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 °C and 1–1450 bars for samples with ∼0.8 to ∼2.5 wt.% total dissolved H2O. The H2O diffusivity is proportional to total H2O contents at ≤0.8 wt.%, but increases exponentially with total H2O at higher H2O contents. The diffusivity of the total dissolved H2O (in μm2/s) can be modeled as D H 2 O t = X ⁢ exp [ 20.463 − 40.433 ⁢ X + ( − 18 , 106 + 69 , 230 ⁢ X ) / T ] , 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 DH2Ot 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 °C and 100 MPa for a total H2O content of 2.5 wt.%).