What's new in the Mantle Stew? More Thoughts on EMII

The basalts of eastern Sao Miguel are founder members of the mantle radiogenic isotope flavour that has come to be known as EMII. The highly radiogenic 87Sr/86Sr (~0.705) and coupled low ɛNd (~+1) have long been taken to be an indication of a sediment contribution to source of these and other EMII basalts. Elevated 208Pb/204Pb and 207Pb/204Pb relative to 206Pb/204Pb in the enriched Sao Miguel samples also apparently smack of sediment. Yet a reassessment of the Sao Miguel samples from a new perspective aided by Hf isotope measurements, and strongly supported by incompatible trace element analyses, suggests that a sedimentary component in the enriched Sao Miguel source is unlikely. The Sao Miguel lavas define a remarkably linear array in Nd-Hf isotope space and point to an unusual composition below the mantle array. This a territory seldom occupied by sediments and the rarer terrigenous sediments that do have such isotopic compositions also have incompatible trace element ratios quite unlike those of the Sao Miguel lavas (even considering the possible effects of subduction zone processing). Simple modelling suggests a moderate (~2%) degree melt of a garnet peridotite, that is aged for >2Ga, can form an appropriate Hf-Nd isotopic endmember for the Sao Miguel array. Certain oceanic basalts, such as those from the island of Reunion, are suitable modern day analogues. Thus we suggest that recycled oceanic basalts form the enriched component beneath Sao Miguel. Incompatible element ratios show no surface alteration nor subduction zone dehydration signatures and so we suggest it is recycled underplated basalts rather than the seawater-altered island edifice itself that contributes to the Sao Miguel source. The extreme radiogenic isotope compositions seen in Sao Miguel are dominantly a result of age rather than extreme parent daughter fractionation, and the rarity of such signatures reflects the efficiency of mantle convection in either hiding or homogenising similar compositions.