Water diffusion in basalt and andesite melts under high pressures

Water diffusion is one of the most important characteristics of many processes dealt with in magmatic geochemistry, petrology, and volcanology. We have experimentally examined water diffusion in Fe-free andesite and basalt melts (stoichiometric mixtures of the minerals albite + diopside + wollastonite) at 3 and 100 MPa, 1300×C, up to approximately 4 wt % water in the melts, and a total (lithostatic) pressure of 100 MPa on a high gas pressure apparatus equipped with a unique internal device. The experiments were conducted simultaneously with the use of two different methods: diffusion hydration and couples. Water solubility in the melts and water concentrations along the diffusion profiles (C _{H 2 O}) were determined by quantitative IR microspectroscopy, using the Beer-Lambert law. A structural chemical model is proposed for calculating and predicting the concentration dependence of the molar absorption coefficient of the hydroxyl group and water molecules in andesite and basalt glasses. The diffusion coefficients of water (D _{H 2 O}) are derived by the mathematical analysis of concentration profiles and the analytical solution of the second Fick diffusion law. Preliminary results indicated D _{H 2 O} is roughly one order of magnitude higher in basaltic melts than in andesitic ones (at the same temperatures and P _{H 2 O}) and significantly (exponentially) increases with increasing water concentrations in andesitic and basaltic melts. The newly obtained experimental data are proved to be fully consistent with the results obtained on the D _{H 2 O} dependence on C _{H 2 O} in melts of acid rocks (rhyolite and obsidian). The derived quantitative dependence between D _{H 2 O} and melt viscosity is used to develope principles of a new method for predicting and calculating the temperature, concentration, and pressure dependences of D _{H 2 O} in magmatic melts of the of acid-basic series (up to 3 wt % C _{H 2 O}) at crustal T, P parameters.