Hydrothermal history of Piton des Neiges volcano (Reunion Island, Indian Ocean)

The Piton des Neiges volcano on Reunion Island represents a unique example of an oceanic volcano where the extreme development of amphitheatre-headed valley erosion has led to the formation of three large cirques. They are so large that the island's volcano-structural and petrological history can be traced from its emergence to the latest stages of its sub-aerial evolution (> 2.1 m.y. to 22,000 years ago).The various magmatic series of the Piton des Neiges are, moreover, abundantly invaded by hydrothermal mineralization. It is this post-magmatic feature, represented by the hydrothermal alteration of the series, which is examined here.Mineralogical studies (X-ray, microprobe, scanning electron microscope) reveal a large number of hydrothermal species. Of these, zeolites are the most common and five are described here for the first time in Reunion (gonnardite, levynite, erionite, garronite, herschelite).Six hydrothermal facies characteristic of weak metamorphism are defined: chabazitephillipsite, natrolite-thomsonite, analcime-thomsonite, laumontite-thomsonite, albiteprehnite and prehnite-pumpellyite. The paleo-temperatures covered by these facies range from 0 to 380°C.On the basis of these data and supporting field observations, three main hydrothermal phases were determined and fitted into the known chronostratigraphy. These three phases have succeeded one another over the last two million years in the Piton des Neiges massif. The extent and mineralogical facies of each phase can be related to the volcanotectonic structures. This sequence has been directly linked to the geological evolution of the massif. The progressive restriction with time of the hydrothermal manifestations to the present Piton des Neiges occurred alongside the focusing of volcanism centralized on this same relief. An attempt is made to reconstruct the island's hydrothermal history.