Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.