THERMAL CONSTRAINTS ON THE SURVIVAL OF PRIMITIVE BLOBS IN THE LOWER MANTLE

Abstract

Geochemical models have frequently divided the mantle into depleted upper and undepleted lower mantle reservoirs, usually taken as indication for a layered style of convection. This is difficult to reconcile with seismological and geodynamical evidence for substantial mass flux between lower and upper mantle. Various models have been proposed to jointly interpret the evidence, including that of G.F. Davies [J. Geophys. Res. 89 (1984) 6017-6040] in which the author suggested that lumps of primitive material may exist in the lower mantle, representing reservoirs for undepleted basalts. Mixing calculations have suggested, however, that such blobs could not survive 4 Ga of convection. Calculations by M. Manga [Geophys. Res. Lett. 23 (1996) 403-406] on the other hand showed that high-viscosity blobs could persist in convective cells for geologically long times without being substantially deformed and mixed with the surrounding flow. We investigate a blob model of convection based on these ideas and consider dynamical, thermal, geochemical and rheological consequences. The radiogenic heat production in the primitive blobs would lead to higher temperatures. However, these would be modest (ΔT < 300 K) for sufficiently small blobs (radius <800 km). The resulting 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 plumes. To satisfy geochemical constraints, blobs would have to fill 30% to 65% of the mantle (less if they are taken to be enriched rather than primitive). Thermal considerations require that they be surrounded by depleted material of lower viscosity that would convectively transport heat to the surface. The thermal decrease in blob viscosity would be about one order of magnitude but constrained to the interior; the stiffer `shell' can then be expected to control the dynamical mixing behavior. On average, the viscosity of 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 would 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 resolution limit of global tomographic models, and the trade-off between temperature and compositional effect on seismic wave speeds tends to blur the signal. However, joint P- and S-wave inversions and scattering studies may ultimately approach the necessary precision to detect blobs. Under the simplifying assumptions employed in this paper, we find that the viscous blob model is internally self-consistent and feasible. The model may explain the outstanding problem of incongruous geochemical and geophysical data.