The history of supercontinents and oceans from the standpoint of thermochemical mantle convection

Abstract

A thermochemical model of mantle convection reproducing both the supercontinental and oceanic history of the Earth is formulated. The model takes into account (i) the stratification of the mantle into the upper and lower layers, (ii) the differentiation, which lead to the formation of light material in D” layer and (iii) the eclogitization, which results in the production of heavy material in the subduction zones. The numerical experiments are carried out in the Boussinesq approximation with isoviscous rheology.It is found that the high degree of spherical symmetry of the young planet promotes the cubic shape of the mantle convection. The hot initial state and rich chemical potential are reflected in active differentiation of the mantle material and formation of the bulk of the continental crust at the early stage of evolution.It is established that after losing a significant part of thermochemical potential, the planet passes into megacyclic regime of evolution, where long periods of predominantly two-layer convection are interrupted by mantle overturns. The overturns render extensive fast mass exchange between the two mantles: part of the cold substance from the upper mantle subsides and hot material lifts up from the lower mantle to fill its place. The numerical experiments demonstrate a remarkable phenomenon of self-organization of a single global convective sink during the mantle overturn. Such a collective sinking of the upper mantle substance is energetically more favorable. The planetary-scale flow, which converges to the global downwelling flow (sink), assembles the supercontinent.Our modeling of the geodynamic evolution of the Earth reproduces five mega-cycles. It is found that the configurations of the overturns tend to a dipole form and the position of the global sinks is being stabilized. Thus, the spherical mantle becomes slightly different: the hemisphere where the global sink is pulsating hosts the continents and the alternating Atlantic-type oceans, while the other hemisphere marked by the superplumes activity is dominated by the formation of the giant floor of the Pacific Ocean. As a result, the Earth acquires an asymmetrical appearance with the continental and oceanic hemispheres.