Abstract Vermischung in 3D sphärischen Konvektionsmodellen des Erdmantels
Vermischung in 3D sphärischen Konvektionsmodellen des Erdmantels
K.-D. Gottschaldt
Abstract The existence of geochemically distinct reservoirs in the mantle is inferred from the observation of worldwide homogeneous MORBs on the one side and heterogeneous OIBs on the other side. It is one of the first order unresolved problems of Geodynamics how these observations could be reconciled with geophysically favoured whole-mantle convection. In this thesis stirring properties of 3D spherical models of convection in the Earth’s mantle are investigated numerically. The code TERRA (Baumgardner, 1983) has been extended and improved. New tools for postprocessing are introduced. The nearly complete energy balance of the core is calculated simultaneously with 3D spherical mantle convection for the first time. However, differenciation in a geochemical sense is not considered yet.
Two fundamentally different regimes of stirring are observed in the models for earthlike parameters. Based on this finding a new hypothesis is proposed as a working assumption for future, more detailed investigations. At a low viscosity level in the mantle and with consideration of a lithosphere, two models show vigorous small-scale convection. The segregation of continental crust is not modelled yet, but is assumed to be confined to a region near the surface. The well stirring small scale convection transports material downwards which has been altered near the surface. A layer of homogeneous depleted material may grow on top of a pristine mantle. The interface between the two regions is irregular and moves slowly downwards. This behaviour was first investigated by Walzer & Hendel (1999) and could characterise the early evolution of the mantle. The viscosity level is rising in a cooling mantle. This time-dependence is not included in the models yet, but at higher viscosities most models develop the kind of convection we observe in the Earth today. There are downwellings going from the surface right to the lower mantle or even to the CMB. Because of 3D spherical geometry the stagnation points of the cell like convective structures lie in rather shallow depths. Laterally averaged, Material just below the lithosphere can preserve its original signature for the longest time in the convecting mantle. Additionally a layering of stirring properties is induced by the viscosity profile of Walzer et al. (2003b). This profile features a second asthenosphere overlying a very high viscous zone in the lower mantle. Stirring is bad in this zone and heterogeneities can persist there longer than in other parts of the mantle.
It is suggested that there may have been a change from small scale convection to large-scale convection during the evolution of the mantle. Under this assumption, geochemical and geophysical observations may be reconciled at least from a fluid dynamical point of view.
Citation: K.-D. Gottschaldt. Vermischung in 3D sphärischen Konvektionsmodellen des Erdmantels, Ph.D. thesis, Friedrich-Schiller-Univ. Jena, 2004.