Water Diffusion in Calc-Alkaline Silicate Melts.

Abstract

Water diffusion in a series of calc-alkaline melts, including rhyolite, dacite, and haploandesite, was investigated at various temperatures, pressures, and water contents. FTIR microspectroscopy was used to analyze profiles of H2O concentration on quenched glasses. Molecular H2O (H2Om), rather than OH, is the dominating diffusion species. For rhyolite, diffusion couple experiments were carried out at 680-1902 K, 0.95-1.9 GPa, and 0.2-5.2 wt.% H2O in a piston-cylinder apparatus. Negative pressure effect on H2O diffusion was observed. With literature data incorporated, H2Om and total H2O (H2Ot) diffusivity models in rhyolite were constructed for 676-1902 K, 0-1.9 GPa, and 0.1-7.7 wt.% H2O. For dacite, diffusion couple experiments were performed at 786-893 K, 0.48-0.95 GPa, and 0-8 wt.% H2O. H2Om and H2Ot diffusivity models in dacite were presented for 786-1798 K, 0-1 GPa, and 0-8 wt.% H2O. For haploandesite, hydrous melts with ≤2.5 wt.% H2O were dehydrated at 743-873 K and 0.1 GPa Ar atmosphere in cold-seal pressure vessels. At a given water concentration and temperature, there is more OH in haploandesite than in rhyolite or dacite. At ≤873 K, H2Ot diffusivity increases from andesite to dacite to rhyolite, which is in contrary to the trend at superliquidus temperatures. These water diffusivity models can be applied to various magmatic and volcanic processes involving the transport of water, such as bubble growth in explosive volcanic eruptions. In addition, the exchange of oxygen isotopes between coexisting minerals during cooling was examined. The mineral pair with the largest isotopic fractionation (PLIF) can bracket their apparent equilibrium temperature (Tae) within their Dodson closure temperatures. This special feature of PLIF may be used to constrain the thermal history of slowly cooled plutonic and metamorphic rocks.