GEOCHEMICAL SIGNATURES OF OCEANIC AND CONTINENTAL BASALTS - A KEY TO MANTLE DYNAMICS

The distinctive trace element and Sr-Nd-Pb isotopic compositions of oceanic island basalts (OIB), believed to be related to the upwelling of deep mantle plumes from the core-mantle boundary, provide important constraints for models of mantle dynamics. Whilst the present-day plume flux is small compared to the plate-scale flow, during the Early Cretaceous there appears to have been a major phase of superplume activity which changed, rather drastically, 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. During such cycles of superplume activity outward flow from the axis of the plumes may provide an important driving force for plate tectonics. Early Cretaceous continental and oceanic flood basalts, possibly related to the initial impact of the superplume heads on the base of the lithosphere, have distinctive Nd-Pb isotopic signatures which suggest that the source of the super-plumes (? D'' layer at the base of the lower mantle) was ancient (c. 2 Ga) 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, whose compositions arc close to those of the HIMU and EM I end-members of the OIB spectrum respectively, can be recognized in the magmatic rocks generated throughout the 130 Ma history of opening of the South Atlantic Ocean.