Reconcilition of the Geophysical and Geochemical Observations: Inferences from Numerical Models of Thermochemical Mantle Convection

Evidence from geophysical observations and numerical models suggested that the Earth’s mantle is a giant heat engine, the surface tectonic plates is a manifestation of the convection within the mantle, the subducted lithosphere can sink into the deep mantle, stirring the mantle from top to bottom, and the source of mid-ocean ridge basalts (MORBs) comprises most of the mantle. However, the geochemical picture of the mantle from oceanic basalt geochemistry is inconsistent with this geophysical picture because it requires a depleted MORB and an enriched oceanic island basalt (OIB). Results from numerical models of thermochemical mantle convection on geochemical processing and oceanic crust segregation show that geophysical models succeed in explaining the age and character of refractory incompatible trace elements and their isotopes. The results also show that the settling of subducted oceanic crust is more likely to comprise a gradational compositional layer at the bottom of the mantle. Models on plume entrainment imply that when chemical density increases with depth, thermal plumes tend to entrain only the less dense upper part of the chemical layer, while the denser lower part is stable and resists the chemical mixing in the layer as a whole. The long-term stability of the denser lower part and its resistance against entrainment might be the key to reconciling the isotope difference and similarity of MORBs and OIBs, and thus the geophysical and geochemical inferences about the mantle.