Core-mantle boundary

K) were investigated. From an equation of state for iron the densities at these temperatures could be predicted to enable the simulations to be performed. A periodic system containing 64 atoms was used and the simulation run for 2 ps after equilibration. The calculated pressure agreed within 10% with the experimental values (330 GPa at the inner core boundary and 135GPa at the core-mantle boundary). Additional parameters could also be calculated, including the viscosity, the values for which were at the low end of previous suggestions. 

Stony Irons. The stony iron meteorites are composed of substantial iron and siUcate components. The paHasites contain cm-sized ohvine crystals embedded ia a soHd FeNi metal matrix and have properties consistent with formation at the core mantle boundary of differentiated asteroids. The mesosiderites are composed of metal and siUcates that were fractured and remixed, presumably ia the near-surface regions of their parent bodies. 

Umemoto et al. wanted to understand what happens to the structure of MgSi03 at conditions much more extreme than those found in Earth s core-mantle boundary. They used DFT calculations to construct a phase diagram that compared the stability of multiple possible crystal structures of solid MgSi03. All of these calculations dealt with bulk materials. They also considered the possibility that MgSi03 might dissociate into other compounds. These calculations predicted that at pressures of 11 Mbar, MgSi03 dissociates in the following way ... 

Irons are mostly fragments of asteroid cores. As with achondrites, their compositional variations reflect differences in parent body chemistry as well as changes wrought by crystallization. Pallasites may represent samples of core-mantle boundaries, and meso-siderites indicate poorly understood mixing of crust and core materials, probably by impact. 

The available data (Bums, 1976a) show a trend towards the stabilization of progressively lower oxidation states at high pressures across the first transition series. Such observations indicate that higher oxidation states characteristic of the Earth s surface (Ti4, Cr34, Fe3+, Ni2+) may become unstable under the high P,T conditions of the Lower Mantle. Exotic oxidation states such as Ti(III), Cr(II), Fe(I), and Ni(I) could be prevalent towards the Core-Mantle boundary, particularly if hosted by sulphide phases. 

Sherman, D. M. (1989) The nature of the pressure-induced metallization of FeO and its implications to the core-mantle boundary. Geophys. Res. Lett., 16, 515-18. 

Silicates and oxides in pallasites Olivine (F080-90). chromite, low-Ca pyroxene (En83 9i) Coarse-grained, rounded to angular Cumulates from core-mantle boundary... 

Can we be sure that the iron meteorites are indeed fragments of cores Since no differentiated asteroid has yet been visited by a spacecraft, we rely on circumstantial evidence. Some M-type asteroids have spectral characteristics expected from exposed metallic cores (Tholen, 1989), while others exhibit basaltic surfaces, a hallmark of global differentiation. Although olivine-rich mantles should dominate the volume of differentiated asteroids, there is an enigmatic lack of olivine-rich asteroids (and meteorites) that could represent mantle material (Burbine et al., 1996). Until we visit an asteroid with parts of a core-mantle boundary exposed, our best evidence supporting a core origin is detailed smdies of iron meteorites. 

Figure 9 The Brenham pallasite contains areas of both olivine-free regions and areas more typical of pallasites. In this specimen, pallasitic material is crosscut by a metallic region, suggesting that silicate-metal mixing at the core-mantle boundary was a dynamic process. Length of specimen is 13 cm (Smithsonian specimen USNM 266).

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... 

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. 

Bird J. M., Meibom A., Frei R., and Nagler T. F. (1999) Osmium and lead isotopes of rare OsIrRu minerals-derivation from the core-mantle boundary region Earth Planet. Set Utt. 170(1-2), 83-92. 

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. 

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... 

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).

Dubrovinsky E., Annerstin H., Dubrovinskaia N., Westman E., Harryson H., Eabrichnaya O., Carlson S. (2001) Chemical interaction of Fe and AI2O3 as a source of heterogeneity at the Earth s core—mantle boundary. Nature 412, 527-529. 

Knittle E. and Jeanloz R. (1991) Earth s core-mantle boundary results of experiments at high pressures and temperatures. Science 251, 1438-1443. 

Poirier J. P., Malavergne V., and Mouel J. L. L. (1998) Is there a thin electrically conducting layer at the base of the mantle In The Core-Mantle Boundary Region (eds. M. Gurnis, M. E. Wysession, E. Knittle, and B. A. Buffet). American Geophysical Union, Washington, DC, pp. 131-137. 

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