Iron speciation using electron microprobe techniques: application to glassy melt pockets within a spinel lherzolite xenolith

TLDR: In this paper, a quantitative determination of Fe 3+ /ΣFe ratios using the electron microprobe is presented for glasses, based on the shift of the Fe-L α peak position between Fe 3 + and Fe 2+, resulting from differences in self-absorption.

Abstract: A quantitative determination of Fe 3+ /ΣFe ratios using the electron microprobe is presented for glasses. The measurements are based on the shift of the Fe- L α peak position between Fe 3+ and Fe 2+ , resulting from differences in self-absorption. We present new calibration curves for glasses with Fe contents as low as 2 wt.% Fe. This method was used for glasses from a metasomatized spinel lherzolite (Massif Central, France) in which highly vesicular glass associated with secondary minerals occurs in reaction pockets around corroded Cr-spinels. The glass was found to be oxidized homogeneously with Fe 3+ /ΣFe ≈ 0.70 (measured with an uncertainty of ±0.06), in contrast with the value of ∼0.15 that would correspond to the former melt under f O 2 conditions inferred from secondary minerals. The glass oxidation probably results from the dissociation of the melt-water content under degassing, prior to local hematite depositions (spinel rims and crack fillings).