Geochemistry and petrology of lavas from the submarine flanks of Reunion Island (Western Indian Ocean): implications for magma genesis and the mantle source

Major and trace element data as well as Sr and Nd isotope compositions for submarine lavas from the flanks of Reunion island are reported. The submarine basalts of the island have major and trace element compositions similar to those of the subaerial basaltic rocks. This implies that no compositional change occurs in the shield-building magmas of the two Reunion volcanoes. Fractional crystallization of lavas from both Piton de la Fournaise and Piton des Neiges begin well within the mantle at pressures up to 1 GPa which is significantly deeper than the crust-mantle boundary at about 12 km depth. The Reunion primary magmas form at an average depth of about 4 GPa in agreement with the thickness of the plate beneath the island. Lavas from both Reunion volcanoes have similar trace element compositions with the exception of lower Th/Ba for Piton des Neiges which implies a relatively homogeneous plume source during, at least, the last 2 Ma. The lack of any variation in the partial melting processes during this time span implies a thermal steady state of the plume centre. The Reunion lavas form either from a source that was enriched by partial melting or that contains recycled enriched MORB. Based on MORB- like Ce/Pb and Nb/U ratios an influence by fluid-metasomatized mantle, sediment or continental crustal material in the Reunion source appears unlikely.