LEAD ISOTOPE EVIDENCE FOR YOUNG TRACE-ELEMENT ENRICHMENT IN THE OCEANIC UPPER MANTLE

ISOTOPIC heterogeneity in ocean island basalts has generally been ascribed to processes related to the long-term cycling of mantle material1-6. A recent study of Cameroon line lavas reported higher Pb-208/Pb-204 and Pb-206/Pb-204 ratios towards the continent/ocean boundary (c.o.b.), but no corresponding increase in Pb-207/Pb-204, indicating large in situ fractionations of uranium and thorium relative to lead in the upper mantle approximately 10(8) years ago7. Here we present neodymium, strontium and lead isotope data for a variety of central Atlantic islands, and show that similar offsets in lead isotope ratios are found in lavas from the islands of Madeira and Trinidade. Like the Cameroon line c.o.b. lavas, these lavas are characterized by high U/Pb and Ce/Pb, low K/U and are located in areas of old oceanic lithosphere8. But in contrast to the Cameroon line7, the Madeira lavas are derived from an enriched MORB-type source with low Pb-207/Pb-204 and Sr-87/Sr-86 and high Nd-143/Nd-144. The lead isotope data can be explained if the U/Pb ratios in the sources are comparable to those observed for the lavas and the U/Pb fractionation occurred at the time of formation of the local oceanic lithosphere. Although we do not have a satisfactory explanation for the U/Pb fractionation, it must have occurred at shallow depths in the mantle, near a spreading ridge; and the resulting enriched source regions have since remained fixed relative to the migrating lithosphere.