Water diffusion in potassium-rich phonolitic and trachytic melts

Water diffusivity was investigated in phonolitic and trachytic melts containing up to 6 wt.% of dissolved water at temperatures between 1373 K and 1673 K for running time of 108 to 1186 s using the diffusion couple technique. The experiments were performed in an internally heated gas pressure vessel (IHPV) at pressures of 0.2 or 0.3 GPa and in a piston cylinder apparatus (PCA) at pressures between 0.5 and 2.5 GPa. 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 1673 K and CH2Ot:logDH2Ot=−7.11−2.07logCH2Ot−4827−4620logCH2OtT.The experimental data are reproduced by this relationship with a standard deviation of 0.07 log 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 2.5 GPa for phonolitic or trachytic melts.

► H2O diffusion in phonolite and trachyte was studied from 1373 to 1673 K.
► Diffusion profiles are roughly consistent with proportionality of DH2Ot on CH2Ot.
► An Arrhenius-type equation is proposed to describe DH2Ot for phonolitic melts.
► A pressure effect could not be resolved for phonolite melts.
► Diffusion in trachytic melt is only slightly slower than in phonolitic melt.