ABSTRACT
We investigate the effect of viscosity and thermal conductivity variations on mixing by thermal convection with the goal of providing constraints on the timescales of stirring in the Earth's mantle, which undergoes convection on geologic timescales. Geophysical observations and high-pressure experiments indicate substantial increases in viscosity and thermal conductivity in the lower mantle, which could slow the rate of heat and mass transfer due to mantle convection and help maintain long-lived geochemical reservoirs in the lower mantle. In the extreme case, heat transport by convection might be negligibly small. However, the increase in viscosity will be somewhat offset by a reduction in viscosity due to increased temperature; moreover, there are large uncertainties in the estimates of these properties. We explore the possible dynamics effects of a substantial increase in viscosity and thermal conductivity, using finite-element models of mantle convection with depth-dependent and temperature-dependent material properties. Tracers are introduced to determine whether isolated reservoirs can be maintained in the lower mantle in this scenario. We observe a wide range of phenomena ranging from rapid mixing to formation of isolated regions. The preservation or destruction of isolated blobs is controlled by the oscillations of downwellings and upwellings.