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

Abstract

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.