Scientific exploitation of deep geothermal drillings on Piton de la Fournaise volcano (La Reunion, Indian Ocean) – Forages PdF

The active volcano of the Reunion Island, Piton de la Fournaise, is among Earth's most active volcanoes. It is the present surface expression of a Mantle plume (hotspot). This type of volcanism thus results from interactions between the Mantle, the Crust and the volcanic edifice. Such a system evolves in a complex way through time. The volcanic products accessible at the surface provide only part of the message. For this reason, the USA have carried out dep scientific drillings on a similar site on the island of Hawaii. Indeed, a drill hole provides continuous information at depth (and therefore in time), thus allowing a detailed study of the evolution of the volcanic system. An opportunity is now being offered to undertake a very ambitious study of the volcanic system of La Réunion. The Regional Council of La Reunion has taken the decision to drill 2 or 3 geothermal reconnaissance, 1.2-1.5 km deep, wells on Piton de la Fournaise volcano. It has also expressed a strong willingness to open the drillings to the scientific community. In addition to the geothermal drilling operation, the Regional Council will pay the coring of at least one well. The drilling operations are expected to start in mid-2008. Deep drill holes on active volcanoes are exceptionally carried out (e.g. Kilauea, Unzen, Iceland). Although the planned dill-holes on Piton de la Fournaise are not intended for scientific purpose, they offer an extraordinary opportunity to acquire highly valuable information on La Réunion volcanism and on hotspot volcanism in general. Many scientific themes can be explored through such wells: magmatic evolution of the volcanism through time, nature of the eruptive products, volcano-tectonic faulting and brecciation, hydrothermal circulation and alteration, physical properties of the rocks, calibration of geophysical models … The project is very advantageous for the scientific community in terms of financial investments. The cost of the drilling operations estimated to be near 5 millions of euros will be paid by the Région Réunion and ICDP (International Continental Scientific Drilling Program) will contribute to 50-80000 euros by providing technical support to the scientific operations. Only the financial support for the scientific projects is submitted to the ANR. The INSU has beforehand collected pre-projects from different French groups that cover a large scientific spectrum. The earlier pre-projects have been discussed during a meeting with the participants and a harmonization of the projects has been made. The individual projects will be exposed below. La Réunion has been the subject of numerous geological and geophysical works since about the 1970s, mostly in the framework of CNRS projects. The scientific results of the project presented here will indisputably have a strong impact in the international community. An international workshop will be organized in La Réunion in 2010 or 2011, after the end of the drilling operations, when preliminary results will be available. The INSU has financially supported all the preliminary expenses related to this project, because it has been considered as a major scientific issue..A project engineer has been hired by the INSU to manage all technical aspects related to the scientific projects and to handle the data and samples databases. The themes that will be treated in the ANR project can be sorted in a few large fields, but interactions obviously exist between them: • Structure and evolution of Piton de la Fournaise volcano • Study of the mantle plume through the nature of the magmas (petrology, mineralogy and geochemistry) • Study of volcano-tectonic processes • Study of the nature and of the circulation of fluids (hydrothermal alteration, dissolution and deposition of minerals) • Microbiological population at depth and its interactions with the fluids and the rocks • Evolution of the magnetic field • Hydrogeological system • Physical properties of the terrains • Calibration of the geophysical methods At the end of the project, the site of La Réunion will be, with Hawaii, the best studied oceanic hotspot system.