H2O diffusion in rhyolitic melts and glasses

H2O diffusion plays a major role in bubble growth and volcanic eruption. We report a comprehensive study of H2O diffusion in rhyolitic melts and glasses. This new study and previous investigations together cover a wide range of conditions: 400–1200°C, 0.1–810 MPa, and 0.1–7.7 wt.% total H2O content (H2Ot). In order to constrain how the diffusivity depends on H2Ot, both the diffusion-couple experiments and the dehydration experiments are carried out in a cold-seal vessel (CSV), an internally heated pressure vessel, and a piston cylinder. H2O concentration profiles are measured by infrared (IR) spectroscopy. Although there are still some experimental and analytical difficulties, our data represent a major improvement over earlier data. The diffusion data have been used to quantify H2O diffusivity as a function of temperature, pressure, and H2Ot. Assuming that molecular H2O (H2Om) is the diffusing species, the H2Om diffusivity (in μm2/s) can be expressed as:DH2Om=exp[(14.08−13,128/T−2.796P/T)+(−27.21+36,892/T+57.23P/T)X],where T is in Kelvin, P is in mPa, and X is the mole fraction of H2Ot on a single oxygen basis. The pressure dependence is not so well-resolved compared to the dependence on T and X. The dependence of DH2Om on X increases with increasing pressure. The results are consistent with the data of Nowak and Behrens (1997) [Nowak, M., Behrens, H., 1997. An experimental investigation on diffusion of water in haplogranitic melts. Contrib. Mineral. Petrol. 126, 365–376.], but different from the assumption of Zhang et al. (1991a) [Zhang, Y., Stolper, E.M., Wasserburg, G.J., 1991a. Diffusion of water in rhyolitic glasses. Geochim. Cosmochim. Acta 55, 441–456.], because the dependence cannot be resolved from their low-H2Ot diffusion data, and because the dependence is not so strong at low pressures. The activation energy for H2Om diffusion decreases as H2Ot increases and depends on P (increases with P at X<0.05 and decreases with P at X>0.05). The results roughly reconcile the different activation energies of Zhang et al. (1991a) and Nowak and Behrens (1997). The total (or bulk) H2O diffusivity (DH2Ot) can be calculated from DH2Ot=DH2OmdXm/dX, where Xm is the mole fraction of H2Om. This approach can reproduce the DH2Ot values to within a factor of 2 in the range of 400–1200°C, 0.1–810 MPa, and 0–7.7% H2Ot. An explicit formula for calculating DH2Ot at H2Ot≤2% is:DH2Ot=CC0exp10.49−10,661T−1.772PT,where C is H2Ot content by weight, and C0 equals 1% H2Ot. A formula for calculating DH2Ot at all conditions covered by this work is:DH2Ot=Xexp(m)1+exp56+m+X−34.1+44,620T+57.3PT−X0.091+4.77×106T2,where m=−20.79−5030/T−1.4P/T. The diffusivities obtained in this work can be used to model bubble growth in explosive and nonexplosive rhyolitic volcanic eruptions in all commonly encountered T, P, and H2Ot conditions.