H<sub>2</sub>O diffusion models in rhyolitic melt with new high pressure data期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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where D₀ is in µm²/s, X is mole fraction of H₂O_t on a single oxygen basis, T is temperature in K, and P is pressure in GPa.H₂O_t diffusivities (D_{H₂O_t}, in µm²/s) can be calculated from H₂O_m diffusivity, or directly from the following expression:

At low H₂O_t content (up to 2 wt.% if an error of a factor of 2 is allowed), H₂O_t diffusivity is approximately proportional to H₂O_t content:

where C is H₂O_t 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.

H₂O diffusion models in rhyolitic melt with new high pressure data

Authors:	Huaiwei Ni Youxue Zhang

Institution:	^aDepartment of Geological Sciences, The University of Michigan, 2534 C. C. Little Bldg., 1100 North University Ave., Ann Arbor, MI 48109-1005, USA

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 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 (H₂O_t) 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 (H₂O_m), the diffusivity of H₂O_m (D_{H₂O_m}) at H₂O_t ≤ 7.7 wt.%, 403–1629 °C, and ≤ 1.9 GPa is given by D_{H₂O_m}=D₀exp(aX),

Keywords:	H₂O diffusion Bubble growth Rhyolite Volcanic eruptions Diffusivity model Pressure effect
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