GEOCHEMICAL STUDIES OF TAHITI, TEAHITIA AND MEHETIA, SOCIETY ISLAND CHAIN

The Society Island chain is located within a region of the South Pacific characterized by anomalously enriched isotopic compositions. The islands of Mehetia and Tahiti, together with the seamount Teahitia, are among the youngest volcanoes in the chain and must be close to the present-day hotspot location. We report here detailed geochemical (major- and trace-element, and isotope) studies of fresh, age-controlled lavas. The data reveal temporal geochemical variations on Tahiti, and close compositional links between Tahiti and both Teahitia and Mehetia. Major-element compositions of the Tahitian lavas define three magmatic fractionation series which coincide with three volcanic phases that have been identified independently. Fractionation-corrected trace-element data also show temporal variations in both absolute concentrations and in the ratios of highly incompatible to moderately incompatible trace elements. Both major- and trace-element results suggest a progressive decrease in the degree of partial melting with time. Mixing of magmas derived from an enriched plume source and from metasomatized lithosphere is consistent with the observed compositional and isotopic variations among the different parental magmas. The earliest recognized (shield-building?) lavas on Tahiti carry the strongest geochemical plume signature, while late-stage lavas were apparently derived partly from lithospheric sources with smaller degrees of partial melting. The similarities in chemical characteristics among lavas from Tahiti, Teahitia and Mehetia suggest common sources and similar magmatic processes for the three volcanoes, consistent with the hotspot mechanism for the origin of these and other islands in the Society chain.