Ar diffusion in hydrous silicic melts: implications for volatile diffusionmechanisms and fractionation

The effect of dissolved water on the diffusivity of Ar in glasses and melts of rhyolitic and albitic compositions was investigated experimentally at p ressures up to 1500 MPa and water contents of 0.1-5 wt%. The data for water -poor rhyolitic composition at 500 MPa can be described in the whole temper ature range of 480-1102 degreesC by a simple Arrhenius relationship D-Ar = 2.14 X 10(-6) m(2)/s exp(-18 883/T), A 4.0 wt% increase in water content in creases the Ar diffusivity by approximately one order of magnitude in both rhyolitic and albitic melts at 1000 degreesC. In contrast to viscosity and total water diffusion, an exponential dependence of Ar diffusivity on water content was observed for the rhyolitic composition in the whole range of w ater contents. For water-poor rhyolite, Ar diffusivity depends on pressure with an apparent activation volume of 13-15 cm(3)/mol at pressures up to 80 0 MPa. For water-rich rhyolite (similar to5 wt% water), there is no signifi cant pressure effect at 1000 degreesC in the range 500 1500 MPa. Combining our data with previous data from Carroll [M.R. Carroll, Earth Planet. Sci. Lett. 103 (1991) 156 168], Ar diffusivity (in 10(-12) m(2)/S) in rhyolitic melts can be expressed as: D-Ar = exp[(14.627-17913/T-2.569P/T) + (35936/T + 27.42P/T)X-water] where T is in K, P in MPa, and X-water is the mol fraction of water on a si ngle oxygen basis. Except for two outlier points, error of estimates is les s than or equal to 0.455 in terms of In D for all data, covering a wide ran ge of temperatures (480-1200 degreesC), pressures (0.1-1500 MPa), and water contents (0.1-5 wt%). The new Ar diffusion data support the assumption tha t molecular H2O diffusivity exponentially increases with water content (C) 2001 Elsevier Science B.V. All rights reserved.