Rapid change of lava composition from 1998 to 2002 at Piton de la Fournaise (Reunion) inferred from Pb isotopes and trace elements: Evidence for variable crustal contamination

After an unusually long quiet period of nearly 6 years, in 1998 the Piton de la Fournaise volcano started a new cycle of intense volcanic activity. We report geochemical data on the first nine events (53 samples), from the long-lived initial eruption (six and a half months) of 1998 to the high-flux picritic eruption of January 2002. Pb isotopes and trace elements display systematic, coupled variations, which are mostly confined to the beginning and the end of the period. Two well-defined binary mixing trends are shown by Pb-Pb and Pb-trace element relationships. These trends indicate a change of end-member components between March and June 2001 that coincides with the transition from steady-state basalts to picrites. A three-component mixing model involving a homogeneous plume and two contaminants successfully explains the data. The Pb-Pb relationship requires that two mixing processes occur successively: plume-derived magma interacts first with altered oceanic crust, and the resulting hybrid product then interacts at shallower level with the old lavas constituting the base of the volcanic edifice. Assimilation of edifice material decreased continuously from 1998 to 2002, whereas assimilation of oceanic crust drastically increased during the late-stage picritic eruption. These results suggest that picrites may have resided for an unusually long time at an oceanic crustal level before ascending rapidly through the volcanic edifice with little interaction with channel walls.