The Earth's mantle

Chemical differentiation of the Earth into a buoyant, olivine-rich upper mantle, along with protocrustal materials, a perovskite-rich deeper layer, and an iron-rich core occurred continuously during accretion. Dense komatiitic liquids and eclogitic solids sank to mid-mantle depths. The large-ion lithophile elements and primordial gases accumulated in the proto-upper mantle. During subsequent evolution, most of the crustal elements were sweated out of the upper mantle; the layer at the base of the mantle collected light dross from the core and dense dregs from the mantle and reacted with the core. This fractionation and gravitational sorting of primordial materials according to density, solubility, silicate compatibility, and melting point became irreversible as the planet grew because of the effect of pressure on thermal expansion. Chemical boundaries are hard to detect by seismic techniques, but evidence favors one such boundary near 1000 km. Below this, the mantle is probably depleted in volatiles and the heat-producing elements, and represents the accreted material minus the buoyant and fusable compounds and the accompanying trace elements. Observations also favor a thick, chemically distinct layer at the base of the mantle that may extend, in places, more than 1000 km from the core-mantle boundary. This layer exhibits large-scale sluggish behavior as appropriate for high Prandtl number, low Rayleigh number convection. This kind of chemical and gravitational stratification resolves various geodynamic and geochemical paradoxes, and is more consistent with petrology and mineral physics than one- and two-layer models, and reversible stratification.

Planetary differentiation

Core–mantle boundary

Crustal recycling

Hotspot (geology)

Post-perovskite

Primitive mantle

10.2747/0020-6814.44.2.97

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Citations (63)

Effect of mantle plumes on the growth of D” by reaction between the core and mantle

Geophysical Research Letters (1993)

L. H. Kellogg Scott D. King

The core‐mantle boundary is a fundamental compositional discontinuity in the Earth, where molten iron alloy from the core meets solid silicate minerals from the mantle. Heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle. At the same time, chemical reactions may create a layer of different composition and density than the overlying mantle. Using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell, we investigate how a layer forms at the base of the mantle. Development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core‐mantle boundary and on the rate at which the material diffuses into the mantle. If the material is less than 3–6% denser than the overlying mantle, assuming reasonable choices of lower mantle thermodynamic parameters, the reactant material will be swept away by upwelling plumes. More dense material, on the other hand, forms an internally convecting layer that entrains material from above. The varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle.

Core–mantle boundary

Planetary differentiation

Hotspot (geology)

Post-perovskite

Crustal recycling

10.1029/93gl00045

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Citations (88)

Mechanisms of Earth differentiation: Consequences for the chemical structure of the mantle

Reviews of Geophysics (1994)

Richard W. Carlson

Compared to the mantles of the Moon and perhaps Mars, the Earth's mantle is much less differentiated chemically. Both the Moon and Mars appear to have undergone a major differentiation accompanying planet formation. The only clear signature of a similar event on the Earth is core formation. While this might imply that the Earth did not experience extensive melting and differentiation during planet formation, the higher pressures and temperatures present in the Earth could have led to a distinctly different chemical evolution for this initial differentiation. The most significant potential outcome of early differentiation on the Earth's mantle is formation of chemically distinct upper and lower mantles distinguished by Mg/Si higher and lower than chondritic, respectively. Plate tectonics on Earth provides a continuing mechanism for planet differentiation that forms crust at the expense of chemical differentiation of the mantle. Plate tectonics, however, also offers a mechanism to return the chemically distinct materials of the crust back into the mantle. Mixing of subducted crustal material into the mantle through the stirring provided by mantle convection can serve to negate the effects of crust formation on the chemical composition of the mantle. Similarly, mixing within the mantle could serve to destroy evidence of early differentiation, if such differentiation occurred on Earth. Completely efficient operation of the plate tectonic cycle would result in remixing of crust and differentiated mantle, with the end result being a homogenous mantle with composition identical to that of the bulk earth minus the materials segregated into the core. In part, this may explain the relatively undifferentiated nature of the Earth's mantle. Plate tectonics has not been completely efficient on Earth, however. Both oceanic and continental crust exist, and there is widespread evidence for chemical variability in the mantle. At least four chemically and isotopically distinct components are observed in mantle‐derived rocks. The nature of these components points to the importance of crust formation and recycling in determining the chemical variability of the mantle. Mapping of the surface expression of chemical heterogeneity in the mantle is providing new views of the chemical structure of the mantle and the geodynamic processes that operate in the Earth's interior.

