Ion microprobe studies of water in silicate melts: Temperature-dependent water diffusion in obsidian |
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| Authors: | JL Karsten JR Holloway JR Delaney |
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| Institution: | 1. School of Oceanography, WB-10, University of Washington, Seattle, WA 98195, U.S.A.;2. Chemistry Department, Arizona State University, Tempem, AZ 85281, U.S.A. |
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| Abstract: | The temperature dependence of water diffusivity in rhyolite melts over the range 650–950°C and PT(H2O] = 700 bars is evaluated from water concentration-distance profiles measured in glass with an ion microprobe. Diffusivities are exponentially dependent on concentration over this temperature range and vary from about 10?8 cm2/s at 650°C to about 10?7 cm2/s at 950°C at 2 wt.% water. Water solubility also varies with temperature at a rate of ?0.14 wt. per 100°C increase. The avtivation energy (Ea) appears to be constant at 19 ± 1kal/mole for 1, 2,and 3 wt.% H2O. Comparison of these data with results for cation diffusion indicates that this value is a minimum Ea for diffusion of any species in a rhyolite melt.Compensation plots of log10D0 (the frequency factor) versus Ea indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts. D0 increases for H2O and Ca2+ 1] as Ea decreases. This suggests that these molecules may diffuse by different mechanisms than do monovalent cations, and that hydration of the melt affects diffusion of Ca2+ and H2O differently than it does monovalent cation diffusion. The results imply that dramatic increases in cation diffusivities by hydration 1] may occur with additions of less than 1 wt.% H2O. |
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