Degassing at La Soufriere de Guadeloupe volcano (Lesser Antilles) since the last eruptive crisis in 1975-77: Result of a shallow magma intrusion?

In the 15 years following the last crisis of phreatic type in 1975-77, the hydrothermal and seismic activity of La Soufriere de Guadeloupe has displayed an episodic behavior superimposed on a long-term decay, as inferred from seismicity and degassing, monitored through fumarolic emissions and Cl content in thermal springs. In particular, the evolution of the Cl content can be explained by a succession of magmatic gas injections into the hydrothermal system. This episodic supply of gas may be associated to the thermodynamic evolution of a batch of magma intruded at shallow depth around 1976. Progressive magma cooling, crystallization, gas exsolution and pressure buildup lead to cyclic failure of host rock around the intrusion, and episodic release of gas, with a decreasing frequency as cooling proceeds. This physical model allows to reproduce the pattern of degassing activity of La Soufriere, including the decreasing frequency of Cl pulses with time. Our preferred forward model of Cl pulses involves a magma intrusion volume in the range 0.01-0.52 km(3) and a tensile strength of the surrounding rock in the range 0.1-0.6 MPa. These results reinforce the hypothesis of a shallow magma intrusion triggering the 1975-77 crisis and provide an explanation for the degassing and seismic activities observed until 1992. (C) 2011 Elsevier B.V. All rights reserved.