Mid-ocean ridge basalts (MORBs) exhibit relatively uniform and depleted rare earth element concentrations compared with ocean island basalts (OIBs). Previous researchers have focused on long-term (billion-year timescale) preservation of an enriched and heterogeneous OIB reservoir within the convecting mantle. Such studies commonly conclude that the OIB reservoir must exist in an area which remains isolated from convection, i.e., D″. Here we investigate the maintenance of MORB reservoir homogeneity over shorter timescales in the face of vigorous upper/lower mantle mass exchange (deep subduction), which may be due to two effects: (1) a high-viscosity lower mantle and/or (2) chaotic mixing due to toroidal flow generated by surface plate motions. We explore this conceptual model using three-dimensional spherical numerical models that include surface plate motions, radial viscosity variation, and a geophysically plausible model of mantle density contrasts. A correlation dimension method is used to characterize mixing of passive tracers. For a uniform viscosity mantle the upper and lower mantles mix on essentially the same timescales. A factor of 100 viscosity contrast results in a relative mixing time for the lower mantle only ~30--60% longer than that of the upper mantle. Therefore neither a strong viscosity contrast nor toroidal mixing significantly affects the relative mixing times of the upper and lower mantle. We conclude that return flow from the lower mantle is of similar (depleted) composition and that the depleted MORB source reservoir constitutes most of the mantle, except for a convectively isolated OIB source region at the base of the mantle.