H2O DIFFUSION IN DACITIC AND ANDESITIC MELTS

Abstract

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 480 s in a piston cylinder apparatus. Concentration profiles of hydrous species (OH groups and H2O molecules) and total water (CH2Ot = 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 (DH2Ot) was derived from profiles of total water using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between DH2Ot and CH2Ot. In dacitic melts DH2Ot is proportional to CH2Ot up to 6 wt.%. In andesitic melts the dependence of DH2Ot on CH2Ot 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 DH2Ot 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 DH2Ot (m²/s) at 1 wt.% H2Ot in melts with rhyolitic to andesitic composition as a function of T (K), P (MPa) and S (wt.% SiO2):