Constraints on hydrothermal processes on basaltic edifices: Inferences on the conditions leading to hydrovolcanic eruptions at Piton de la Fournaise, Réunion Island, Indian Ocean
Résumé
The hydrothermal processes that occur on the active basaltic edifice of Piton de la Fournaise are investigated. The present-day volcanic activity concentrates in a west-east-oriented collapsed area, 13 km long and 6-8 km large,called Enclos Fouque. Enclos Fouque is open to the sea on its eastern side, while a horseshoe rim delimits its extension to the west, south and north. This forms a continuous cliff, 100-200 m high above the floor of Enclos Fouque. Inside Enclos Fouque, a 400 m high basaltic cone with two coalescent summit craters constitutes the most active area of the volcano. Beyond the western wall of Enclos Fouque,a wide basaltic plateau called the Plaine des Sables stretches west for a few kilometers, until it reaches a cliff limiting its western part. To the north and south of this plateau, rivers notch the basaltic edifice,and along the slopes of the induced valleys springsflow. Geochemical data from these springs indicate hydrothermal activity with indeep fractured media inside the plateau generated owing to west-east regional extension. On the floor of the Plaine des Sables, near the western wall of Enclos Fouque, extensive ash deposits from a violent hydrothermal eruption have been recognized. Hydrothermal activity has also been detected in the deep (s100m) porous layers of Enclos Fouque, in an area surrounding the central cone. This circulation occurs in layers of vesiculated rocks that constitute a highly permeable and quasi-isotropic medium. Physical models are presented to illustrate the basic differences between the thermal regime of each hydrothermal system. We show that, In the periphery of the volcano, the fracture walls stabilize the hydrothermal circulation inside the fissure network , thus steadily draining the heat along the impermeable walls. Consequently, hot(230-240°C) rising convective currents with a fingershape, 150m distant, reach the uppermost parts of the fracture. By contrast, inside the isotropic porous layers of the central area, the circulation is likely to be chaotic/turbulent because of the high permeability of the medium.This scatters the thermal energy through the whole porous media. Numerical models of the transient evolution of the hydrothermal circulation in the fractured medium are presented. They reveal that the fingers generated in the deep levels of the edifice move upward at av elocity of 15-150m/yr, depending on the amplitude of the fracture permeability. The extensive character of the stresses in the Plaine des Sables, the velocity and the high temperature inside the hydrothermal flow constitute necessary conditions for the disruption of the fracture walls during boiling. Using a model of Germanovich and Lowell [J.Geophys.Res.100(1995)8417-434], these conditions are shown to be sufficient for the generation of huge (km3 of deposits) hydrothermal eruptions in the periphery of the edifice, while only small (a few m3 of deposits) hydrovolcanic events are shown to develop at the summit area. Accordingly, the ash deposits (0.6km3) recovered near the western wall of Enclos Fouque areascribed to the hydrothermal flow in fractures generated by the east-west extensional stresses.