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 8C 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 V0.8 wt.%, but increases exponentially with total H2O at higher H2O contents. The diffusivity of the total dissolved H2O (in Am /s) can be modeled as DH2Ot 1⁄4 X exp 20:463 40:433X þ 18; 106þ 69; 230X ð Þ=T 1⁄2 ; 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 N740 8C and 100 MPa for a total H2O content of 2.5 wt.%). D 2004 Elsevier B.V. All rights reserved.