New calibration of infrared measurement of dissolved water in rhyolitic glasses

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: (rho/rho(0))C-1 = a(0) (A) over bar(523), (rho/rho(0))C-2 = (b(0) + b(1) (A) over bar(523) + b(2) (A) over bar(452))(A) over bar(452), and C = C-1 + C-2, where C-1, C-2, and C are the mass fractions of molecular H2O, H2O present as OH, and total H2O, rho/rho(0) is the ratio of the density of the hydrous glass to that of the anhydrous glass and is approximately 1 - C, (A) over bar(523) and (A) over bar(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, b(1) = -0.02011 mm(2), and b(2) = 0.0522 mm(2). 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 less than or equal to 5.5 wt% and in retrieving molecular H2O and OH concentrations for H2Ototal less than or equal to 2.7 wt%. Using the new calibration, the equilibrium coefficient K for the reaction H2O + O = 2OH is independent of H2Ototal (for H2Ototal less than or equal to 24 wt%) at a given temperature and can be expressed as lnK = 1.876 - 3110/T, 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. Copyright (C) 1997 Elsevier Science Ltd.