H2O diffusion models in rhyolitic melt with new high pressure data

Water diffusion in silicate melts is important for understanding bubble growth in magma, magma degassing and eruption dynamics of volcanos. Previous studies have made significant progress on water diffusion in silicate melts, especially rhyolitic melt. However, the pressure dependence of H2O diffusion is not constrained satisfactorily. We investigated H2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H2Ot) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H2O diffusion in rhyolitic melt is controlled by the mobility of molecular H2O (H2Om), the diffusivity of H2Om (DH2Om) at H2Ot ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given byDH2Om=D0exp(aX),withD0=exp(13.375+1.8875P−12939+3625.6PT),anda=−37.256+75884T,where D0 is in µm2/s, X is mole fraction of H2Ot on a single oxygen basis, T is temperature in K, and P is pressure in GPa. H2Ot diffusivities (DH2Ot, in µm2/s) can be calculated from H2Om diffusivity, or directly from the following expression:ln(DH2Ot/X)=13.47−49.996X+7.0827X+1.8875P−9532.3−91933X+13403X+3625.6PT. At low H2Ot content (up to 2 wt.% if an error of a factor of 2 is allowed), H2Ot diffusivity is approximately proportional to H2Ot content:DH2Ot=CC0exp(9.5279+1.8875P−9698.5+3625.6PT),where C is H2Ot content in wt.% and C0 is 1 wt.%. The new expressions for H2O diffusion not only reproduce our own data, but also match data in literature from different laboratories and using different methods, indicating good inter-laboratory and multi-method consistency. The new expressions cover a wide range of geological conditions, and can be applied to H2O diffusion in rhyolitic melts in various volcanic and magmatic processes.