Experimental dehydration of natural obsidian and estimation of D H 2 O at low water contents

Details Experimental dehydration of natural obsidian and estimation of D H 2 O at low water contents Experimental dehydration of natural obsidian and estimation of D H 2 O at low water contents

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.2931j&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 56, Issue 7, pp.2931-2935

Physics

3

Scientific paper

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, . 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 , except for two replicate runs at 510°C which give values much lower than that defined by the above equation. When interpreted according to a model of water speciation in which molecular H 2 O is the diffusing species with concentration-independent diffusivity while OH units do not contribute to the transport but react to provide H 2 O, 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 2 O diffusivities because the fraction of molecular H 2 O 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 2 O interconversion at low temperature.

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