Thermal constraints on the survival of primitive blobs in the lower mantle

Geochemical models have frequently divided the mantle into depleted upper a nd undepleted lower mantle reservoirs, usually taken as indication for a la yered style of convection. This is difficult to reconcile with seismologica l and geodynamical evidence for substantial mass flux between lower and upp er mantle. Various models have been proposed to jointly interpret the evide nce, including that of G.E Davies [J. Geophys. Res. 89 (1984) 6017-6040] in which the author suggested that lumps of primitive material may exist in t he lower mantle, representing reservoirs for undepleted basalts. Mixing cal culations have suggested, however, that such blobs could not survive 4 Ga o f convection. Calculations by M. Manga [Geophys. Res. Lett. 23 (1996) 403-4 06] on the other hand showed that high-viscosity blobs could persist in con vective cells for geologically long times without being substantially defor med and mixed with the surrounding flow. We investigate a blob model of con vection based on these ideas and consider dynamical, thermal, geochemical a nd rheological consequences. The radiogenic heat production in the primitiv e blobs would lead to higher temperatures. However, these would be modest ( Delta T < 300 K) for sufficiently small blobs (radius < 800 km). The result ing thermal buoyancy can be offset by a small intrinsic density excess (< 1 %) so that blob material is hidden from the ridges but sampled by rising pl umes. To satisfy geochemical constraints, blobs would have to fill 30% to 6 5% of the mantle (less if they are taken to be enriched rather than primiti ve). Thermal considerations require that they be surrounded by depleted mat erial of lower viscosity that would convectively transport heat to the surf ace. The thermal decrease in blob viscosity would be about one order of mag nitude but constrained to the interior; the stiffer 'shell' can then be exp ected to control the dynamical mixing behavior. On average, the viscosity o f the lower mantle would be increased by the presence of the blobs; if they were 100 times more viscous than the surrounding mantle the net effect wou ld be to increase the effective viscosity approximately 5-fold. The origin of the proposed blobs is an unresolved problem. We suggest that perovskite/ magnesiowustite ratio variations could be the reason, which would yield an intrinsic density contrast as well. Blob geometries are at the current reso lution limit of global tomographic models, and the trade-off between temper ature and compositional effect on seismic wave speeds tends to blur the sig nal. However, joint P- and S-wave inversions and scattering studies may ult imately approach the necessary precision to detect blobs. Under the simplif ying assumptions employed in this paper, we find that the viscous blob mode l is internally self-consistent and feasible. The model may explain the out standing problem of incongruous geochemical and geophysical data. (C) 1999 Elsevier Science B.V. All rights reserved.