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

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 a bundance ratios and they arise from the combination of partial melting, seg regation, ascent of the melt, differentiation of the melt, and lateral tran sport. The fractionation generates the chemical and isotopic diversity, whi le the solid-state convection works toward homogenization in particular in mantle areas with high gradient of creeping velocity perpendicular to the v elocity vector. The thermal and chemical evolution of the Earth's mantle an d crust has been modeled simultaneously by a fractionation mechanism plus 2 D-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: Waiter, U., Hendel, R., 1997b. Tecton ic episodicity and convective feedback mechanisms. Phys. Earth Planet. Inte riors 100, 167-188], layered convection is not an assumption but the minera l phase changes are introduced for 410 and 660 km depth using customary val ues of the Clapeyron slope, the density contrast, etc. So we have heat sour ces and sinks at 410 and 660 km, respectively, in addition to the usual Ray leigh number, Rq, for internal heating by radioactivity and bottom healing at the core-mantle boundary (CMB). The viscosity is a function of the tempe rature field and the pressure. Segregation takes place if the asthenospheri c 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 firs t feedback mechanism. A growing continent is produced by accretion and furt her fractionation, consuming the older oceanic plateaus. The lateral movabi lity of the growing continent causes a second feedback mechanism. The menti oned mechanisms generate acceptable distributions of the convective vigor a nd of the growth of juvenile continent material over the lime axis. These d istributions are stable for a moderate variation of the parameters only. Th e solutions of the system of differential equations show acceptable values for the distributions of the temperature, viscosity, heat flow, mantle-cree p velocity and the continent's velocity. The following new result is insens itive 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 arr ive at a mainly depleted upper half of the mantle including the uppermost p arts of the lower mantle and a predominantly pristine lower half of the man tle in the Phanerozoic at the latest. There is no sharp interface between t he halves. (C) 1999 Elsevier Science B.V. All rights reserved.