Dynamics of Magma Transfer at Piton de la Fournaise Volcano (Réunion Island, Indian Ocean)

Piton de la Fournaise (Réunion Island) is a basaltic oceanic intraplate volcano. On the basis of geological and geophysical data gathered at the volcanological Observatory since its installation in 1980, and of a detailed geological survey of its activity since 1930 (Bachelery, 1981), a preliminary model of its magma supply and transfer is presented. For the period 1930–1985 the mean magma output rate was 0.34 m3/s, but for shorter time intervals this rate varied between about 0.15 and several m3/s. The cumulative output volume is represented by an irregular sawtooth curve. We distinguish a type A behavior that applies to periods of high magma output rates and a type B behavior corresponding to the intervening periods of lower output rates. The type A periods are relatively short and are associated with eruptions of oceanites and olivine basalts that are both less evolved lavas than the other lavas of the Enclos caldera. It is suggested that the type A periods mark the arrival of new batches of magma in the shallow magma storage complex. According to this model the three oceanite eruptions of 1931, 1961 and 1977, which account for at least half of the erupted volume of lavas of about 50 eruptions between 1930 and 1985, would reflect major pulses of internal magma transfer while less considerable eruptions of oceanites or olivine basalts would represent smaller pulses. The type B period observed since 1977 is analysed; the physical behavior (seismicity and deformation) of Piton de la Fournaise since 1980 and detailed petrological and geochemical studies (Boivin et al., 1984) of the lavas of the eruptions that followed the April 1977 oceanite eruption lead us to assume that these eruptions have been fed by a shallow magma reservoir that has not been replenished since 1977. Similar geophysical and petrological data are not available for the other type B periods. It is therefore not possible to assume that they also correspond to successive partial discharges of a non-replenished reservoir as suggested by the pattern of the cumulative volume curve.The internal magma transfer to the sub-volcanic reservoir of Piton de la Fournaise appears to be a complex, discontinuous and irregular process for a period of some tens of years. This contrasts with the well-known sub-continuous supply of magma to the shallow reservoirs of Kilauea and Krafla volcanoes.