Micro X-ray absorption near edge structure at the sulfur and iron K-edges in natural silicate glasses

Both sulfur and iron chemical environments were studied in natural compounds; sulfides, sulfates and silicate basaltic glasses which are supposed to have variable oxidation states, using XANES (X-ray absorption near edge structure) spectroscopy. Although X-ray absorption methods are well suited for such investigations, the size of natural glass inclusions trapped in volcanic minerals, ranging from a few micrometers to several tenths of micrometers, makes micro-spectroscopy necessary. Hence, we present the μ-XANES spectra at the sulfur and iron K-edges of olivine-hosted glass inclusions containing from 1000 to 1650 ppm S, and between 5.5 and 7.8 wt.% Fe. By combining both high energy and spatial resolutions, we demonstrate the ability of the μ-XANES to produce quantitative determination of sulfur and iron valence states. We have identified various species for sulfur, specifically S^II−, S^VI and possibly S^IV, in basaltic glass inclusions hosted in olivine grains. We propose a method to calibrate the proportion of Fe dissolved as Fe³⁺ in basaltic glasses for which the Fe³⁺/ΣFe ratio varies between 0.05 and 0.48 with a relative precision of less than 10%.