Abstract: Time-dependent thermal convection, mantle differentiation and continental-crust growth
Inhalt
U. Walzer and R. Hendel. Time-dependent thermal convection, mantle differentiation, and continental crust growth. Geophys. J. Int., 130:303-325, 1997a.
Time-dependent thermal convection, mantle differentiation and continental-crust growth
Uwe Walzer1, Roland Hendel1,
1 Institut für Geowissenschaften, Friedrich-Schiller-Universität, Burgweg 11, 07749 Jena, Germany
Abstract.
The thermal evolution of the Earth is controlled by radioactive elements whose heat production rate decays with time and whose spatial distribution depends on chemical segregation processes.
We present a 2-D and finite-difference Boussineq convection model with temperature-dependent viscosity and time- and space-dependent radioactive heat sources. We used Newtonian rheology, boxes of aspect ratio 3, and heating from within. Starting from the geochemical results of Hofmann (1988), it is assumed that the radioactive heat sources of the mantle were initially distributed homogeneously. In a number of calculations however, higher starting abundances of radioactive sources were assumed in the upper mantle. For the present geological situation, this also results in a depleted upper mantle. It was assumed that, if the viscosity falls below a certain critical value, chemical segregation will take place. In this way, model continental crust develops, leaving behind areas of a depleted mantle. We obtained the heat source, flow line, temperature, viscosity and heat-flow distribution as a function of time with realistic values, especially for the present time. The present time viscosity of the upper mantle is approximately at the standard value obtained for postglacial uplift modelling; the deeper-mantle viscosity is considerably higher. The time dependence of the computed mean of kinetic energy of mantle convection bears a resemblance to that of the magmatic and orogenetic activity of the Earth. We assumed that the 670 km discontinuity cannot be penetrated by the flow.
Key words: Earth, mantle, convection, mantle convection, thermal convection, mantle differentiation, continental crust, chemical segregation, thermal evolution, crust growth, viscosity, chemical evolution, numerical model, chemical fractionation, evolution, heat-producing elements, orogeny.