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Nucleation of gas bubbles triggered by shear in magmas: an experimental approach

Olivier Roche 1, Jean-Michel Andanson2, Alain Dequidt2, Christian Huber3, Olivier Bachmann4, David Pinel2

  • Affiliations:  1 Laboratoire Magmas et Volcans, University Clermont Auvergne, CNRS, IRD, OPGC, Clermont-Ferrand, France; 2 Institut de Chimie de Clermont-Ferrand, University Clermont Auvergne, CNRS, Clermont-Ferrand, France; 3 Earth, Environmental, and Planetary Sciences, Brown University, USA; 4 Department of Earth Sciences, ETH, Zurich, Switzerland

  • Presentation type: Talk

  • Presentation time: Monday 14:45 - 15:00, Room S150

  • Programme No: 3.6.3

  • Theme 3 > Session 6

Abstract

The nucleation of gas bubbles in magmas is fundamental to controlling the dynamics of volcanic eruptions. It is commonly assumed that nucleation is essentially the consequence of pressure drop that causes supersaturation of volatile species dissolved in the melt. Here, we examine a different mechanism: the formation of gas bubbles promoted by shear, which is ubiquitous in volcanic systems. We present analogue experiments in which liquid polyethylene oxide (PEO) containing dissolved supersaturated CO2 is subjected to shear, in a configuration that enables us to observe nucleation in situ. At a given supersaturation pressure, gas bubbles nucleate in the PEO at a critical shear rate, which corresponds to a critical shear stress given the known liquid viscosity. This result is supported by complementary molecular dynamic simulations we performed under conditions that mimic those in experiments. Overall, our experimental results reveal that the critical shear stress for nucleation decreases as the initial volatile supersaturation increases. By expressing our results in dimensionless form in order to apply them to real cases, we show that nucleation events caused by shear are possible at a capillary number greater than 10-5. This condition is met in volcanic conduits and lava flows, particularly for the most viscous magmas. These results suggest that bubble nucleation is not only caused by decompression or nanolite crystallization during ascent, but that shear also plays a role. Our results have implications for magma degassing processes, estimates of decompression rates from observed bubble size distributions, and predictions of eruptive styles.