New insights into the interplay between water diffusion and viscosity in magma fragmentation
Final Report Abstract
To get insights in the mechanisms of magma degassing and fragmentation, we performed dehydration experiments at ambient pressure with dacitic glasses containing between 1.42 - 5.28 wt% H2O. The focus was on the degassing behavior in the range of glass transition, i.e., from temperatures 50 K below to 50 K above the glass transition temperature Tg. Duration of the experiments covered a range of 5 - 125 hours. Water contents and initial water speciation was determined by FTIR spectroscopy. Diffusion depth profiles were measured by Raman spectroscopy. We developed an equation to calculate the total water content along the depth profiles from the ratio of the fundamental OH stretching vibration band at 3550 cm-1 and the T-O-T bending band near 500 cm-1. Glasses with high water content (4.66 – 5.28 wt%) showed huge alterations, i.e., crack formation below Tg and crystallization at and above Tg, but glasses with water contents up to 2.5 wt% were visually unchanged and suitable for water diffusion analysis. Contrary to previous studies we found evidence that total water did not drop to zero at the surface of the glasses. Furthermore, both the surface concentration in the glass and the apparent total water diffusivity derived from error function fitting decrease with increasing duration of the experiment. These observations suggest that the interconversion between OH groups and H2O molecules is too slow to adjust local equilibrium of water species in the glasses. We tested the numerical model of diffusion and speciation of water proposed by Coumans et al. (2020) to reproduce the measured total water profiles. Consistent diffusion coefficients and reaction rate constants at different experimental time for a given condition suggest that this approach is suitable to model dehydration of dacitic glasses in the highviscosity range. Our results show the importance of considering the interconversion of water species when modeling water diffusion at high viscosities. Due to the slow interconversion of OH groups to mobile water molecules, dehydration is much slower in the range of glass transition than predicted by diffusion models based on experiments performed on stable melts at magmatic temperatures.
Publications
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Water release from dacitic melts near the glass transition and implications for volcanic eruptions. Chemical Geology, 645, 121885.
Pohl, Florian; Behrens, Harald & Dohmen, Ralf
