Understanding H20 Degassing in Natural Phonolite Melts: An Experimental Study

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Magmatic degassing begins with bubble nucleation and growth, and is linked to eruptive style. What exactly causes explosive volcanism will not be fully understood until the fundamentals of degassing are known in all magma compositions that erupt explosively. At this point, only two experimental studies of H2O bubble nucleation and growth exist in non-rhyolite melts. More experiments are needed to understand how the bubble nucleation and growth processes change as a function of melt composition. This project will use experiments on natural samples of the 79 AD Vesuvius phonolite to measure: 1) Critical H2O super-saturation needed for homogeneous and heterogeneous bubble nucleation. 2) The amount of excess (disequilibrium) H2O retained in the melt as a function of decompression rate and path. 3) Bubble growth rates as a function of melt temperature, composition, and crystal contents. The results will be used as input for dynamical models focusing specifically on the 79 AD eruption, and on vesiculation and eruption dynamics of alkalic magmas in general. One goal is to compare phonolite eruption processes with calc-alkaline rhyolites and dacites at a range of modeled ascent rates. This work will help provide a better understanding of the degassing process in K phonolite melts that are prevalent at very dangerous and heavily populated volcanoes, such as Vesuvius in Italy. It will also provide insight into whether the build-up of extreme melt super-saturation during magmatic ascent is reasonable to explain the eruption dynamics of alkalic magmas, as has been suggested for rhyolites. Data from this research will be made available to other researchers interested in phonolite magmas at Vesuvius and other alkalic systems, and will provide experimental constraints to be used in interpreting vesicle textures in pumices. The results will be broadly disseminated in publications and meeting presentations. The publications will focus not only on the experiments, but also on eruption dynamical models, and comparisons with the 79 AD pumice textures. This award will provide an avenue for new collaborations between the PI and Italian researchers and the theoretical modeling community.