A general model of water diffusivity in calc-alkaline silicate melts and glasses

The calc-alkaline magma series (basalt, andesite, dacite, and rhyolite) commonly occurring at subduction zones are typically hydrous. Water diffusivity in silicate melt is a crucial parameter controlling the transport and evolution of water throughout magma differentiation. Based on experimental data accumulated over the past three decades, we develop a unified, mechanistic and quantitative model of water diffusion and diffusivity in calc-alkaline silicate melts and glasses. This model involves the following features: (1) the coupling of water diffusion and water speciation; (2) an exponential increase of molecular H2O (H2Om) diffusivity with the mole fraction of total water (X), DH2Om = D0 exp(aX), and OH diffusivity (DOH) independent of total water content; (3) the influence of temperature and melt/glass composition on water speciation, the parameter a, and the ratio DOH/D0; and (4) the influence of temperature, composition and pressure on D0. Using the mole fraction of Si among all cations (YSi) to index melt/glass composition, we find that total water (H2Ot) diffusivity at 1 wt% H2Ot decreases with increasing YSi (i.e., from basaltic to rhyolitic composition) at high temperature (T > 1300 K) but this trend is reversed in the low temperature range (T < 900 K). A vast majority of diffusivity data, including those used for model development and those excluded, can be reproduced by our model within a factor of 2, only slightly greater than experimental uncertainty. This new model allows to trace the evolution of water diffusivity along the liquid line of descent, and is particularly useful when the composition of interest is intermediate between the specific compositions previously investigated.