Water diffusion in silicate melts

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  • OKUMURA Satoshi
    Department of Earth and Space Science, Osaka University Present address: Institute of Mineralogy, Petrology and Economic Geology, Tohoku University

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  • シリケイトメルト中の水の拡散
  • シリケイトメルト チュウ ノ ミズ ノ カクサン

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Abstract

Water diffusion in silicate melts is a controlling factor of magma ascent process in volcanic eruption, because it dominates the rates of bubble nucleation and growth in magmas and the decrease of water contents by water diffusion causes drastic changes in the physical properties of silicate melts (viscosity and density). This paper reviews experimental studies for water diffusion in rhyolitic to basaltic melts and glasses. In rhyolitic and dacitic melts (polymerized silicate melts), water mainly dissolves as hydroxyl group (OH) and molecular water (H2O), but only H2O is considered as the mobile species. The H2O diffusivities in rhyolite and dacite increase with water contents and temperature. The mechanism of water diffusion in andesite to basalt (depolymerized silicate melts) is still unclear, but some studies presented the data of total water (OH + H2O) diffusivities in andesite and basalt by assuming the diffusion of one component water. Those data show that the diffusivity of total water increases with melt depolymerization (from rhyolite to basalt) at high temperatures (e.g., > 1400 °C at 0.7 wt% water), but the magnitude relation inverts at the lower temperatures due to the difference of temperature dependences of total water diffusivity in rhyolite to basalt. Recently, a few studies explored the compositional dependence of total water diffusivity and expressed the total water diffusivity as a function of the SiO2 content or the NBO/T value. To test and develop those models, additional experimental data are needed in the future studies, and the developed-comprehensive model for water diffusion in silicate melts and glasses will make a significant contribution to understanding of volcanic eruptions.

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