Models mantle chemical structure

Allegre, C. J., Hart, S. R. Minster, J.-F. (1983). Chemical structure and evolution of the mantle and the continents determined by inversion of Nd and Sr isotopic data, I. Theoretical models. Earth Planet. Sci. Letters, 66, 177-90. 

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).

Chemical mass balance models can actually only provide information on the relative amounts of different reservoirs, not on their spatial distribution. A mantle structure in which blobs of relict primitive mantle are maintained throughout a variously depleted and enriched lower mantle may be possible (e.g., Becker et al. (1999) Figure 9(c)), or true primitive mantle... 

In the first part of this chapter we shall examine the structure and composition of the modern mantle in order to establish how it works. In so doing we will find tantalizing clues which relate to the mantle s earlier history. It is these clues that we shall explore in the second part of this chapter and use to identify the nature and chemical evolution of the Archaean mantle. These data are then used in the third part of the chapter to constrain models for the Archaean mantle. 

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