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: (ƍƍ_0)C_1 = a_(0-523); (ƍƍ_0)C_2 = (b_0 + b_(1-523) + b_(2-452)_452, ardC = C_1 + C_2, where C_1, C_2, and C are the mass fractions of molecular H_2O, H_2O present as OH, and total H_2O, πƍ_0 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, a_0 = 0.04217 mm, b_0 = 0.04024 mm, _1 = −0.02011 mm^2, and b_2 = 0.0522 mm^2. The new calibration has a high internal reproducibility in calculating H_2O_(total), six times better than the calibration of Newman et al. (1986). We expect the new calibration to be accurate in retrieving H_2O_(total) for H_2O_(total) ≤ 5.5 wt% and in retrieving molecular H_2O and OH concentrations for H_2O_(total) ≤ 2.7 wt%. Using the new calibration, the equilibrium coefficient K for the reaction H_2O + O = 2OH is independent of H_2O_(total) (for H_2O_(total) ≤ 2.4 wt%) at a given temperature and can be expressed as ln K = 1.876 - 3110T, where T is in K. The bulk water diffusivity reported before is not affected by the new calibration, but the molecular H_2O diffusivity will be roughly 4–30% greater.

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