Continental growth and thermal convection in the Earth's mantle

Abstract

The main subject of this paper is the numerical simulation of the chemical differentiation of the Earth's mantle. This differentiation induces the generation and growth of the continents and, as a complement, the formation and augmentation of the depleted MORB mantle. Here, we present for the first time a solution of this problem by an integrated theory in common with the problem of thermal convection in a 3-D compressible spherical-shell mantle. The whole coupled thermal and chemical evolution of mantle plus crust was calculated starting with the formation of the solid-state primordial silicate mantle. No restricting assumptions have been made regarding number, size and form of the continents. It was, however, implemented that moving oceanic plateaus touching a continent are to be accreted to this continent at the corresponding place. The model contains a mantle-viscosity profile with a usual asthenosphere beneath a lithosphere, a highly viscous transition zone and a second low-viscosity layer below the 660-km mineral phase boundary. The central part of the lower mantle is highly viscous. This explains the fact that there are, regarding the incompatible elements, chemically different mantle reservoirs in spite of perpetual stirring during more than 4.49 × 109 a. The highly viscous central part of the lower mantle also explains the relatively slow lateral movements of CMB-based plumes, slow in comparison with the lateral movements of the lithospheric plates. The temperature-and pressure-dependent viscosity of the model is complemented by a viscoplastic yield stress, σy. The paper includes a comprehensive variation of parameters, especially the variation of the viscosity-level parameter, rn, the yield stress, σy, and the temporal average of the Rayleigh number. In the rn-σy plot, a central area shows runs with realistic distributions and sizes of continents. This area is partly overlapping with the rn-σy areas of piecewise plate-like movements of the lithosphere and of realistic values of the surface heat flow and Urey number. Numerical problems are discussed in Sect. 3.