Iron oxidation states in silicate glass fragments and glass inclusions with a XANES micro-probe

The iron oxidation states and its environment in volcanic silicate glasses have been studied by combining micro-X-ray absorption near edge structure (XANES) experiments at the iron K-edge in silicate glass fragments and glass inclusions trapped during the crystal growth. Experiments were performed at the ESRF (ID22 beam-line) – a third generation synchrotron – using a focused beam (2×10 μm2) together with a high brilliance and a good energy resolution. The selected silicate glasses, basaltic in composition, contain between 5.5 and 8 wt% total Fe. In all XANES spectra, pre-peaks exhibit a multi-component structure, which is the convolution of information related to the valence state of iron (2+,3+), the site geometry and transition types. The Fe3+/ΣFe ratio of silicate glass fragments deduced from the XANES spectra is correlated to values determined by chemistry on bulk samples within the range 0.05–0.85. XANES spectra from glass inclusions hosted in natural olivine crystals may be interpreted considering the glass fragments as reference samples.