Iron oxidation state in lower mantle mineral assemblages - I. Empirical relations derived from high-pressure experiments

The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe3+/SigmaFe, synthetic assemblages of (Mg,Fe)(Si,Al)O-3 perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650-1850 degreesC using a multianvil press. (Mg,Fe)(Si,Al)O-3 majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mossbauer spectra were measured for each run product, and a robust fitting method was developed for Mossbauer spectra using EELS results as a standard that enabled Fe3+/SigmaFe of (Mg,Fe)(Si,Al)O-3 perovskite to be determined from Mossbauer spectra of multiphase assemblages. There is a close to linear variation between Fe3+/SigmaFe and Al concentration in (Mg,Fe)(Si,Al)O-3 perovskite, independent of oxygen fugacity. The concentration of Fe3+ in (Mg,Fe)O increases with increasing iron concentration along curves of constant oxygen fugacity, where higher oxygen fugacity stabilises greater Fe3+ concentrations. Fe2+/Mg partition coefficients calculated from chemical composition data corrected for measured Fe3+/SigmaFe showed values nearly identical within experimental error for all samples, and independent of Al concentration and oxygen fugacity. Simple empirical relations were derived to calculate Fe3+/SigmaFe in (Mg,Fe)(Si,Al)O-3 perovskite and (Mg,Fe)O ferropericlase samples for which no Mossbauer or EELS data were available, and tested by applying them to calculation of Fe2+/Mg partition coefficients from literature data for (Mg,Fe)(Si,Al)O-3 perovskite-(Mg,Fe)O assemblages where only total iron concentrations had been measured. Results showed Fe2+/Mg partition coefficients that were equal to existing values within experimental error, hence confirming the validity of the empirical relations. (C) 2004 Elsevier B.V. All fights reserved.