Geochemical signatures of oceanic and continental basalts; a key to mantle dynamics?

The distinctive trace element and Sr-Nd-Pb isotope compositions of OIB, believed to be related to the upwelling of deep mantle plumes from the core-mantle boundary, provide important constraint for models of mantle dynamics. During the early Cretaceous there appears to have been a major phase of plume activity which changed the pattern of convection within the upper mantle, may have modulated the reversal frequency of the Earth's magnetic field, and considerably accelerated plate motions. Major foci of upwelling occurred in the Pacific and Indian Oceans and beneath Gondwana, along the site of the future South Atlantic rift. Early Cretaceous continental and oceanic flood basalts have distinctive Nd-Pb isotope signatures which suggest that the source of the super-plumes was ancient (approx 2000 m.y.) recycled oceanic crust plus subducted pelagic sediment. In addition the Pacific super-plume may also have entrained lower mantle material with distinctive Nd-Pb isotopic characteristics. Within the Atlantic domain, the distinctive isotopic fingerprint of two plumes, St Helena and Tristan da Cunha, can be recognized in the magmatic rocks generated throughout the 130 m.y. history of the opening of the South Atlantic Ocean.