HR: 11:10h
AN: V42B-04    [Abstracts]
TI: A model for episodic degassing of an andesitic magma intrusion and application to La Soufriere de Guadeloupe volcano (Lesser Antilles) since its last eruptive crisis in 1975- 77.
AU: * Boichu, M
EM: mb632@cam.ac.uk
AF: University of Cambridge, Department of Geography, Downing Place, Cambridge, CB2 3EN, United Kingdom
AU: Villemant, B
EM: villemant@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu , Paris, 75005, France
AU: Boudon, G
EM: boudon@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu , Paris, 75005, France
AB: Episodic magmatic degassing has been observed at numerous volcanoes, especially those of intermediate composition. It can span timescales from years to decades. We propose here a physical model for the degassing of a shallow magma intrusion to explain this phenomenon. The magma cools by convection, which leads to melt crystallization, volatile exsolution and magma overpressure. When the pressure reaches a critical value, wall rocks fracture and the exsolved gas escapes. The intrusion then returns to the initial lithostatic pressure and a new cooling-crystallization-degassing cycle occurs. A series of such cycles leads to an episodic degassing. The trend and time scale of the degassing process are mainly governed by magma cooling. Two degassing regimes, with a high gas pulse frequency for the first and a lower for the second, are exhibited. The transition between these two regimes is caused by the strong viscosity increase when the magma crystallinity exceeds the crystal percolation threshold. We find that the time to this transition is dependent on magma volume, to a first approximation. This model is applied to interpret chemical analyses of spring waters sampled on La Soufriere de Guadeloupe volcano over the past 30 yr, since its last eruptive crisis. This study enables to give a global interpretation for the evolution of the spring water composition and the fumarolic and seismic activities since 1975-77. Moreover, it provides constraints on key aspects of the sub-surface magmatic system, including estimation of stored magma volume and host rock strength. This model therefore represents a relevant tool for volcanic surveillance and hazard assessment.
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly