Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57 Fe 2 O 3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 ∞C. The Fe 3+ /SFe ratios were determined previously by 57 Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe 3+ /SFe ratios from XANES spectra were investigated. The energy of the 1s AE 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s AE 4s and crest (1s AE 4p) transitions. The Fe 3+ /SFe ratios determined from linear combinations of end-member spectra (Fe 3+ /SFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe 2+ or Fe 3+ coordination with the Fe 3+ /SFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe 3+ /SFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

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