Hafnium isotopic studies of the Cameroon line and new HIMU paradoxes

We present the first detailed Pb, Nd, Sr and Hf isotopic study of a classic HIMU system. The Hf isotopic compositions of 18 representative basaltic lavas from the length of the Cameroon volcanic line have been determined using the new technique of inductively coupled plasma magnetic sector multiple collector mass spectrometry. The Hf isotopic compositions are found to be the same on average in continental, oceanic, and continental-oceanic boundary sectors. The three islands from the oceanic sector, Pagalu, Sao Tome and Principe, display a large spread in Pb and Sr isotopic compositions but have Hf and Nd isotopic compositions that are both uniform and identical to the HIMU islands of Tubuai and St. Helena. The Hf and Nd isotopic compositions of HIMU centers appear to be a more reproducible and characteristic feature than Pb and Sr isotopic compositions. On this basis, basalts along the entire Cameroon line have a similar origin. The data imply that Pagalu, with its unradiogenic Pb-206/Pb-204 (similar to 19) has incorporated the same Hf and Nd as Principe, Bioko, Etinde, Mt. Cameroon, St. Helena and Tubuai, all with average Pb-206/Pb-204 > 20. These data are consistent with the conclusion that the increase in Pb-206/Pb-204 toward the COB is due to young fractionation in U/Pb within the upper mantle. Neither the Hf isotopic composition of Cameroon line basalts, nor that of HIMU basalts in general, can be explained by recycling and storage of subducted N-MORB. Storage of more enriched components is required. Furthermore, the uniformity in Hf-Nd isotopic space is difficult to explain by any recycling models and is more consistent with the HIMU source representing a distinctive and potentially important reservoir with specific Lu/Hf, Sm/Nd and age. (C) 1997 Elsevier Science B.V.