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Published September 2010 | public
Journal Article

Experimental study of water diffusion in haplobasaltic and haploandesitic melts


The diffusion of water in polymerized silicate melts has been studied extensively, but only limited data are available for water diffusion in depolymerized melts such as andesite and basalt. We report here results of eight experiments to determine the diffusivity of water (D_(H_(2)O), defined to be the diffusivity of total H_(2)O; i.e., regardless of speciation) in haplobasalt and haploandesite melts at 1300 °C and P_(H_(2)O)=0.3–100 MPa using hydration and diffusion-couple methods. Water contents of the melts ranged from < 0.1 to ~4 wt.%. Our results contribute to understanding the dependence of water diffusivity on melt composition, especially the concentration of water itself. Diffusion experiments were conducted for 420–1500 s using an internally heated pressure vessel. Water concentration vs. distance profiles in quenched glasses was determined using Fourier transform infrared (FTIR) spectroscopy. D_(H_(2)O) values were determined by fitting these profiles using either Boltzmann–Matano methods, simple functional forms for the relationship between water content and D_(H_(2)O), or the D_(H_(2)O) vs. water content function implied by a simple water speciation model. For both of the compositions studied, D_(H_(2)O) increases with increasing water content. The increase in D_(H_(2)O) with water content can be modeled as exponential over the range investigated, and it is generally consistent with simple models of speciation in which dissolved molecular water is mobile and hydroxyl groups are immobile. The following equations can be used to calculate water diffusivity at various water contents in haplobasaltic and haploandesitic melts at 1300 °C: D_(H_(2)O) = 1.27⋅ exp(0.538 · C_(H_(2)O))⋅10^(−10) haplobasalt D_(H_(2)O) = 4.90⋅ exp(0:493 · C_(H_(2)O))⋅10^(−11) haploandesite where D_(H_(2)O) is in m^2/s and C_(H_(2)O) is in weight percent. Comparison of our results with those in the literature for other compositions suggest an overall correlation between water diffusivity and melt viscosity for C_(H_(2)O)≤3 wt.%. Given the availability of algorithms relating viscosity to melt composition, this correlation can be used to estimate D_(H_(2)O) in melts for which it has not been experimentally determined and to model igneous processes for which D_(H_(2)O) plays a role over a range of magma composition spanning rhyolite to basalt. The following equations can be used to calculate water diffusivity from the viscosity of depolymerized and polymerized melt compositions: logD_(H_(2)O) = −8.87−0.578⋅ log η depolymerized melt logD_(H_(2)O) = −9.65−0.269⋅ log η polymerized melt where η is in Pa⋅s and D_(H_(2)O) in m^2/s.

Additional Information

© 2010 Elsevier B.V. Received 23 December 2009; revised 12 May 2010; accepted 21 June 2010. Editor: B. Bourdon. Available online 1 July 2010. This work was supported by the Department of Energy (DE-FG02-85ER13445 and DE-FG02-06ER15773) and the Russian Foundation for Basic Research (RFBR, projects 06-05-64631 and 09-05-00417). We thank G. R. Rossman for his generous help in the use of the FTIR spectrometer and Y. Zhang for helpful discussions and the use of his Microsoft® QuickBASIC computer programs for calculating functional fits to the diffusion profiles. We appreciate the comments of S. Kohn, F. Gaillard and Don Baker on an earlier version of this paper.

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