Vermischung in 3D sphärischen Konvektionsmodellen des Erdmantels The existence of geochemically distinct reservoirs in the Earths mantle is inferred from the observation of worldwide rather homogeneous mid ocean ridge basalts (MORB) on the one side and heterogeneous ocean island basalts (OIB) on the other side. How can these observations be reconciled with geophysically favoured large scale convection ... is an unresolved problem of global geodynamics. In this thesis stirring properties of 3 D spherical models of convection in the Earths mantle are investigated numerically. Attempts to make the models more earthlike are described. It is proposed as a working assumption for future more detailed investigations that there may have been a change from small scale to large scale convection during the evolution of the mantle.