FTIR MICRO-REFLECTANCE MEASUREMENTS OF THE CO32- ION CONTENT IN BASANITE AND LEUCITITE GLASSES

Infrared spectroscopy provides a reliable method for rapid, non-destru ctive quantitative analysis of volatile species in silicate glasses, w ith applications to geochemistry and volcanology. The technique has be en 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, i ncluding the need for double polishing of parallel faces on thin sampl es, the errors associated with sample thickness measurement, and total absorption of the IR beam intensity for samples with high volatile co ntent. We have tested an alternative method for quantitative IR determ ination of volatile concentrations in silicate glasses, based on analy sis of the IR reflectivity signal. The reflectivity method requires pr eparation of a single polished glass surface, and no thickness measure ment of the sample is necessary. The technique is applied easily as a microbeam technique using apertures as small as a few mu m in diameter . The method should be particularly useful for volatile analysis of gl ass 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% CO 2), which could not be easily analyzed via IR transmission. We have us ed SIMS to standardize the technique. Two features observed in the ref lectance spectra near 1400 cm(-1) and 1500 cm(-1) are due to resonance of the infrared beam with the asymmetric stretching vibrations of car bonate groups. The contribution of these species to the total reflecti vity is directly correlated with the carbon abundance in the samples. This forms the basis for an empirical quantitative analysis. The optic al constants, including the IR absorption coefficients associated with the CO32- stretching vibrations, have been extracted by Kramers-Kroni g 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) f or the 1400 and 1500 cm(-1) bands, respectively, in excellent agreemen t with results of previous transmission studies, after orientation eff ects are taken into account.