Dehydration experiments on basaltic, andesitic and dacitic glasses with 1.1, 0.7 and 1.0wt% total water, respectively, were performed at 400 to 675C and 0.1MPa. The dehydration rates were measured by in situ infra-red spectroscopy and modeled by assuming the diffusion of one component water (total water). The diffusivities of total water in basalt and dacite were found to be linearly dependent upon total water contents, but that in andesite was weakly dependent upon, or independent of, total water contents within the water-content and temperature ranges used here. The total water diffusivities in basalt and dacite were determined by assuming a diffusivity that was linearly dependent upon the total water content. In andesite, it was was obtained by assuming a constant diffusivity. The resultant diffusivities (figure 4) of total water, D(μm2/s), could be described by:

DBasalt = C/C0exp[(20.2)−(166000)/RT]

DAndesite = exp[(12.4)−(138000)/RT]

DDacite = C/C0exp[(13.9)−(133000)/RT],

where C was the total water content in wt% and C0 was a reference total water content, which was equal to 1.1 and 1.0wt% for basalt and dacite, respectively. These equations covered the temperature ranges: 400 to 575, 500 to 675 and 500 to 675C, and water-content ranges of <1.1, 0.7 and 1.0wt% for basalt, andesite and dacite, respectively. The resultant diffusivities of total water in basalt, andesite and dacite, and previous data for water diffusivity in rhyolite, showed that at 0.7wt% and 400 to 675C, the total water diffusivity decreased from rhyolite to andesite, while the total diffusivity in basalt was higher than that in dacite and lower than that in rhyolite. The results of the study predicted that the diffusivities of total water in basalt to rhyolite increased with de-polymerization of silicate structures at higher temperatures, which resulted from a cross-over in diffusivity versus temperature trends.

Water Diffusion in Basaltic to Dacitic Glasses. S.Okumura, S.Nakashima: Chemical Geology, 2006, 227[1-2], 70-82

Figure 4

Diffusivity of Total Water in Basalt, Dacite and Andesite