H 2O diffusion models in rhyolitic melt with new high pressure data

Abstract

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 H 2O diffusion is not constrained satisfactorily. We investigated H 2O diffusion in rhyolitic melt at 0.95–1.9 GPa and 407–1629 °C, and 0.2–5.2 wt.% total water (H 2O t) content with the diffusion-couple method in a piston-cylinder apparatus. Compared to previous data at 0.1–500 MPa, H 2O diffusivity is smaller at higher pressures, indicating a negative pressure effect. This pressure effect is more pronounced at low temperatures. Assuming H 2O diffusion in rhyolitic melt is controlled by the mobility of molecular H 2O (H 2O m), the diffusivity of H 2O m ( D H 2O m ) at H 2O t ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by D H 2 O m = D 0 e x p ( a X ) , with D 0 = e x p ( 13.375 + 1.8875 P − 12939 + 3625.6 P T ) , and a = − 37.256 + 75884 T , where D 0 is in µm 2/s, X is mole fraction of H 2O t on a single oxygen basis, T is temperature in K, and P is pressure in GPa. H 2O t diffusivities ( D H 2O t , in µm 2/s) can be calculated from H 2O m diffusivity, or directly from the following expression: l n ( D H 2 O t / X ) = 13.47 − 49.996 X + 7.0827 X + 1.8875 P − 9532.3 − 91933 X + 13403 X + 3625.6 P T . At low H 2O t content (up to 2 wt.% if an error of a factor of 2 is allowed), H 2O t diffusivity is approximately proportional to H 2O t content: D H 2 O t = C C 0 e x p ( 9.5279 + 1.8875 P − 9698.5 + 3625.6 P T ) , where C is H 2O t content in wt.% and C 0 is 1 wt.%. The new expressions for H 2O 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 H 2O diffusion in rhyolitic melts in various volcanic and magmatic processes.