Determination of Fe^{3 +}/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB

To improve the accuracy of X-ray absorption near-edge structure (XANES) calibrations for the Fe^{3 +}/ΣFe ratio in basaltic glasses, we reevaluated the Fe^{3 +}/ΣFe ratios of glasses used as standards by Cottrell et al. (2009), and available to the community (NMNH catalog #117393). Here we take into account the effect of recoilless fraction on the apparent Fe^{3 +}/ΣFe ratio measured from room temperature Mössbauer spectra in that original study. Recoilless fractions were determined from Mössbauer spectra collected from 40 to 320 K for one basaltic glass, AII_25, and from spectra acquired at 10 K for the 13 basaltic glass standards from the study of Cottrell et al. (2009). The recoilless fractions, f, of Fe^{2 +} and Fe^{3 +} in glass AII_25 were calculated from variable-temperature Mössbauer spectra by a relative method (RM), based on the temperature dependence of the absorption area ratios of Fe^{3 +} and Fe^{2 +} paramagnetic doublets. The resulting correction factor applicable to room temperature determinations (C₂₉₃, the ratio of recoilless fractions for Fe^{3 +} and Fe^{2 +}) is 1.125 ± 0.068 (2σ). Comparison of the spectra at 10 K for the 13 basaltic glasses with those from 293 K suggests C₂₉₃ equal to 1.105 ± 0.015 (2σ). Although the 10 K estimate is more precise, the relative method determination is believed to be more accurate, as it does not depend on the assumption that recoilless fractions are equal at 10 K. Applying the effects of recoilless fraction to the relationship between Mössbauer-determined Fe^{3 +}/ΣFe ratios and revised average XANES pre-edge centroids for the 13 standard glasses allows regression of a new calibration of the relationship between the Fe XANES pre-edge centroid energy and the Fe^{3 +}/ΣFe ratio of silicate glass. We also update the calibration of Zhang et al. (2016) for andesites and present a more general calibration for mafic glasses including both basaltic and andesitic compositions. Recalculation of Fe^{3 +}/ΣFe ratios for the mid-ocean ridge basalt (MORB) glasses analyzed previously by XANES by Cottrell and Kelley (2011) results in an average Fe^{3 +}/ΣFe ratio for MORB of 0.143 ± 0.008 (1σ), taking into account only analytical precision, and 0.14 ± 0.01(1σ), taking into account uncertainty on the value of C₂₉₃. This revised average is lower than the average of 0.16 ± 0.01 given by Cottrell and Kelley (2011). The revised average oxygen fugacity for MORB based on the database of Cottrell and Kelley (2011) is − 0.18 ± 0.16 log units less than the quartz-fayalite-magnetite buffer of Frost (1991) at 100 kPa (∆ QFM = − 0.18 ± 0.16).