COMPARISON OF MICROANALYTICAL METHODS FOR ESTIMATING H2O CONTENTS OF SILICIC VOLCANIC GLASSES

Three methods of estimating H2O contents of geologic glasses are compa red: (1) ion microprobe analysis (secondary ion mass spectrometry), (2 ) Fourier-transform infrared spectroscopy (FTIR), and (3) electron mic roprobe analysis using the Na decay-curve method. Each analytical meth od has its own advantages under certain conditions, depending on the r elative importance of analytical accuracy, precision, sensitivity, spa tial resolution, and convenience, and each is capable of providing rea sonably accurate estimates of the H2O, or total volatile, content of g eologic glasses. The accuracy of ion microprobe analyses depends criti cally on the availability of well-characterized hydrous standard glass es. Precision is often better than 0.2 wt% (1 sigma). The method provi des good spatial resolution (similar to 15 mu m) and the capability to determine simultaneously the abundance of other volatile species of i nterest (e.g., F, B). FTIR spectroscopy provides excellent analytical sensitivity (similar to 10 ppm), accuracy and precision (<0.1 wt%), an d the capability to determine the abundance of H2O and CO2 species (H2 O, OH-, CO2, CO32-) in analyzed glasses, although the spatial resoluti on (>25-35 mu m) is not as good as that of the ion microprobe. The mai n advantages of the estimation of H2O contents of hydrous glasses usin g the electron microprobe are excellent spatial resolution (similar to 10 mu m) and analytical convenience. The disadvantages are that accur acy and precision (>0.5 wt%) are not as good as those associated with the other methods, but, for certain applications, these uncertainties may be acceptable for the estimation of H2O contents of H2O-rich (> 1 wt%) samples.