Abstract
New strontium, lead and neodymium isotope compositions of volcanics from two volcanic island chains of French Polynesia (Marquesas, Australes) are presented. The data from Marquesas fall in the well-established oceanic islands field and are very similar to those of the Society Islands of French Polynesia. In contrast, Tubuaii Island basalts from the Australes chain exhibit very low 87Sr/86Sr ratios (∼0.7028) and the highest 206Pb/204Pb (∼21.1) and 208Pb/204Pb (∼40.4) ratios ever reported for an oceanic island. Their ɛNd value is ∼+4.9. These data are similar to those of St Helena, although even more extreme. This convergence indicates that a comparable and as yet undefined process took place in the mantle beneath the Atlantic and Pacific oceans. The isotopic data for the Polynesian volcanics encompass almost the entire range of Sr and Nd isotopic ratios found in oceanic islands (87Sr/86Sr = 0.7028−0.7083; 143Nd/144Nd = 0.51257−0.51297) and more than half of the Pb isotope range (206Pb/204Pb = 18.9−21.1; 208Pb/204Pb = 38.9−40.4).
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