Diffusion of water in rhyolitic glasses

Abstract

Water dehydration experiments on rhyolitic glasses have been carried out at 400–550°C under a N_2 atmosphere. Concentration profiles of both H_2O molecules and OH groups were measured by Fourier transform infrared spectroscopy. As found in previous studies of water diffusion in rhyolitic melts, the measured total water concentration profiles do not match expectations based on a single constant diffusion coefficient for total water. The diffusion of total water is described by considering the diffusion of both H_2O molecules and OH groups and the reaction between them. The concentration relationship between the two species has been obtained from direct infrared measurement on quenched experimental charges. The quench is inferred to be rapid enough to preserve concentrations of both species at experimental temperature based on experimental results designed to examine reaction kinetics. The measured species concentrations along diffusion profiles show that local equilibrium between H_2O and OH is approximately reached at high temperatures and high water contents. However, at lower water content or lower temperature, local equilibrium is not reached. In treating the diffusion problem, this disequilibrium effect is partially compensated by using empirical relationships between H_2O and OH concentrations based on measurements, instead of using an equilibrium relationship. It is thus possible to obtain diffusion coefficients for both species from their concentration profiles. The diffusion coefficient of OH is found to be negligible compared to that of H_2O at 403–530°C (D_(OH) < 0.02D_(H2O) and could be much smaller); i.e., H_2O is the dominant diffusing species even at total water concentration as low as 0.2 wt%. The variation of OH concentration along the diffusion profile is inferred to be due to the local interconversion between OH groups and H_2O molecules; the reaction also provides the diffusing H_2O species. D_(H2O) values are found to vary by less than a factor of 2 over a total water concentration range of 0.2 to 1.7 wt%. This simple model, coupled with the assumption of local equilibrium between H_2O and OH, yields a very good fit to the data from diffusion-couple experiments of Lapham et al. (1984) at 850°C. When our data are combined with D_(H2O) obtained from that fit, D_(H2O) (in m^2/s) is given by In D_(H2O) = (−14.59 ± 1.59) − (103000 ± 5000)/RT 673 K ≤ T ≤ 1123 K where T is temperature in K and R is the gas constant in J K^(−1) mol^(−1). This equation also approximates well D_(H2O) values calculated from previous measurements of concentration-dependent bulk water diffusion coefficients of Karsten et al. ( 1982). The diffusion of H_2O is also compared to the diffusion of the noble gas elements. The activation energy for diffusion in rhyolitic glasses is well correlated with neutral species radii of He, Ne, H_2O, and Ar. This supports the contention that the diffusing species for "water" is neutral molecular H_2O. The role of speciation may also be important in understanding the diffusion of many other multi-species components, and the effect can be treated in a similar fashion as that during water diffusion.

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