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

摘要

A quantitative determination of Fe~(3+)/SFe ratios using the electron microprobe is presented for glasses. The measurements are based on the shift of the Fe-Lα 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 highlyvesicular glass associated with secondaryminerals occurs in reaction pockets around corroded Cr-spinels. The glass was found to be oxidized homogeneouslywith Fe~(3+)/SFe ≈0.70 (measured with an uncertaintyof ±0.06), in contrast with the value of ～0.15 that would correspond to the former melt under fO_2 conditions inferred from secondaryminerals. 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).