Comparison of microanalytical methods for estimating H₂O contents of silicic volcanic glasses

Abstract

Three methods of estimating H20 contents of geologic glasses are compared: (1) ion
\nmicroprobe analysis (secondary ion mass spectrometry), (2) Fourier-transform infrared
\nspectroscopy (FTIR), and (3) electron microprobe analysis using the Na decay-curve method.
\nEach analytical method has its own advantages under certain conditions, depending
\non the relative importance of analytical accuracy, precision, sensitivity, spatial resolution,and convenience, and each is capable of providing reasonably accurate estimates of the H20, or total volatile, content of geologic glasses. The accuracy of ion microprobe analyses
\ndepends critically on the availability of well-characterized hydrous standard glasses. Precision is often better than 0,2 wt% (10). The method provides good spatial resolution (-15
\n#m) and the capability to determine simultaneously the abundance of other volatile species of interest (e.g., F, B). FTIR spectroscopy provides excellent analytical sensitivity (-10
\nppm), accuracy and precision «0.1 wt%), and the capability to determine the abundance
\nof H20 and C02 species (H20, OH-, C02' eOj-) in analyzed glasses, although the spatial
\nresolution (> 25-35 #m) is not as good as that of the ion microprobe. The main advantages
\nof the estimation of H20 contents of hydrous glasses using the electron microprobe are
\nexcellent spatial resolution (- 10 #m) and analytical convenience. The disadvantages are
\nthat accuracy and precision (>0.5 wt%) are not as good as those associated with the other
\nmethods, but, for certain applications, these uncertainties may be acceptable for the estimation of H20 contents of H20-rich (> 1 wt%) samples.