A XANES calibration for determining the oxidation state of iron in mantle garnet

Iron K-edge X-ray absorption near edge structure (XANES) spectra were recorded for synthetic garnets of the almandine-skiagite (Fe₃Al₂Si₃O₁₂-Fe₃Fe₂Si₃O₁₂), andradite-skiagite (Ca₃Fe₂Si₃O₁₂-Fe₃Fe₂Si₃O₁₂), Fe-knorringite-skiagite (Fe₃Cr₂Si₃O₁₂-Fe₃Fe₂Si₃O₁₂), and andradite-grossular (Ca₃Fe₂Si₃O₁₂-Ca₃Al₂Si₃O₁₂) solid solutions. Fe³⁺/ΣFe varied systematically in these samples from 0 to 1. The variation in the energy and intensity of spectral features as a function of Fe³⁺/ΣFe was investigated to identify correlations that could be used to determine Fe³⁺/ΣFe of unknowns. The pre-edge energy, which is commonly used for quantification, was found to be relatively insensitive for garnet, particularly at low values of Fe³⁺/ΣFe. The best correlation was for the absorption edge energy, which may provide an accurate and precise method for determining Fe³⁺/ΣFe of garnets in metapelitic rocks. The resulting calibration curve, however, significantly and non-systematically overestimates the Fe³⁺/ΣFe value of mantle garnets (from xenoliths and megacrysts) for which Fe³⁺/ΣFe had been determined previously by Mössbauer spectroscopy. This is probably a result of differences in composition between the synthetic and natural garnets. For the mantle garnets, Fe³⁺/ΣFe is strongly correlated with the intensity ratio of post-edge features at 7138.4 and 7161.7eV. The deviation of the Mössbauer results from a linear fit is less than 0.01 (1 σ). This suggests a new method for determining Fe³⁺/ΣFe from XANES spectra that does not require precise energy calibration or spectral fitting. The accuracy and precision for mantle garnets (within the compositional range studied: 0.75≤Mg/(Mg+Fe)≤0.86, 3.7-6.3wt.% CaO, and 0.3-7.4wt.% Cr₂O₃) is comparable to those of Mössbauer spectrosopy, however, XANES spectra can be acquired in ~15min, allowing large numbers of samples to be analysed or the distribution of Fe³⁺/ΣFe to be mapped with micron spatial resolution.