Mantle plumes from ancient oceanic crust

We propose the following model for the origin of "hot-spot" volcanism: Oceanic crust is returned to the mantle during subduction. It is separated from the surrounding peridotite, it sinks into the deeper mantle and accumulates at some level of density compensation, possibly the core-mantle boundary. The accumulated layerlocally reaches thicknesses exceeding 100 km. Eventually, it becomes unstable as a consequence of internal heating, and the resulting diapirs become the source plumes of ocean island basalts (OIB) and other hot-spot volcanism. This mechanism may trigger upper mantle convection as suggested by Morgan [ ¹]. Our model provides possible explanations for (1) the high traceelement concentrations of OIB, (2) the seemingly contradictory isotopic evidence for both enrichment and depletion of magmaphile elements in OIB sources, (3) the phenomenon of apparent mantle isochrons in oceanic basalts, and (4) the apparently episodic nature of continental igneous activity. The model can be tested further, as more knowledge accumulates about the actual bulk composition of the oceanic crust, including its alteration products and sediment cover.