Plate Tectonics, Convection, Phase Transitions and Heterogeneity in the Mantle

Plate tectonics and mantle convection are one and the same. The process of convection, partial melting and differentiation at mid-ocean ridges generates the oceanic lithosphere, which is thermally, lithologically and rheologically distinct and heterogeneous. During the course of its lifetime, the oceanic plate is further modified by tectonic interactions, rehydration at transform faults and fracture zones, volcanism and other geological processes. When the oceanic lithosphere returns to the mantle via subduction it continuously introduces its inherited and acquired thermal, compositional and rheological heterogeneity into the mantle. Such heterogeneity has clear seismic signals and convective consequences, distinct from a homogenous mantle. I review previous work on the possible seismic signals of a mechanically mixed mantle resulting from the delamination of the oceanic crust from the underlying mantle during subduction. I examine the possible accumulation through time of the delaminated crust, paying close attention to its evolving composition as the Earth cools, and the consequences for the composition of LLSVP. There are interesting convective consequences of the presence of such heterogeneity for the temperature of ridges and hotspots.