Experimental dehydration of natural obsidian and estimation of D_(H2O) at low water contents

Abstract

Water diffusion experiments were carried out by dehydrating rhyolitic obsidian from Valles Caldera (New Mexico, USA) at 510–980°C. The starting glass wafers contained ~0.114 wt% total water, lower than any glasses previously investigated for water diffusion. Weight loss due to dehydration was measured as a function of experiment duration, which permits determination of mean bulk water diffusivity, D_w. These diffusivities are in the range of 2.6 to 18 × 10^(−14) m^2/s and can be fit with the following Arrhenius equation: In D_w(m^2/s) = -(25.10 ± 1.29)-(46,480 ± 11,400) (J/mol)/RT, except for two replicate runs at 510°C which give D_w values much lower than that defined by the above equation. When interpreted according to a model of water speciation in which molecular H_2O is the diffusing species with concentration-independent diffusivity while OH units do not contribute to the transport but react to provide H_2O, the data (except for the 510°C data) are in agreement with extrapolation from previous results and hence extend the previous data base and provide a test of the applicability of the model to very low water contents. Mean bulk water diffusivities are about two orders of magnitude less than molecular H_2O diffusivities because the fraction of molecular H_2O out of total water is very small at 0.114 wt% total water and less. The 510°C experimental results can be interpreted as due to slow kinetics of OH to H_2O interconversion at low temperature.

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