Fe³⁺. We find that the volume reduction by the spin transition of the B-site Fe³⁺ leads to a minimum Fe³⁺/σFe in Mg-Pv at mid-mantle pressures. The model shows that configurational entropy is a key driving force to create Fe³⁺ and therefore Fe³⁺ content is highly temperature sensitive. The temperature sensitivity may lead to a maximum Fe³⁺/σFe in Mg-Pv in warm regions at the core-mantle boundary region, such as Large Low Shear Velocity Provinces (LLSVPs), potentially altering the physical (e.g., bulk modulus) and transport (e.g., thermal and electrical conductivities) properties of the heterogeneities.