An assessment of the quality of microanalysis of natural silicate glass using scanning electron microscope-based energy dispersive spectrometry (SEM-EDS)
The composition of volcanic glass records important clues into the origin and evolution of magmatic systems. However, the analysis of volcanic glass presents challenges when performed using electron-beam techniques, particularly due to Na mobility. While microanalysis of geological materials is usually performed using electron probe microanalyzer-based wavelength-dispersive spectrometry (EPMA-WDS), we present here results of glass analysis using energy-dispersive spectrometry with a scanning electron microscope (SEM-EDS). We use three U.S. Geological Survey (USGS) whole-rock reference materials (i.e., RGM-1, STM-1, QLO-1) that were fused into glass using a double-fusion technique. Results for 12 sessions over two months using the SEM-EDS reveal excellent reproducibility, agreement with the expected values for RGM-1 and STM-1, and excellent counting statistics when using live acquisition times of 15 s; results for QLO-1 suggest that the fused material is somewhat inhomogeneous. For longer acquisition times (30, 60, 90 s), significant Na migration is observed; we conclude that minimal Na migration takes place with acquisition times of 15 s using our analytical instrumentation. We also compile data for RGM-1 obtained using the same instrument over the last decade, and we further demonstrate the reliability of SEM-EDS for microanalysis of silicate glasses. Comparison of our results with EPMA-WDS results for a range of reference materials published by researchers at the USGS Alaska Volcano Observatory suggest that both EPMA-WDS and SEM-EDS can yield adequate analysis of volcanic glass, but SEM-EDS can potentially lead to higher precision for major elements (>1 wt.% concentration), while EPMA-WDS typically leads to higher precision for minor elements. The higher precision of SEM-EDS results for major elements, combined with the more widespread distribution, lower usage cost, and higher ease of use of such systems, makes analysis via SEM-EDS an attractive option for measuring major-element compositions of volcanic glass. However, it is critical to test the quality of each instrument given differences in hardware and software between SEM and EDS systems.