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

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

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004e%26psl.222..435m&link_type=abstract

Earth and Planetary Science Letters, Volume 222, Issue 2, p. 435-449.

Physics

10

Mössbauer Spectroscopy, Electron Energy Loss Spectroscopy, Ferric Iron, Cation Partitioning, Perovskite, Ferropericlase

Scientific paper

The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe3+/∑Fe, synthetic assemblages of (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650-1850 °C using a multianvil press. (Mg,Fe)(Si,Al)O3 majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mössbauer spectra were measured for each run product, and a robust fitting method was developed for Mössbauer spectra using EELS results as a standard that enabled Fe3+/∑Fe of (Mg,Fe)(Si,Al)O3 perovskite to be determined from Mössbauer spectra of multiphase assemblages. There is a close to linear variation between Fe3+/∑Fe and Al concentration in (Mg,Fe)(Si,Al)O3 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+/∑Fe 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+/∑Fe in (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase samples for which no Mössbauer 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)O3 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.

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