Water diffusion in potassium-rich phonolitic and trachytic melts

Abstract

Water diffusivity was investigated in phonolitic and trachytic melts containing up to 6wt.% of dissolved water at temperatures between 1373K and 1673K for running time of 108 to 1186s using the diffusion couple technique. The experiments were performed in an internally heated gas pressure vessel (IHPV) at pressures of 0.2 or 0.3GPa and in a piston cylinder apparatus (PCA) at pressures between 0.5 and 2.5GPa. A newly developed rapid heating and rapid quench device was used for short term experiments in the IHPV. Concentration profiles of hydrous species (OH groups and H2O molecules) and total water (bulk water concentration as sum of OH and H2O molecules) were measured along the cylindrical axis of the diffusion sample using IR micro-spectroscopy. The IR spectroscopic technique was calibrated using a set of samples with bulk water contents measured by Karl–Fischer titration. Electron microprobe traverses show no significant change in relative proportions of anhydrous components along H2O profiles, indicating that our data can be treated as effective binary diffusion between H2O and the rest of the silicate melt.Bulk water diffusivity was derived from profiles of total water using a modified Boltzmann–Matano method as well as using fittings assuming a functional relationship between the total water diffusivity (DH2Ot) and the total water concentration (CH2Ot). The fitting of the profiles indicates that for phonolitic melt the water diffusivity is roughly proportional to water content. The following formulation was derived for the prediction of total water diffusivity (m2/s) as a function of T (K) in the T-range of 1373 to 1673K and CH2Ot:logDH2Ot=−7.11−2.07logCH2Ot−4827−4620logCH2OtT.The experimental data are reproduced by this relationship with a standard deviation of 0.07log units. Water diffusivity in trachytic melts is similar at the same conditions. A pressure effect on water diffusivity could not be resolved in the range 0.2 to 2.5GPa for phonolitic or trachytic melts.