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0.3) buoyancy ratios induce stable layering. For buoyancy ratio be-tween 0.15 and 0.25, and ifno other ingredient is add-ed, dense material is rapidly entrained upwards. (ii) For strong ($104) thermal viscosity contrasts, large pools of dense material are generated at the bottom ofthe system and survive convection. (iii) Small chemical viscosity contrasts induce rapidmixing, whereas large chemical viscosity contrasts lead to stable layering. The influence ofthe chemical viscosity contrast is however of 2nd order compared to those of the buoyan-cyratio and of the thermal viscosity contrast. (iv) A 660-km viscosity contrast of 30 or morereduce the vertical mass transfer around this depth. (v) An endo-thermic phase transition at660-km with a Clapeyron slope around –3.0 to –1.5 MPa/K, strongly inhibits the rise of densematerial above 660 km (thermal plumes dramatically thin around this depth, and only a verysmall amount of dense material is entrained upwards), but still allow the penetration ofdownwellings in the lower mantle. Thus, models that includes both a large thermal viscositycontrast and an endothermic 660-km phase transition (Figure 1) are able to create and maintainlarge pools of dense material at the bottom of the system. Interestingly, the power spectraof these ther-mo-chemical structure are in good agreement with those deduced probabilistictomography. Furthermore, such models globally support the hypothesis that Oceanic IslandBasalt (OIB) partially sample an iso-lated reservoir of dense material located in the lowermantle.Perspectives. Next steps will focus on building models with increased complexity(spherical geometry, self-generation of plate tectonics and the recycling of these plates in thedeep mantle), and contraining the balance between the primitive (dense) and recycled materialin the thinned plumes.References[1] Iishi M. and Tromp J. (1999) Science, 285, 1231–1236.[2] Trampert J. et al. (2004) Science, 306, 853-856.[3] Labrosse S. et al. (2007) Nature, 450, 866-869.[4] Deschamps F. and Tackley P. (2008), Phys. Earth Planet. Inter., 171, 357-373.[5] Deschamps F. and Tackley P. (2009), Phys. Earth Planet. Inter., in press.[6] Tackley P.J. (2002) AGU Geodynamical Ser., 28, 231-253.Direct numerical simulation of two-phase flow: Effective rheology andpattern formation of particle suspensionsY. Deubelbeiss 1,2 , B. J.P. Kaus 2 and J. A.D. Connolly 11 Institute for Mineralogy and Petrology, ETH Zurich, Switzerland2 Institute of Geophysics, Geophysical Fluid Dynamics, ETH Zurich, Switzerlandyolanda.deubelbeiss@erdw.ethz.chWe analyzed the mechanical behavior of a two-phase system consisting of high viscositygrains and an interconnecting fluid of much smaller viscosity. For this purpose we use 2Ddirect numerical simulations for either Newtonian or non-Newtonian rheology. Directnumerical simulations solve two-phase flow problems on the spatial scale of individual grains.Effective properties of a two-phase particle suspension can be characterized by using thedirect stress-strain rate relationship. On the basis of these results, we derived expressions forthe effective viscosity for both Newtonian and non-Newtonian rheologies. We demonstratethat, for large viscosity contrasts between grains and interconnecting fluids, the effective42