H O diffusion in rhyolitic melts and glasses

H O diffusion plays a major role in bubble growth and volcanic eruption. We report a comprehensive study of H O 2 2 diffusion in rhyolitic melts and glasses. This new study and previous investigations together cover a wide range of Ž . conditions: 400–12008C, 0.1–810 MPa, and 0.1–7.7 wt.% total H O content H O . In order to constrain how the 2 2 t diffusivity depends on H O , both the diffusion-couple experiments and the dehydration experiments are carried out in a 2 t Ž . cold-seal vessel CSV , an internally heated pressure vessel, and a piston cylinder. H O concentration profiles are measured 2 Ž . 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 O diffusivity as a function of 2 Ž . Ž temperature, pressure, and H O . Assuming that molecular H O H O is the diffusing species, the H O diffusivity in 2 t 2 2 m 2 m 2 . mm rs can be expressed as: w x D sexp 14.08y13,128rTy2.796 PrT q y27.21q36,892rTq57.23PrT X , Ž . Ž . H O 2 m where T is in Kelvin, P is in mPa, and X is the mole fraction of H O on a single oxygen basis. The pressure dependence is 2 t not so well-resolved compared to the dependence on T and X. The dependence of D on X increases with increasing H O 2 m Ž . w pressure. The results are consistent with the data of Nowak and Behrens 1997 Nowak, M., Behrens, H., 1997. An x experimental investigation on diffusion of water in haplogranitic melts. Contrib. Mineral. Petrol. 126, 365–376. , but Ž . w different from the assumption of Zhang et al. 1991a Zhang, Y., Stolper, E.M., Wasserburg, G.J., 1991a. Diffusion of water x in rhyolitic glasses. Geochim. Cosmochim. Acta 55, 441–456. , because the dependence cannot be resolved from their low-H O diffusion data, and because the dependence is not so strong at low pressures. The activation energy for H O 2 t 2 m Ž . diffusion decreases as H O increases and depends on P increases with P at X-0.05 and decreases with P at X)0.05 . 2 t Ž . Ž . The results roughly reconcile the different activation energies of Zhang et al. 1991a and Nowak and Behrens 1997 . The Ž . Ž . total or bulk H O diffusivity D can be calculated from D sD d X rd X, where X is the mole fraction of 2 H O H O H O m m 2 t 2 t 2 m ) Corresponding author. Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109-1063, USA. Ž . E-mail address: [email protected] Y. Zhang . 0009-2541r00r$ see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2541 99 00231-4 ( ) Y. Zhang, H. BehrensrChemical Geology 169 2000 243–262 244 H O . This approach can reproduce the D values to within a factor of 2 in the range of 400–12008C, 0.1–810 MPa, 2 m H O 2 t and 0–7.7% H O . An explicit formula for calculating D at H O F2% is: 2 t H O 2 t 2 t C 10,661 1.772 P D s exp 10.49y y , H O 2 t ž / C T T 0 where C is H O content by weight, and C equals 1% H O . A formula for calculating D at all conditions covered by 2 t 0 2 t H O 2 t this work is: 6 44,620 57.3P 4.77=10 ' D sXexp m 1qexp 56qmqX y34.1q q y X 0.091q , Ž . H O 2 2 t ž / 1⁄2 5 ž / T T T where msy20.79y5030rTy1.4PrT. 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 O conditions. q 2000 2 t Elsevier Science B.V. All rights reserved.