Effect of alkalis on water diffusion in silicate melts based on new experimental data for a shoshonitic melt

Abstract

Water diffusion is an important mechanism affecting magmatic water content, vapor bubble growth, and the dynamics of volcanic eruptions. Water diffusivity in silicate melts is dependent on temperature, pressure, H2O content and speciation, and melt composition. While a general model has recently been developed for water diffusivity in calc-alkaline (subalkaline) melts as a function of silica content, the effect of alkalis on water diffusion is less clear due to the lack of experimental data for alkali-rich melts and a systematic evaluation. Here, water diffusion in shoshonitic melt with up to 1.6 wt% H2O was investigated at 1654–1926 K and 1 GPa in a piston cylinder apparatus using both hydration and diffusion couple methods. Diffusion profiles measured by FTIR microspectroscopy are in accordance with a modified speciation-based diffusion model which takes into account different mobility of hydroxyl (OH) and molecular H2O (H2Om). The diffusivity ratio DOH/DH2Om in shoshonitic melt increases with increasing temperature and falls into the range of 0.24–0.55, greater than the values for all of the melts previously investigated. The remarkable contribution of OH to water diffusion in shoshonitic melt is attributed to a substantial presence of free OH species (OH bonding with network-modifying cations but here especially with K or Na). The total H2O diffusivity in shoshonitic melt can be expressed as:DH2Ot=e-12.11-16765T1+Ce-11.14+17381-262.6CT,where D is in m2/s, T is temperature in K, and C is H2Ot content in wt% (DH2Ot reduced to DOH at C = 0). The above equation replicates the experimental data within 0.1 natural log unit. The total H2O diffusivities at 1 wt% H2Ot in three alkali-rich melts, including shoshonite, trachyte and phonolite, appear to fall onto a single Arrhenian trend line over a broad temperature range of 1100–1900 K. Compared to the general water diffusivity model for calc-alkaline melts, water diffusivities in alkali-rich silicate melts are generally higher and roughly twofold at the same P-T-H2Ot-XSi conditions, with XSi being the mole fraction of Si among all cations. Excess alkalis decrease degree of melt polymerization and promote formation of mobile K-OH and Na-OH species, thereby having a positive influence on water diffusion and vapor bubble growth in magma.