Ion microprobe studies of water in silicate melts: Temperature-dependent water diffusion in obsidian

Abstract

The temperature dependence of water diffusivity in rhyolite melts over the range 650–950°C and [ P T(H 2O] = 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 cm 2/s at 650°C to about 10 −7 cm 2/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 ( E a) appears to be constant at 19 ± 1kal/mole for 1, 2,and 3 wt.% H 2O. Comparison of these data with results for cation diffusion indicates that this value is a minimum E a for diffusion of any species in a rhyolite melt. Compensation plots of log 10 D 0 (the frequency factor) versus E a indicate that hydrous rhyolite melts follow the same trend as anhydrous basalts. D 0 increases for H 2O and Ca 2+ [1] as E a decreases. This suggests that these molecules may diffuse by different mechanisms than do monovalent cations, and that hydration of the melt affects diffusion of Ca 2+ and H 2O 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.% H 2O.