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 microprobe analysis (secondary ion mass spectrometry), (2) Fourier-transform infrared spectroscopy (FTIR), and (3) electron microprobe analysis using the Na decay-curve method. Each analytical method has its own advantages under certain conditions, depending on 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 depends 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 #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 ppm), accuracy and precision «0.1 wt%), and the capability to determine the abundance of H20 and C02 species (H20, OH-, C02' eOj-) in analyzed glasses, although the spatial resolution (> 25-35 #m) is not as good as that of the ion microprobe. The main advantages of the estimation of H20 contents of hydrous glasses using the electron microprobe are excellent spatial resolution (-10 #m) and analytical convenience. The disadvantages are that accuracy 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 H20 contents of H20-rich (> 1 wt%) samples.

Gas‐saturated crystallization and degassing in large‐volume, crystal‐rich dacitic magmas from the Altiplano‐Puna, northern Chile

“…Other elements were calibrated on natural and synthetic mineral standards. EPMAs for hydrous rhyolitic reference glasses are presented in Table 2 and show excellent agreement with previously published values [Devine et al, 1995]. showed that unless additional corrections are applied, the indirect methods will yield erroneously high results.…”

Magma plumbing beneath Anak Krakatau volcano, Indonesia: evidence for multiple magma storage regions

“…The pre-eruptive volatile content of a rock can be approximated from the mass deficiency in EMP analyses of groundmass glass and melt inclusions ("the difference method"), as described in Devine et al (1995), provided that the volatiles make up >1%. Anak Krakatau melt inclusions represent the melt composition of the magma during different stages of its evolution prior to degassing and eruption.…”

“…The mass deficiency in the glass inclusions measured here ranges from 1.2 to 4.9 wt% (average = 2.4 wt%; n = 9). However, the precision of the "difference method" is not very high (generally around ±0.5 %; Devine et al 1995). Mandeville et al (1996a), also using the "difference method", estimated the pre-eruptive volatile content in 1883 Krakatau rhyolites and dacites to be 4 ± 0.5 wt.…”

Effects of fO2 and H2O on andesite phase relations between 2 and 4 kbar

“…In this paper the effects of fo2 and melt H20 content on <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 +1 <1 <1 <1 <1 <1 <1 -1 <1 <1 <1 +1 -1 <1 <1 <1 -1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 3.96 + 1 g1(75), pl(16), opx(6), mt(3), ame 0.21 < 1 g1(66), p1(24), opx(7), mt(3), ame 0.63 < 1 gl(60), pl(30), opx(7), mt(3), ame 0.12 < 1 g1(95), pl(1), mt(4), ame 2.75 < 1 g1(63), p1(29), opx(6), mt(2), ame 0.15 < 1 g1(62), p1(28), opx(7), mt(3), ame 1.03 < 1 g1(59), pl(31), opx(5), cpx(3), mt(2), ame 0.57 < 1 all20 calculated from H20 in glass using the model of Burnham [1979] (see also text); log fo2 calculated from experimental fH2 (see text) and calculated fH20 (obtained from aH20); ANNO = log fo2 -log fo2 of the NNO buffer calculated at P and T [Chou, 1987] (Table 2), infrared spectroscopy could not be used as a routine method for H20 analysis. The glass H20 contents were thus measured using the "by-difference" method [Devine et al, 1995]. The difference from 100% of electron microprobe analyses (after correction of the alkalies) was calibrated against the dissolved glass H20 content by using the three hydrous glasses described above as standards, analyzed together with the experimental glasses during each microprobe session.…”