Iron redox variations in Australasian Muong Nong‐type tektites

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Abstract Two thin sections of Muong Nong‐type tektites from the Australasian tektite strewn field have been analyzed by Fe K‐edge X‐ray absorption near edge spectroscopy (XANES), using a hundreds‐of‐micrometers–sized beam suitable for spatially resolved analysis of the Fe oxidation state across distinct regions of the samples. Earlier analyses with an unfocused beam were inconclusive regarding different amounts of oxidized iron in the Muong Nong‐type tektites, but did indicate different chemical compositions of the lighter and darker colored layers. Experimental XANES spectra are very similar in shape to those of other tektites. However, small and reproducible changes were found in the pre‐edge peak involving the centroid energy: the pre‐edge peak of the spectra collected within the dark layers are reproducibly 0.2 eV at higher energy than those of the spectra collected within the light matrix. This difference in energy position is four times the estimated energy reproducibility and, therefore, is significant. By comparison with pre‐edge peak data of Fe model compounds, we estimate the Fe 3+ /(Fe 2+ +Fe 3+ ) ratios in the light matrix and dark layers to be 5% and 15% (±5), respectively. The heterogeneous distribution of the Fe oxidation state in Muong Nong‐type tektites, as opposed to the homogeneous Fe oxidation state distribution in splash‐form tektites, is consistent with previous hypotheses, based on volatile contents, of Muong Nong‐type tektites resulting from melts that experienced lower temperatures compared to those of splash‐form tektites.

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