Ultradeep continental roots and their oceanic remnants : a solution to the geochemical "mantle reservoir" problem?

High-resolution global seismic tomography (Vs) models reveal high-velocity domains beneath cratonic crust in Africa that extend to depths of 300–400 km. These high-velocity domains show a distinct contrast with the characteristics of “normal” asthenosphere and are interpreted as depleted, buoyant roots that formed in the Archean and have been metasomatised over time, but have remained attached to the overlying crust. Such deep roots are impediments to free horizontal convection in the upper mantle. The movement of magmas and other fluids in such regions may be more vertically constrained (a shallow lava lamp regime), creating a geodynamic environment conducive to interaction of such magmas with the boundaries of deep mantle domains that would carry old “crustal” geochemical signatures. The tomographic models and the new world magnetic-anomaly map show that these continental roots, overlain by thinned continental crust, locally extend well out under the deep Atlantic Ocean basin. However, such high-velocity domains are not confined to the basin margins, but are scattered randomly through the basin, some quite distant from the continental margins of South America and Africa. These high-velocity domains are interpreted to be remnant lithospheric fragments isolated by disruption of the ancient continental regions during rifting. This interpretation is supported by the old Os depletion ages of mantle peridotites from mid-ocean ridges and oceanic islands. Basaltic magmas near such high-velocity domains carry the geochemical signatures (EM1, EM2) of interaction with refertilised cratonic mantle. The interaction of rising magmas with fragments of ancient lithospheric mantle can explain such geochemical signatures and obviates the need for complex models in which these geochemical reservoirs are isolated and preserved in the convecting mantle.12 page(s