FTIR micro-reflectance measurements of the CO₃^2- ion content in basanite and leucitite glasses

Infrared spectroscopy provides a reliable method for rapid, non-destructive quantitative analysis of volatile species in silicate glasses, with applications to geochemistry and volcanology. The technique has been extensively calibrated for transmission measurements, in which the species concentration present is correlated with the height or area of characteristic absorption peaks, for doubly polished samples of known thickness. There are several drawbacks associated with this method, including the need for double polishing of parallel faces on thin samples, the errors associated with sample thickness measurement, and total absorption of the IR beam intensity for samples with high volatile content. We have tested an alternative method for quantitative IR determination of volatile concentrations in silicate glasses, based on analysis of the IR reflectivity signal. The reflectivity method requires preparation of a single polished glass surface, and no thickness measurement of the sample is necessary. The technique is applied easily as a microbeam technique using apertures as small as a few um in diameter. The method should be particularly useful for volatile analysis of glass inclusions in phenocrystals, or standard samples in thin section. We have developed the methodology for the technique using a series of basanite and leucitite glasses with high carbonate contents (>1 wt% CO₂), which could not be easily analyzed via IR transmission. We have used SIMS to standardize the technique. Two features observed in the reflectance spectra near 1400 cm^-1 and 1500 cm^-1 are due to resonance of the infrared beam with the asymmetric stretching vibrations of carbonate groups. The contribution of these species to the total reflectivity is directly correlated with the carbon abundance in the samples. This forms the basis for an empirical quantitative analysis. The optical constants, including the IR absorption coefficients associated with the CO₃^2- stretching vibrations, have been extracted by Kramers-Kronig analysis of the reflectivity data. The molar extinction coefficients are 1119 ± 138 L mol^-1 cm^-1 and 1198 ±145 L mol^-1 cm^-1 for the 1400 and 1500 cm^-1 bands, respectively, in excellent agreement with results of previous transmission studies, after orientation effects are taken into account.