H 2O diffusion in rhyolitic melts and glasses

Abstract

H 2O diffusion plays a major role in bubble growth and volcanic eruption. We report a comprehensive study of H 2O 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 H 2O content (H 2O t). In order to constrain how the diffusivity depends on H 2O t, 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. H 2O 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 H 2O diffusivity as a function of temperature, pressure, and H 2O t. Assuming that molecular H 2O (H 2O m) is the diffusing species, the H 2O m diffusivity (in μm 2/s) can be expressed as: D H 2 O m = 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 H 2O t on a single oxygen basis. The pressure dependence is not so well-resolved compared to the dependence on T and X. The dependence of D H 2O m 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-H 2O t diffusion data, and because the dependence is not so strong at low pressures. The activation energy for H 2O m diffusion decreases as H 2O t 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) H 2O diffusivity ( D H 2O t ) can be calculated from D H 2O t = D H 2O m d X m/d X, where X m is the mole fraction of H 2O m. This approach can reproduce the D H 2O t values to within a factor of 2 in the range of 400–1200°C, 0.1–810 MPa, and 0–7.7% H 2O t. An explicit formula for calculating D H 2O t at H 2O t≤2% is: D H 2 O t = C C 0 exp 10.49− 10,661 T − 1.772P T , where C is H 2O t content by weight, and C 0 equals 1% H 2O t. A formula for calculating D H 2O t at all conditions covered by this work is: D H 2 O t =X exp(m) 1+ exp 56+m+X −34.1+ 44,620 T + 57.3P T − X 0.091+ 4.77×10 6 T 2 , where m=−20.79−5030/ T−1.4 P/ 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 H 2O t conditions.