Toward a Thermochemical Model of the Evolution of the Earth’s Mantle Uwe Walzer 1 , Roland Hendel 1 , and John Baumgardner 2 1 Institut f¨ ur Geowissenschaften, Friedrich-Schiller-Universit¨ at, Burgweg 11, 07749 Jena, Germany u.walzer@uni-jena.de 2 Los Alamos National Laboratory, MS B216 T-3, Los Alamos, NM 87545, USA Summary. This is a report on first steps for a combination of two numerical models of the evolution of the Earth’s mantle: The first one, K3, is a new 2-D convection- fractionation model that simulates the growth of continents and of the geochem- ically complementary depleted mantle reservoir. The second model shows the 3-D generation of oceanic lithospheric plates and subducting sheet-like downwellings in a spherical-shell mantle. Based on the abundances of the present-day geochemical reservoirs of Hofmann (1988) we developed a numerical dynamical model of convec- tion and of chemical differentiation in the Earth’s mantle. It is shown that a growing and additionally laterally moving continent and a growing depleted mantle evolved from an initially homogeneous primordial mantle. The internal heat production den- sity of the evolving mantle depends on the redistribution of the radioactive elements by fractionation and convection. The fractionation generates separate geochemical reservoirs. However, the convection blurs the reservoirs by mixing. Although we take into account also the effects of the two phase transitions in 410 and 660 km depth, it is essentially the dependence of the viscosity on radius which guarantees the conser- vation of the major geochemical reservoirs. This model has no internal compulsory conditions. The principal idea of this first model is to compute the relative vis- cosity variations as a function of depth from observable quantities. We develop a self-consistent theory using the Helmholtz free energy, the Ullmann-Pan’kov equat- ion of state, the free volume Gr¨ uneisen parameter and Gilvarry’s formulation of Lindemann’s law. In order to receive the relative variations of the radial factor of the viscosity, we insert the pressure, P , the bulk modulus, K, and ∂K/∂P from PREM. For mantle layers deeper than 771 km we used the perovskite melting curve by Zerr and Boehler (1993, 1994) in order to estimate the relative viscosity. For the calibration of the viscosity we have chosen the standard postglacial-uplift viscosity beneath the continental lithosphere. Furthermore, we took into account the depen- dence of the viscosity on temperature and on the degree of depletion of volatiles. An essential first new result of this paper is a high-viscosity transition layer and a second low-viscosity layer below it. Although our model mantle is essentially heated from within, we assume additionally a small heat flow at the CMB. This is nec- essary because of the dynamo theory of the outer core. The second main result of this first model is a more distinct bipartition of the mantle in a depleted upper part and a lower part rich in incompatible elements, yet. This result is rather insensit-