57Fe Mossbauer spectroscopy analysis of spinels: Fe3+/Fetot quantification accuracy and consequences on fO2 estimate

In this study spinel data from literature were treated in order to highlight the influence of Fe3+/Fe-tot quantification accuracy on estimate of upper mantle f(O2). Particular regard was given to the importance of using recoil-free fraction f on Fe3+/Fe-tot ratios quantification derived from Fe-57 Mossbauer spectroscopy analysis (MS). For this aim Fe3+/Fe-tot ratios obtained by Fe-57 Mossbauer spectroscopy at 298 K (RT) were corrected with f values calculated by De Grave and Van Alboom (1991). Uncorrected and corrected Fe3+/Fe-tot ratios allowed to calculate f(O2) by using oxygen geobarometric method. Oxygen fugacities derived by the uncorrected ratios are revealed systematically higher than those derived by corrected ones, with a difference up to 0.7 log f(O2) units. By using chemical data from two literature works it was highlighted that f(O2) overestimate is not dependent neither on rock chemical composition nor on the equilibrium phases involved in the calculation, but only on the Fe3+/Fe-tot ratios quantification accuracy.