A new convection–fractionation model for the evolution of the principal geochemical reservoirs of the Earth's mantle

Abstract

There are geochemical reservoirs (CC, DM, PM, EM1, EM2, HIMU) in the Earth's mantle and crust. They are distinguished by their isotopic and chemical abundance ratios and they arise from the combination of partial melting, segregation, ascent of the melt, differentiation of the melt, and lateral transport. The fractionation generates the chemical and isotopic diversity, while the solid-state convection works toward homogenization in particular in mantle areas with high gradient of creeping velocity perpendicular to the velocity vector. The thermal and chemical evolution of the Earth's mantle and crust has been modeled simultaneously by a fractionation mechanism plus 2D-FD Oberbeck–Boussinesq thermal convection. In contrast with the published model K1 [Walzer, U., Hendel, R., 1997a. Time-dependent thermal convection, mantle differentiation and continental-crust growth. Geophys. J. Int. 130, 303–325; geological interpretation: Walzer, U., Hendel, R., 1997b. Tectonic episodicity and convective feedback mechanisms. Phys. Earth Planet. Interiors 100, 167–188], layered convection is not an assumption but the mineral phase changes are introduced for 410 and 660 km depth using customary values of the Clapeyron slope, the density contrast, etc. So we have heat sources and sinks at 410 and 660 km, respectively, in addition to the usual Rayleigh number, Rq, for internal heating by radioactivity and bottom heating at the core–mantle boundary (CMB). The viscosity is a function of the temperature field and the pressure. Segregation takes place if the asthenospheric viscosity falls below a certain threshold. Oceanic plateaus, enriched in incompatible elements, develop leaving behind depleted parts of the mantle (DM). The resulting inhomogeneous heat-source distribution generates a first feedback mechanism. A growing continent is produced by accretion and further fractionation, consuming the older oceanic plateaus. The lateral movability of the growing continent causes a second feedback mechanism. The mentioned mechanisms generate acceptable distributions of the convective vigor and of the growth of juvenile continent material over the time axis. These distributions are stable for a moderate variation of the parameters only. The solutions of the system of differential equations show acceptable values for the distributions of the temperature, viscosity, heat flow, mantle-creep velocity and the continent's velocity. The following new result is insensitive to a strong variation of Rq: After an initially rather complex mixing process of the depleted parts and the pristine parts of the mantle, we arrive at a mainly depleted upper half of the mantle including the uppermost parts of the lower mantle and a predominantly pristine lower half of the mantle in the Phanerozoic at the latest. There is no sharp interface between the halves.