Ar di¡usion in hydrous silicic melts: implications for volatile di¡usion mechanisms 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 pressures 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 temperature range of 480^11023C by a simple Arrhenius relationship DAr = 2.14U1036 m2/s exp(318 883/T). A 4.0 wt% increase in water content increases the Ar diffusivity by approximately one order of magnitude in both rhyolitic and albitic melts at 10003C. 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 water contents. For water-poor rhyolite, Ar diffusivity depends on pressure with an apparent activation volume of 13^15 cm3/mol at pressures up to 800 MPa. For water-rich rhyolite (V5 wt% water), there is no significant pressure effect at 10003C 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 10312 m2/s) in rhyolitic melts can be expressed as: DAr ˆ exp‰…14:627317913=T32:569P=T† ‡ …35936=T ‡ 27:42P=T†X waterŠ where T is in K, P in MPa, and Xwater is the mol fraction of water on a single oxygen basis. Except for two outlier points, error of estimates is 9 0.455 in terms of ln D for all data, covering a wide range of temperatures (480^12003C), pressures (0.1^1500 MPa), and water contents (0.1^5 wt%). The new Ar diffusion data support the assumption that molecular H2O diffusivity exponentially increases with water content [Y. Zhang, H. Behrens, Chem. Geol. 169 (2000) 243^ 262]. ß 2001 Elsevier Science B.V. All rights reserved.