Compositional dependence of molar absorptivities of near-infrared OH- and H2O bands in rhyolitic to basaltic glasses

Molar absorption coefficients (molar absorptivities) of the near infrared c ombination bands at 4500 and 5200 cm(-1) assigned to OH groups and H2O mole cules, respectively, were determined for glasses of dacitic, andesitic and basaltic compositions. Total water contents (range 1.5-6.3 wt.%) of the sam ples used in the calibration were determined by pyrolysis and subsequent Ka rl-Fischer Titration. Different combinations of baseline types and intensit y measure (peak height/area) were applied to investigate the effect of eval uation procedure on infrared spectroscopic determination of apparent specie s concentrations and total water. The best reproducibility of total water w as obtained by modeling the baseline of the combination bands by two gaussi ans at similar to 5700 and similar to 4000 cm(-1) (GG type baseline) and ev aluating peak heights (maximum deviation of +/-10.17 wt.% water). Plots of normalized absorbances are consistent with identical ratios of the absorpti on coefficients, epsilon (H2O)/epsilon (OH), for dacitic, andesitic and bas altic compositions as well as for a rhyolitic composition (data from Wither s and Behrens [Withers, A.C., Behrens, H., 1999. Temperature induced change s in the NIR spectra of hydrous albitic and rhyolitic glasses between 300 a nd 100 K. Phys. Chem. Minerals, 27, 119-132]). A parabolic equation is prop osed to predict the molar linear and integrated absorption coefficients as a function of the SiO2, content of the glass within the range of water cont ents used in the calibration. For example, using the GG type baseline and e valuating peak heights, we obtained epsilon (H2O)/epsilon (OH) = 1.13 and e psilon (H2O) [in 1 mol(-1) cm(-1)] = 2.290 X 10(-4) x (wt.% SiO2)(2). At a given water content and quench rate, OH concentrations are higher in andesi tic than in dacitic glasses which is consistent with higher fictive tempera tures of hydrous andesitic glasses containing more than 1.5 wt.% water. (C) 2001 Elsevier Science B.V. All rights reserved.