Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published July 1992 | public
Journal Article

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


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.

Additional Information

© 1992 Pergamon Press. Received 3 December 1991. Accepted 10 April 1992. We acknowledge the comments ofF. J. Ryerson, D. R. Baker, and two anonymous reviewers. YZ thanks J. Blank for help in C02 analyses. This research was supported by CNRS URA 736 to AJ and by NSF grant EAR-8916707 to EMS. Contribution 5034 of Division of Geological and Planetary Sciences, Caltech. Editorial handling: G. Faure

Additional details

August 22, 2023
October 18, 2023