Heterogeneities lower mantle

Hager BH, Clayton RW, Richards MA, Comer RP, Dziewonski AM (1985) Lower mantle heterogeneity, dynamic topography and the geoid. Nature 313 541-545... 

Seismic scatterers within the lower mantle are more likely to represent chemical than thermal heterogeneities, with subducted slab material (especially the basaltic crustal component thereof) constituting a likely candidate. The very base of the mantle, nearest the core-mantle boundary, may also be characterized by significant major-element chemical heterogeneity. 

Kaneshima S. and Helffrich G. (1999) Dipping low-velocity layer in the mid-lower mantle evidence for geochemical heterogeneity. Science 283, 1888-1892. 

Ono S., Ito E., and Katsura T. (2001) Mineralogy of subducted basaltic crust (MORE) from 25 to 37 GPa and chemical heterogeneity of the lower mantle. Earth Planet. Sci. Lett. 190, 57-63. 

Kellogg et al. (1999), however, have suggested, on the basis of a transition in seismic heterogeneity observed at —1,600 km depth, the possibility of a very deep layer extending hundreds of kilometers above the core-mantle boundary. One possibility is that a relic layer of dense, primordial crystalline differentiates (e.g., magnesium- and calcium-silicate perovskite) may have remained buried in the deep lower mantle until the present. Such a layer is a potential storehouse for trace elements, including radioactive heat-producing elements, and potentially could provide an important reservoir for bulk silicate Earth chemical mass balance... 

In summary, we have recently witnessed a shift away from the classically layered mantle model in favor of whole mantle convection models, where the buoyancy of sinking slabs is the dominant driving force. Slabs can penetrate deep into the lower mantle and with the induced return flow we would expect the mantle to mix efficiently. This leaves us with an interesting dilemma. If the mantle convects as a whole, how can it preserve the large-scale and long-hved heterogeneity seen in the geochemistry of oceanic basalts ... 

Dziewonski A. M. (1984) Mapping the lower mantle determination of lateral heterogeneity in P velocity up to degree and order 6. J. Geophys. Res. 89, 5929-5952. 

Figure 9 Examples of models proposed for the chemical structure of the terrestrial mantle, (a) Whole mantle convection with depletion of the entire mantle. Some subducted slabs pass through the transition zone to the coremantle boundary. Plumes arise from both the core-mantle boundary and the transition zone. This model is not in agreement with isotopic and chemical mass balances, (b) Two-layer mantle convection, with the depleted mantle above the 660 km transition zone and the lower mantle retaining a primitive composition, (c) Blob model mantle where regions of more primitive mantle are preserved within a variously depleted and enriched lower mantle, (d) Chemically layered mantle with lower third above the core comprising a heterogeneous mixture of enriched (mafic slabs) and more primitive mantle components, and the upper two-thirds of the mantle is depleted in incompatible elements (see text) (after Albarede and van der Hilst, 1999).

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