Models core-mantle boundary

If we assume a core-mantle boundary origin (see Mittlefehldt et al., 1998, for references suggesting alternative models), how can these... 

Bertka and Fei (1997) experimentally determined mantle mineral stabilities using the Wanke and Dreibus (1988) model composition. The mineral stability fields and resulting mantle density profile, as well as core densities and positions of the core-mantle boundary for a range of model core compositions, are illustrated in Figure 9. The moment of inertia calculated from these experimental data (0.354) is consistent with the Mars Pathfinder measurement (Bertka and Fei, 1998). However, this model requires an unrealistically thick crust. 

Figure 1 Depth-varying phase proportions in a pyrolite model mantle after the manner of Ringwood (1989), Ita and Stixmde (1992), and Bina (1998h). Phases are (a) ohvine, (fi) wadsleyite, (y) ringwoodite, (opx) orthopyroxene, (cpx) clinopyroxene, (gt-mj) garnet-majorite, (mw) magnesiowiistite, ((Mg,Fe)-pv) ferromagnesian sihcate perovskite, and (Ca-pv) calcium silicate perovskite. Patterned region at base denotes likely heterogeneity near core-mantle boundary.

Aside from the core-mantle boundary region, a pyrolite lower-mantle composition appears to be consistent with seismological constraints. Silica enrichment of the lower mantle can be accommodated if the lower mantle is hotter than expected for a simple adiabat rooted at the 660 km y— pv + mw transition (Figure 9). Because any chemical boundary layer between the upper and lower mantle would be accompanied by a corresponding thermal boundary layer, such a model... 

Calculated bulk rock trace-element systematics of eclogites have wider implications for mantle recycling models and bulk silicate earth mass balance. The subchondritic Nb/Ta, Nb/La, and Ti/Zr of both continental cmst and depleted mantle require the existence of an additional reservoir with superchondritic ratios to complete the terrestrial mass balance. Rudnick et al. (2000) have shown that rutile-bearing eclogites from cratonic mantle have suitably superchondritic Nb/Ta, Nb/La, and Ti/Zr such that if this component formed 1 -6% by weight of the bulk silicate earth, this would resolve the mass imbalance. This mass fraction far exceeds the likely mass of eclogite in the continental lithosphere and so the material is proposed to reside in the lower mantle, possibly at the core-mantle boundary. 

There are several possible descriptions of a layered mantle. The possibilities that have been incorporated into noble gas models include a boundary layer at the 670 km seismic discontinuity, a deeper layer of variable thickness, and a boundary layer at the core-mantle boundary. 

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).

Figure 6 A range of mantle models for the distribution and fluxes of noble gases in the Earth. Layered mantle models with the atmosphere derived from the upper mantle involve either progressive unidirectional depletion of the upper mantle (A) or an upper mantle subject to inputs from subduction and the deeper mantle, and has steady state concentrations (B). Whole mantle convection models involve degassing of the entire mantle, with helium with high He/ He ratios found in OIB stored in either a deep variable-thickness layer (C), a layer of subducted material at the core-mantle boundary (D), or the core (E). The models are discussed in the text and Chapter 2.06 (source Porcelli and...

FIGURE 3.17 Schematic diagram illustrating the two different models of mantle convection, (a) A two-layer mantle in which the upper mantle is chemically isolated from the primitive, lower mantle. In this model the plume source is in the upper mantle and subducting slabs do not penetrate the 660 km upper mantle-lower mantle boundary, (b) The whole mantle model in which there is mass exchange between the upper and lower mantle. In this case subduction penetrates the 660 km discontinuity and plumes are sourced in the D" layer, at the core-mantle boundary. 

This model is the development of an idea originally proposed by Hofmann and White in 1982. In its original form this model was proposed as an alternative to the then prevalent view that OIB is from a primitive mantle source. In it mantle plumes were derived from subducted oceanic lithosphere, accumulated and stored at the core-mantle boundary. 

In this present version of the model the D" layer is thought to have originated very early in Earth history, as an early, incompatible element- and metal-rich basaltic crust, enriched during late accretion (4,540-4,000 Ma) with chondritic material. There is support from Nd-and Hf-isotopes for the existence of this very early differentiate of the mantle (see Sections 3.2.3.1 and 3.2.3.2). This crust, when subducted, had a bulk density which exceeded that of the mantle and numerical modeling experiments confirm that it would have stabilized at the core-mantle boundary (Davies, 2006). 

Different authors have variously located primitive mantle in the present-day lower mantle beneath the 660 km discontinuity, in a deep layer beneath a 1,600 km discontinuity, in the D" layer at the core-mantle boundary, or as "blobs" within the lower mantle (Becker et al., 1999). It is arguable whether any modern basalts have been derived from such a primitive reservoir, and it is possible that such a reservoir does not exist. In fact models of whole mantle convection, such as that of Helffrich and Wood (2001), in which the majority of the mantle has been processed through the subduction process, render the preservation of primitive mantle most unlikely. 

Tolstikhin, I.N., Kramers, J.D., and Hofmann, A., 2006. A chemical Earth model with whole mantle convection the importance of a core-mantle boundary layer (D") and its early formation. Chem. Geol, 226, 79-99. 

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