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We have studied the ferrous (Fe2+) and ferric (Fe3+) iron concentrations inAl-free Fe containing Mg-silicate perovskite (Mg-Pv) at pressure (P),temperature (T), and oxygen fugacity (fO2) conditions related to the lowermantle using a thermodynamic model based on ab-initio calculations. We considerthe oxidation reaction and the charge disproportionation reaction, both ofwhich can produce Fe3+ in Mg-Pv. The model shows qualitatively good agreementwith available experimental data on Fe3+/{\Sigma}Fe ({\Sigma}Fe = total Fe insystem), spin transitions, and equations of state. We predict that underlower-mantle conditions Fe3+/{\Sigma}Fe determined by the chargedisproportionation is estimated to be 0.01-0.07 in Al-free Mg-Pv, suggestingthat low Al Mg-Pv in the uppermost pyrolitic mantle (where majoritic garnetcontains most of the Al) and in the harzburgitic heterogeneities throughout thelower mantle contains very little Fe3+. We find that the volume reduction bythe spin transition of the B-site Fe3+ leads to a minimum Fe3+/{\Sigma}Fe inMg-Pv at mid-mantle pressures. The model shows that configurational entropy isa key driving force to create Fe3+ and therefore Fe3+ content is highlytemperature sensitive. The temperature sensitivity may lead to a maximumFe3+/{\Sigma}Fe in Mg-Pv in warm regions at the core-mantle boundary region,such as Large Low Shear Velocity Provinces (LLSVPs), potentially altering thephysical (e.g., bulk modulus) and transport (e.g., thermal and electricalconductivities) properties of the heterogeneities.

Origin of Fe3+ in Fe-containing, Al-free Mantle Silicate Perovskite