H2O diffusion in dacitic melts

We report the first experimental study of the diffusion of H2O in dacitic melts. Dehydration experiments were conducted at 551-637 degreesC and 1-1450 bars for samples with similar to0.8 to similar to2.5 wt.% total dissolved H2O. The H2O diffusivity is proportional to total H2O contents at:less than or equal to0.8 wt.%, but increases exponentially with total H2O at higher H2O contents. The diffusivity of the total dissolved H2O (in mum(2)/s) can be modeled as where X is the mole fraction of the total dissolved H2O on a single oxygen basis and T is in Kelvin. In the investigated T-range, H2O diffuses more slowly in hydrous dacitic melt than in rhyolitic melt, by a factor of 2 to 12. When compared to H2O diffusion in rhyolite, the activation energy in dacite is greater (124-142 vs. 87-91 kJ/ mol). Therefore, extrapolation of D-H2O,D- in dacite to higher T suggests that above certain T, H2O diffusivity in dacite would be greater than that in rhyolite (e.g., at >740 degreesC and 100 MPa for a total H2O content of 2.5 wt.%). (C) 2004 Elsevier B.V. All rights reserved.