High spatial resolution electron probe analysis of H 2O-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 ions migration (particularly Na+ and K+) under the high current densities typical of micron-beam spot. Furthermore, correction procedures have been 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 optimized conditions: 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 demonstrate that Na2O loss correlates linearly with both current intensity and H2O content, exhibiting consistent proportionality across varying water contents and current conditions. In contrast, K2O loss exhibited threshold-dependent behavior, becoming significant loss (≥5% loss) only in glasses with ≥4wt.% 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 the increases of the Al2O3 and SiO2 concentrations. These finding indicate that alkali mobility is controlled by both external factors (beam parameters) and internal conditions (glass composition, particularly volatile content). To minimize the effect, we developed a correction protocol utilizing standard-derived calibration factors based on measured analyzed and known concentration ratios. We recommend optimal analytical conditions (1 μm spot beam, 3–4 nA current) combined with matrix-matched H2O-bearing standards. This methodology maintains the superior spatial resolution of EPMA while significantly improving analytical accuracy for H2O-bearing glasses. This approach is especially advantageous for analyzing minute melt inclusions in minerals and experimental melt quench products.