Abstract
The remarkable distinction between the compositions of ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) provides an important constraint on models of mantle composition and structure1–3. Previous studies of OIBs4–8, however, have emphasized regional isotopic variations, often relying on a small number of samples from many separate volcanoes. Although this type of sampling has now established the basic range of isotopic variations, with the notable exception of the Hawaiian Islands6,9,10, there is little information on either spatial or temporal variations within a single volcano. Here we report isotopic (Sr, Nd and Pb) and K–Ar age measurements for tholeiites, alkali basalts and differentiated rocks from the island of Ua Pou. In this island volcanism spanned the interval from 5.6 to 1.8 Myr, with the ratio of highly to moderately incompatible trace elements increasing with time; however, in contrast to Hawaii, 87Sr/86Sr and 207Pb/204Pb increased while 143Nd/144Nd decreased from the tholeiitic to alkalic magmas. The total variation in isotopic composition within this single island is nearly as great as within the entire French Polynesian region8,11,12, and argues against systematic geographical correlations13.
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