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 (Fe 3Al 2Si 3O 12–Fe 3Fe 2Si 3O 12), andradite–skiagite (Ca 3Fe 2Si 3O 12–Fe 3Fe 2Si 3O 12), Fe–knorringite–skiagite (Fe 3Cr 2Si 3O 12–Fe 3Fe 2Si 3O 12), and andradite–grossular (Ca 3Fe 2Si 3O 12–Ca 3Al 2Si 3O 12) solid solutions. Fe 3+/ΣFe varied systematically in these samples from 0 to 1. The variation in the energy and intensity of spectral features as a function of Fe 3+/ΣFe was investigated to identify correlations that could be used to determine Fe 3+/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 Fe 3+/ΣFe. The best correlation was for the absorption edge energy, which may provide an accurate and precise method for determining Fe 3+/ΣFe of garnets in metapelitic rocks. The resulting calibration curve, however, significantly and non-systematically overestimates the Fe 3+/ΣFe value of mantle garnets (from xenoliths and megacrysts) for which Fe 3+/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, Fe 3+/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 Fe 3+/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.% Cr 2O 3) 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 Fe 3+/ΣFe to be mapped with micron spatial resolution. ► New method for determining Fe 3+/SFe of mantle garnets from XANES spectra. ► Fe 3+/SFe quantified within ± 0.01. ► Potential for mapping the oxidation state of Fe with micron spatial resolution.