Water content and iron oxidation state analysis of silicate glass using the electron probe

The water content (H₂O) and iron oxidation state (Fe³⁺/Fe_T) of silicate glass are useful compositional parameters to measure in volcanology and igneous petrology due to: (1) their influence on the chemical and physical properties of magmas, and (2) their use in constraining the pressure-temperature-composition conditions of magma storage and ascent. We present techniques using electron probe microanalysis (EPMA) that carefully mitigate for the effects of sub-surface charging, which causes beam damage and modifies X-ray emission. The calibrated volatiles-by-difference technique quantifies H₂O (assuming that this is the dominant volatile species) in silicate glass at a spatial resolution of 5-10 μm diameter with uncertainties of ±0.5-0.7 wt% and has been tested on basaltic glasses. The time-dependent-ratio flank method quantifies Fe³⁺/Fe_T at a spatial resolution of 20-60 μm diameter with uncertainties of ±0.1 and has been tested on a wide range of basaltic and peralkaline rhyolitic glasses. EPMA often requires straightforward sample preparation and is more accessible than other techniques used to quantify both H₂O and Fe³⁺/Fe_T (e.g., SIMS, FTIR, Raman, XANES, Mössbauer), although uncertainties are typically larger using EPMA. For H₂O, the spatial resolution of EPMA is often higher than other techniques (e.g., SIMS, FTIR), whereas for Fe³⁺/Fe_T it is often lower (e.g., Raman, XANES). Both EPMA techniques can be used on natural (e.g., melt inclusion and matrix glass) and experimental glasses, in addition to standard EPMA for quantification of major and minor element concentrations, for extensive chemical characterisation using EPMA.