Changes in the spin state of iron in both magnetowustite and perovskite at lower mantle conditions may result in increases in radiative thermal transport and viscosity that could suppress convection in the lowermost mantle (Badro et al. 2003, 2004). It has been suggested that such a stagnant layer in the lower mantle could serve as a reservoir for a significant portion of the mantle's incompatible elements, accounting for the isotopic characteristics of hot spots linked to proposed deep-rooted mantle plumes. We investigate the possible effects on mantle dynamics of increases in thermal conductivity and viscosity, using finite-element models of mantle convection in 2-D. Our previous results (Naliboff et al. 2003) showed that increases in thermal conductivity in the lower mantle up to 250 times that in the upper mantle, with otherwise uniform physical properties, fail to isolate a stagnant layer beneath a mid-mantle phase change. When both the viscosity and thermal conductivity increase in the lower mantle, flow velocities through the lower layer and across the boundary decrease. To investigate the rate of mass exchange and mixing in the presence of a partially stagnant layer, we injected tracer particles into the models. We examine mixing in three different classes of models: two models have a viscosity and thermal conductivity change at the mantle mid-point; the third has a viscosity increase at 660 km and a viscosity and thermal conductivity change near 2000 km depth. In models in which the viscosity and thermal conductivity increases by a factor of 10 at the mid-mantle, multi-cell whole-mantle convection rapidly produces a marble cake mantle, leaving no isolated reservoir of material in the lower mantle. Increasing the viscosity and thermal conductivity in the lower mantle by a factor of 50 or 100 produces a relatively stable pattern of convection with a few strong upwellings and downwellings. Although mixing rates decrease and the residence time of material in the lower mantle increases with increasing viscosity and thermal conductivity in these models, no long-term stagnation of material occurs beneath the phase change. When the lower mantle viscosity and thermal conductivity increase by a factor of 150 in the lower mantle, the size of upwellings decreases, with multiple small plume-like structures forming at the interface. This transition marks the change to a more stable mid-mantle boundary between material above and below the interface. Increasing the viscosity and thermal conductivity in the lower mantle by a factor of 250 further decreases the size and material transport rate, leading to longer residence times of material in the bottom half of the mantle. In summary, a phase change with large magnitude changes in viscosity and thermal conductivity at the mid-mantle would fail to maintain isolated lower mantle geochemical reservoirs over significant geologic time. Mixing rates, however, decrease with increasing viscosity and thermal conductivity.

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