Compositional dependence of molar absorptivities of near-infrared OH- and H 2O bands in rhyolitic to basaltic glasses

Abstract

Molar absorption coefficients (molar absorptivities) of the near infrared combination bands at 4500 and 5200 cm −1 assigned to OH groups and H 2O molecules, respectively, were determined for glasses of dacitic, andesitic and basaltic compositions. Total water contents (range 1.5–6.3 wt.%) of the samples used in the calibration were determined by pyrolysis and subsequent Karl–Fischer Titration. Different combinations of baseline types and intensity measure (peak height/area) were applied to investigate the effect of evaluation procedure on infrared spectroscopic determination of apparent species concentrations and total water. The best reproducibility of total water was obtained by modeling the baseline of the combination bands by two gaussians at ∼5700 and ∼4000 cm −1 (GG type baseline) and evaluating peak heights (maximum deviation of ±0.17 wt.% water). Plots of normalized absorbances are consistent with identical ratios of the absorption coefficients, ε H 2O / ε OH, for dacitic, andesitic and basaltic compositions as well as for a rhyolitic composition (data from Withers and Behrens [Withers, A.C., Behrens, H., 1999. Temperature induced changes in the NIR spectra of hydrous albitic and rhyolitic glasses between 300 and 100 K. Phys. Chem. Minerals, 27, 119–132]). A parabolic equation is proposed to predict the molar linear and integrated absorption coefficients as a function of the SiO 2 content of the glass within the range of water contents used in the calibration. For example, using the GG type baseline and evaluating peak heights, we obtained ε H 2O / ε OH=1.13 and ε H 2O [in l mol −1 cm −1]=2.290×10 −4×(wt.% SiO 2) 2. At a given water content and quench rate, OH concentrations are higher in andesitic than in dacitic glasses which is consistent with higher fictive temperatures of hydrous andesitic glasses containing more than 1.5 wt.% water.