H2O diffusion in dacitic and andesitic melts

The diffusion of water in dacitic and andesitic melts was investigated at temperatures of 1458 to 1858 K and pressures between 0.5 and 1.5 GPa using the diffusion couple technique. Pairs of nominally dry glasses and hydrous glasses containing between 1.5 and 6.3 wt.% dissolved H2O were heated for 60 to 480s in a pis on cylinder apparatus. Concentration profiles of hydrous species (OH groups and H2O molecules) and total water (C-H2O = sum of OH and H2O) were measured along the cylindrical axis of the diffusion sample using IR microspectroscopy. Electron microprobe traverses show no significant change in relative proportions of anhydrous components along H2O profiles, indicating that our data can be treated as effective binary interdiffusion between H2O and the rest of the silicate melt. Bulk water diffusivity (D-H2Ot) was derived from profiles of total water using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between D, C, and In dacitic melts D-H2Ot is proportional to C-H2Ot up to 6 wt.%. In andesitic melts the dependence of D-H2Ot on C-H2Ot is less pronounced. A pressure effect on water diffusivity could not be resolved for either dacitic or andesitic melt in the range 0.5 to 1.5 GPa. Combining our results with previous studies on water diffusion in rhyolite and basalt show that for a given water content D-H2Ot increases monotonically with increasing melt depolymerization at temperatures >1500 K. Assuming an Arrhenian behavior in the whole compositional range, the following formulation was derived to estimate D-H2Ot (m(2)/s) at 1 wt.% H2Ot in melts with rhyolitic to andesitic composition as a function of T(K) P(MPa) and S(wt.% SiO2): log D-H2Ot = (-0.757-0.0868 (.) S) + (-14.785 + 131.7 (.) S)/T + (3.079-0.0490 (.) S) (.) P/T The experimental data (69 in total. covering 803 to 1848 K and 0.1 to 1500 MPa) are. reproduced by this relationship with a standard error of 0.12 too units. Using proportionality between water content and bulk water diffusivity. the above equation can also be used to estimate D-H2Ot in rhyolite to dacite containing up to 2 wt.% H2Ot at magmatic temperatures. For andesitic melts the functional relationship between D-H2Ot and C-H2Ot is not known at magmatic temperatures and. hence, application of our model remains uncertain for such conditions. As the activation energy for water diffusion increases from rhyolite to andesite, the diffusivities become similar at intermediate temperatures. Below 1000 K (depending on H2O content and pressure), water diffusion may be faster in rhyolite than in dacite than in andesite. Copyright (C) 2004 Elsevier Ltd.