High spatial resolution electron probe analysis of H2O-bearing aluminosilicate glasses

Abstract

Accuracy of the in situ major element analysis of silicate glasses is critical for insights into magmatic evolution, anatectic processes, and melt–fluid interactions. Electron probe microanalysis (EPMA), a widely used technique for microanalysis, faces limitations due to alkali ion migration (particularly Na⁺ and K⁺) under high current densities typical of micron-beam spots. Furthermore, the correction procedures are constrained by standard availability and reference data. This study addresses these challenges by analyzing a suite of H₂O-bearing aluminosilicate glass standards using a JEOL JXA-8530F EPMA under the optimized conditions of 1 μm beam spot size, 15 kV accelerating voltage, 1–5 nA beam currents, and counting times of 10 s on peak and 5 s on background. Our results demonstrated that Na₂O loss correlated linearly with both current intensity and H₂O content, exhibiting consistent proportionality across varying water contents and current conditions. In contrast, K₂O loss exhibited a threshold-dependent behavior, exhibiting a significant loss (≥5% loss) only in glasses with ≥4 wt% H₂O or under higher beam currents (≥3 nA). Notably, Na⁺ migration occurred more readily than K⁺ migration under identical analytical conditions. The observed alkali depletion was accompanied by an increase in Al₂O₃ and SiO₂ concentrations. These findings indicate that alkali mobility is controlled by both external factors (beam parameters) and internal conditions (specifically, glass composition, with volatile content playing a particularly important role). To minimize the impact of measurement variablility, we developed a correction protocol utilizing standard-derived calibration factors based on measured, analyzed and known concentration ratios. We recommended optimal analytical conditions (1 μm spot beam and 3–4 nA current) combined with matrix-matched H₂O-bearing standards. This methodology maintains the superior spatial resolution of EPMA while significantly improving the analytical accuracy of H₂O-bearing glasses. This approach is especially advantageous for analyzing minute melt inclusions in minerals and experimental melt quench products.