Water diffusion in basaltic to dacitic glasses

Dehydration experiments on basaltic, andesitic and dacitic glasses with 1.1, 0.7 and 1.0 wt.% total water, respectively, were performed at 400–675 °C and 0.1 MPa. The dehydration rates were measured by in situ infrared (IR) spectroscopy and modeled by assuming the diffusion of one component water (total water). The diffusivities of total water in basalt and dacite were found to be linearly dependent on total water contents, but that in andesite was weakly dependent on or independent of total water contents at water content and temperature ranges of this study. The total water diffusivities in basalt and dacite were determined by assuming the linear-dependent diffusivity on total water contents and that in andesite was obtained by assuming the constant diffusivity. The obtained diffusivities of total water D (μm2/s) can be expressed by the following equations:DBasalt=C/C0⁢exp⁡[(20.2±2.1)−(166,000±13,000)/R⁢T]DBasalt=C/C0⁢exp⁡[(20.2±2.1)−(166,000±13,000)/R⁢T]DAndesite=exp⁡[(12.4±1.4)−(138,000±10,000)/R⁢T]DAndesite=exp⁡[(12.4±1.4)−(138,000±10,000)/R⁢T]DDacite=C/C0⁢exp⁡[(13.9±0.9)−(133,000±6000)/R⁢T],DDacite=C/C0⁢exp⁡[(13.9±0.9)−(133,000±6000)/R⁢T],where C is the total water contents in weight percent; C0, reference total water contents of 1.1 and 1.0 wt.% for basalt and dacite, respectively; and T, temperature in Kelvin. These equations cover the temperature ranges of 400–575, 500–675 and 500–675 °C, and the water content ranges of < 1.1, 0.7 and 1.0 wt.% for basalt, andesite and dacite, respectively. The obtained diffusivities of total water in basalt, andesite and dacite and previous data for water diffusivity in rhyolite show that at 0.7 wt.% and 400–675 °C, the total water diffusivity decreases from rhyolite to andesite, but the total diffusivity in basalt is higher than that in dacite and lower than that in rhyolite. On the other hand, the results of this study predict that the diffusivities of total water in basalt to rhyolite increase with depolymerization of silicate structures at higher temperatures, which results from the crossovers of diffusivity–temperature trends.