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 7161.7 eV. 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, 3.7–6.3 wt.% CaO, and 0.3–7.4 wt.% Cr2O3) is comparable to those of Mössbauer spectrosopy, however, XANES spectra can be acquired in ~ 15 min, allowing large numbers of samples to be analysed or the distribution of Fe3+/ΣFe to be mapped with micron spatial resolution.