Mechanisms and dynamics of bubble coalescence in silicic magmas

Mechanisms and dynamics of bubble coalescence in silicic magmas 1 Jonathan M. Castro (University of Mainz) 2 Alain Burgisser (ISTO-CNRS) 3 Ian Schipper (ISTO-CNRS) 4 Simona Mancini (ISTO-Université d’Orléans) 5 6 Abstract 7 Bubble coalescence is an important process that strongly affects magmatic degassing. 8 Without coalescence, bubbles remain isolated from one another in the melt, severely 9 limiting gas release. Despite this fact, very little has been done to identify coalescence 10 mechanisms in textures of magmatic rocks, or to quantify the dynamics of bubble 11 coalescence in melts. In this paper, we present a systematic study of bubble-coalescence 12 mechanisms and dynamics in natural and experimentally produced bubbly rhyolite 13 magma. We have used a combination of natural observations aided by high-resolution 14 X-Ray computed tomography, petrological experiments, and physical models to identify 15 different types of bubble-bubble interaction that lead to coalescence on the timescales of 16 magma ascent and eruption. Our observations and calculations suggest that bubbles 17 most efficiently coalesce when inter-bubble melt walls thin by stretching rather than by 18 the commonly invoked mechanism of melt drainage from between converging bubble 19 walls. Orders of magnitude more rapid than melt drainage, bubble wall stretching 20 produces walls thin enough such that inter-bubble pressure gradients may cause the 21 melt wall to dimple the melt wall, further enhancing coalescence. To put these results 22 into volcanogical context, we have identified magma ascent conditions where each 23 coalescence mechanism should act, and discuss the physical conditions for preserving 24 coalescence structures in natural pumice. The timescales we propose have the potential 25 to improve volcanic eruption models, which currently do not account for bubble 26 coalescence. Although we do not address the effect of shear strain on bubble 27 Manuscript Click here to download Manuscript: Castroetal.pdf Click here to view linked References