Title: New calibration of infrared measurement of dissolved water in rhyolitic glasses
Abstract: This paper presents a new calibration for infrared analyses of dissolved water and its species concentrations in rhyolitic glasses. The new calibration combines infrared/manometry measurements and infrared study of hydrous rhyolitic glasses heated at different temperatures. The heating experiments show that the ratio of the molar absorptivity of the 5230 cm−1 band to that of the 4520 cm−1 band varies with water concentration. Therefore, earlier calibrations assuming constant molar absorptivities are not accurate. Using our new calibration, total water concentration, and species concentrations can be calculated as follows: (ϱϱ0)C1 = a0−523, (ϱϱ0)C2 = (b0 + b1−523 + b2−452)−452, and C = C1 + C2, where C1, C2, and C are the mass fractions of molecular H2O, H2O present as OH, and total H2O, πϱ0 is the ratio of the density of the hydrous glass to that of the anhydrous glass and is approximately 1 − C, −523 and −452 are the absorbances (peak heights) of the 5230 cm−1 and 4520 cm−1 bands per mm sample thickness and relative to a baseline that was fit by a flexicurve, a0 = 0.04217 mm, b0 = 0.04024 mm, b1 = −0.02011 mm2, and b2 = 0.0522 mm2. The new calibration has a high internal reproducibility in calculating H2Ototal, six times better than the calibration of Newman et al. (1986). We expect the new calibration to be accurate in retrieving H2Ototal for H2Ototal ≤ 5.5 wt% and in retrieving molecular H2O and OH concentrations for H2Ototal ≤ 2.7 wt%. Using the new calibration, the equilibrium coefficient K for the reaction H2O + O = 2OH is independent of H2Ototal (for H2Ototal ≤ 2.4 wt%) at a given temperature and can be expressed as lnK = 1.876 − 3110T, where T is in K. The bulk water diffusivity reported before is not affected by the new calibration, but the molecular H2O diffusivity will be roughly 4–30% greater.