Planetary differentiation

Core–mantle boundary

Crustal recycling

Hotspot (geology)

Early Earth

10.1029/94rg01874

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Citations (68)

Pressure regimes and core formation in the accreting earth

H. E. Newsom

Recent work suggests that a large degree of melting is required to segregate metal from silicates, suggesting a connection with the formation of magma oceans. At low pressures metallic liquids do not wet silicate minerals, preventing the metal from aggregating into large masses that can sink. At high pressures, above 25 GPa, the dihedral angles of grains in contact with oxygen-rich metallic liquids may be reduced enough to allow percolation of metal, but this has not been confirmed. Physical models of core formation and accretion may therefore involve the formation of magma oceans and the segregation of metal at both high and low pressures. Models of core formation involving different pressure regimes are discussed as well as chemical evidence bearing on the models. Available geophysical data is ambiguous. The nature of the 670 km boundary (chemical difference or strictly phase change) between the upper and lower mantle is in doubt. There is some evidence that plumes are derived from the lower mantle, and seismic tomography strongly indicates that penetration of subducting oceanic crust into the lower mantle, but the tomography data also indicates that the 670 km discontinuity is a significant barrier to general mantle convection. The presence of the D' layer at the base of the lower mantle could be a reaction zone between the mantle and core indicating core-mantle disequilibrium, or D' layer could be subducted material. The abundance of the siderophile elements in the mantle could provide clues to the importance of high pressure processes in Earth, but partition coefficients at high pressures are only beginning to be measured.

Planetary differentiation

Core–mantle boundary

Post-perovskite

Source

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Citations (0)

Speculations on the composition and origin of the earth.

GEOCHEMICAL JOURNAL (1982)

Lin-gun Liu

The distribution pattern of the chemical elements of the Earth's surface indicates that the bulk composition of the Earth as a whole is similar to that of volatile-depleted CI chondrites. On the basis of the above bulk composition of the Earth, the known chemical composition of the Earth's crust, and the well accepted compositional model for the upper mantle, the chemical compositions of the lower mantle and the core have been calculated. It has been found that the silica content (the most abundant chemical component of the mantle) of the lower mantle is about 20wt% more enriched than the upper mantle. Furthermore, the iron content of the lower mantle is likely to be depleted relative to that of the upper mantle, since iron is chemically incompatible with the major mineral phase in the lower mantle, which is probably composed of 95 wt% of silicates with perovskite modifications. The possible stable mineral assemblages of the various parts of the mantle are given. It has also been calculated that the outer core contains about 15wt% of a light element, which is in line with, but independent of, previous estimates based primarily on geophysical constraints. Various reasons suggest that oxygen is the major light element in the core. The simple and direct correlations in the abundances of the major and minor elements, and in the general distribution of the chemical elements between the Earth and CI chondrites suggest a comparatively simple model for the origin of the Earth. Only some volatile and light elements in the CI composition were volatilized and escaped from the accreting Earth. The core of the Earth was developed by high-pressure disproportionation reactions in iron-rich silicates simultaneously with accretion.

Core–mantle boundary

Post-perovskite

Outer core

Planetary differentiation

10.2343/geochemj.16.287

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Citations (29)

Core cooling by subsolidus mantle convection

Physics of The Earth and Planetary Interiors (1979)

G. Schubert P. Cassen Richard E. Young

Planetary differentiation

Core–mantle boundary