SOURCE COMPOSITIONS AND MELTING PROCESSES IN THE SOCIETY AND AUSTRAL PLUMES (SOUTH-PACIFIC OCEAN) - ELEMENT AND ISOTOPE (SR, ND, PB, TH) GEOCHEMISTRY

We present comprehensive geochemical analyses on samples from the active volcanoes of the Society and Austral hotspot chains, including data for major, trace and rare-earth elements, and Sr, Nd, Pb and Th isotopes. The latter can be used to determine the Th/U in the source at the time of melting, and so give a constraint on the absolute amount of incompatible-element fractionation occurring during melting. SiO2 vs. MgO variations show evidence for variable amounts of a nephelinitic melt component (low SiO2, low MgO) in all the magmas studied. The nephelinite is probably produced in the presence Of CO2 during melting. Correlations between SiO2 and the degree of Th-U fractionation (derived from Th isotope measurements) imply that the CO2-driven, nephelinitic melting is also responsible for fractionating the Th/U ratio. Comparing the Th isotopes with Sr, Nd and Pb isotopes, it is possible to place limits on the Th/U ratios in the EM II (Societies) and HIMU (Australs) sources. These are approximately 3.4 and less-than-or-equal-to 2.5, respectively. The Nb/U ratio, which was previously thought to be relatively constant in all oceanic volcanics (47 +/- 10), is shown to be anomalously low (25 +/- 5) in some of the Society Seamounts and high (> 60) in some of the Australs. This confirms the presence of continent-derived material in the Society source and suggests that HIMU volcanics may have elevated Nb/U ratios. The change in Pb isotope compositions in the Australs, from HIMU (e.g., Tubuai) before 6 Ma to the more recent, less extreme compositions seen for example at Macdonald, is the result of a small amount of subducted sediment contaminating the pure old recycled oceanic crust component which, before 6 Ma, yielded HIMU.