Modelling of bubble nucleation in trachy-phonolitic magmas: implications for the dynamics of ash-rich eruptions

Nucleation of water gas bubbles in trachyphonolitic magmatic melts has been investigated integrating theory and numerical modelling with decompression experiments and analysis of natural ash samples of explosive eruptions. Bubble nucleation, considered the natural response of magmas to decompression, is strongly dictated by the gas-melt surface tension. Here, I use an integrated approach to quantify the role of the surface tension in the nucleation process combining high pressure - high temperature nucleation experiments with a numerical modelling based on the gradient theory (Cahn and Hilliard, 1959). This theory, successfully applied in several studies of industrial polymers (Poser and Sanchez, 1981; Harrison et al., 1999; Kahl and Enders, 2000; Enders et al., 2005) was never been used before to study systems of volcanological interest. I show that surface tension is far to be a constant, but it decreases with in- creasing nucleation pressure (i.e. the confining pressure). Entering the values of surface tension into the classical theory of nucleation, I obtain a variable supersaturation pressure triggering nucleation. The decreasing value of the gas-melt surface tension with increasing pressure, facilitate bubble nucleation at high pressure, thus enhancing the explosivity of eruptive events from deeper reservoirs. Instead, the hindered nucleation at relatively low pressure, due to high bubble surface tension, implies that the generation of explosive eruptions from shallow reservoirs requires high decompressions. Finally the vesiculation, in terms of nucleation and growth, of natural samples of ash-rich eruptions has been studied by applying a novel technique able to take 3D measurements of bubbles preserved on ash particle’s surface. The Bubble Size Distributions (BSD), together with the field evidence, suggest that the ash production in these ash-rich eruptions, rather than to magma-water explosive interaction, is related to the high decompression necessary to nucleate bubbles in a shallow reservoir.