Pressure Effect on H2O Diffusion and a General Model of H2O Diffusivity in Dacite

Diffusion rate of H2O plays an important role in magma dynamics, such as bubble growth, magma degassing and volcanic eruptions. H2O diffusivity in dacitic melts at 1 bar and low temperatures (550-640 degree C) has been reported, as well as that at high P and high T conditions (1200-1500 degree C). Extrapolation of high temperature data to low T yields diffusivity 2 magnitudes lower than low T data, suggesting a prominent pressure effect on H2O diffusion in dacite. Pressure effect on H2O diffusion in dacite was studied by carrying out diffusion couple experiments at low temperatures (500-600 C) and 10 kbar in piston cylinder apparatus. The hydrous dacite half contains H2O from 1 to 8.3 weight percent. The diffusion profiles of H2O concentration were attained by measuring the absorbance in MIR (at H2O lower than 0.5 weight percent) or adding the intensity of H2O molecules and hydroxyl groups absorbance in NIR region at higher H2O concentration. The diffusion profiles were modeled by assuming that H2O molecules dominate the H2O diffusion, whereas the concentration divergence of OH is simply the correspondence of local interconvertion. The molecular H2O diffusivity is found to be lower within 1 magnitude at 10 kbar compared to what has been reported under the same temperature and H2O concentration but 1 bar. Therefore the pressure effect is not as large as expected. Combined with high temperature data, the diffusivity of H2O in dacite can be fit into a general formula which accommodates most geological conditions.