New insights into the interplay between water diffusion and viscosity in magma fragmentation

The efficiency of magma degassing depends on the interplay of water diffusion and viscous flow in the melt. Volatile oversaturation in the melt generated during magma ascent initiates bubble formation with accumulation of excess volatiles (i.e. H2O) in the bubbles. The decrease of the water content in the melt, on the other hand, strongly reduces the melt viscosity. As a consequence, magma fragmentation may occur in particular in silicic magmas.In such magmas, the bulk water diffusivity is determined by the mobility of water molecules but also by the interconversion rate between OH groups and H2O molecules. In models describing water diffusivity in silicic melts, the common assumption has been that local equilibrium of water species is established in the melt. However, this assumption holds only at high temperature far beyond the glass transition and at sufficiently long time scales. Our hypothesis is that the efficiency of magma degassing strongly decreases towards low temperature and short time scales (rapid decompression), simply because structural relaxation (melt viscosity) is too slow for water species interconversion. Thus, application of published water diffusion equation at such conditions may strongly overestimates the release of water.To test this hypothesis, we will perform dehydration experiments in the range of glass transition with dacitic glasses containing 1 - 4 wt% dissolved water. Micro Raman spectroscopy will allow to measure concentration-distance profiles with high spatial resolution using a depth profiling modus. Water contents near the glass surface will provide information on the fraction of immobile water species. Furthermore, the analysis of the measured profiles will yield information about the concentration dependence of water diffusivity, which reflects the influence of melt relaxation on degassing kinetics. The obtained data are of interest to improve water diffusion equations at low temperature and/or at short time scales. This has implications in modelling of magma degassing and fragmentation.