Earth core-mantle boundary

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 ... 

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. 

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. 

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... 

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. 

Anderson D. E., Sammis C., and Jordan T. (1971) Composition and evolution of mantle and core. Science 171, 1103. Anderson O. E. (2002) The power balance at the core-mantle boundary. Phys. Earth Planet. Inter. 131, 1-17. 

Kellogg L. H. (1997) Growing the Earth s D layer effect of density variations at the core-mantle boundary. Geophys. Res. Lett. 24, 2749-2752. 

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...

There may also be a significant amount of carbon in the core, with values of up to 4% possible, although this depends upon the amount available in the early Earth (Wood, 1993 Halliday and Porcelli, 2001). The amount of carbon that is supplied across the core-mantle boundary into the mantle is not known. 

Nanoparticles may also be important within planetary interiors. For example, phase transitions within the deep Earth may generate materials that are composites of nanoparticles (e.g., within the spinel phase at the olivine-spinel transition at the 400-km discontinuity). These grain sizes may affect both kinetics and rheology (e.g., of ice in planetary interiors Stern et al. 1997). Chemical reactions in the deep Earth, perhaps between metal and silicate near the core-mantle boundary, may be impacted by nanocrystals. 

Core-mantle boundary Inner-outer core boundary Mean radius of the Earth... 

The phase identified in the deep mantle, dose to the core-mantle boundary, which represents the very deep transformation of the mineral perovskite The composition of the Earth s original mantle, after core formation but before crust formation. It is synonymous with bulk silicate Earth... 

FIGURE 3.2 Tomographic image of the Earth s mantle beneath the Japanese Arc, down to the core-mantle boundary showing the distribution of slow and fast seismic waves. The wave velocity distribution also reflects temperature distribution and shows the penetration of a cold subducting slab through the transition zone into the lower mantle (after Fukao et al., 2001). 

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). 

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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