Gravitational depletion of the early Earthʹs upper mantle and the viability of early plate tectonics

Numerical modelling of stirring of subducted mafic (oceanic) crust in the hot early mantle yields an upper mantle strongly depleted of mafic components due to gravitational settling, while the lower mantle becomes marginally enriched, with a thin dense layer at the base. These results accord with the very early and strong depletion of incompatible trace elements in the mantle source of the oldest rocks. They support a recent geochemical argument that the depletion is due more to the extraction of mafic material rather than of continental crust. The strong depletion would yield a thin oceanic crust, despite the high temperature of the mantle, which would make early plate tectonics more viable.