Layered mantle

The close-packed-spheron theory of nuclear structure may be described as a refinement of the shell model and the liquid-drop model in which the geometric consequences of the effectively constant volumes of nucleons (aggregated into spherons) are taken into consideration. The spherons are assigned to concentric layers (mantle, outer core, inner core, innermost core) with use of a packing equation (Eq. I), and the assignment is related to the principal quantum number of the shell model. The theory has been applied in the discussion of the sequence of subsubshells, magic numbers, the proton-neutron ratio, prolate deformation of nuclei, and symmetric and asymmetric fission. 

Creation of magnetic field to protect Earth from Sun s particle emission Resulted in a fixed rotational axis (no wobble) and an ever lengthening day from 5 to 24 h Creation of layers, mantle and crust Re-melting of mantle. Possibly introduction of water to give an acidic sea Introduction of carbon compounds such as amino-acids ... 

To explain the imbalance, O Nions and Oxburgh (1983) and Oxburgh and O Nions (1987) proposed that a barrier, which is suggested to exist between the upper and the lower mantle from seismic observation, has trapped helium in the lower mantle and retarded the heat transport from the lower mantle to the upper mantle. O Nions et al. (1983) suggested, from a semiquantitative discussion, that delayed heat transfer from the lower mantle to the upper mantle with a time constant of about 2Ga would enhance the present heat flow by a factor of two. McKenzie and Richter (1981) made numerical calculation on a two-layered mantle convection and showed that heat transfer from the lower mantle to the upper mantle is considerably retarded to give rise to an enhancement of the present surface heat flow up to a factor of two. If the thermal barrier not only retards the heat transfer and hence enhances the present surface heat flow but also essentially prevents the 4He flux from the lower to the upper mantle, this would qualitatively explain the imbalance. If this indeed were the case, we would expect a large amount of 4He accumulation in the lower mantle. However, it is difficult to conclude such a large accumulation of 4He in the lower mantle from the currently available scarce noble gas data derived from mantle-derived materials. 

Coltice N. and Ricard Y. (1999) Geochemical observations and one layer mantle convection. Earth Planet. Sci. Lett. 174(1-2), 125-137. 

Hofmann A. W., "White W. M., and "Whitford D. J. (1978) Geochemical constraints on mantle models the case for a layered mantle. Carnegie Inst. Wash. Year Book 11, 548-562. 

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. 

DePaolo, 1979 O Nions et al., 1979), divide the mantle into two convectively isolated layers with a boundary at 670 km. Such models incorporate the degassing of the upper mantle reservoir to the atmosphere. In order to explain the high OIB He/ He ratios, the underlying gas-rich reservoir is isolated from the degassing upper mantle. Therefore, these layered mantle models can be considered to incorporate two separate systems the upper mantle-atmosphere and the lower mantle. There is no interaction between these two systems, and the lower mantle is completely isolated except for a minor flux to OIB that marks its existence. It is further assumed that the mantle was initially uniform in noble gas and parent isotope concentrations, so that both systems had the same starting conditions. Note that various modifications to this basic scheme have been proposed, and are discussed below (Allegre et a/., 1983, 1986). 

Note that if such a scheme is viable, the evolution of mantle noble gases, reflecting degassing and interaction between reservoirs, may still follow those calculated in layered mantle models. An important issue that must be resolved is that there must be a mechanism for the preferential involvement of this previously melted material at OIB. This would link the involvement of these reservoirs in OIB source regions with where such material is stored. In this case, high He/ He ratios at the surface will trace the transfer of material... 

The formation of basalts by partial melting of the upper mantle at mid-oceanic ridges and hot spots provides the opportunity to determine mantle composition. Early studies of radiogenic isotopes in oceanic basalts (e.g., Eaure and Hurley, 1963 Hart et al, 1973 Schilling, 1973) showed fundamental chemical differences between OIBs and MORBs (see Chapter 2.03). This led to the development of the layered mantle model, which consists essentially of three different reservoirs the lower mantle, upper mantle, and continental cmst. The lower mantle is assumed primitive and identical to the bulk silicate earth (BSE), which is the bulk earth composition minus the core (see also Chapters 2.01 and 2.03). The continental cmst is formed by extraction of melt from the primitive upper mantle, which leaves the depleted upper mantle as third reservoir. In this model, MORB is derived from the depleted upper mantle, whereas OIB is formed from reservoirs derived by mixing of the MORB source with primitive mantle (e.g., DePaolo and Wasserburg, 1976 O Nions et al., 1979 Allegre et al., 1979). 

In summary, we have recently witnessed a shift away from the classically layered mantle model in favor of whole mantle convection models, where the buoyancy of sinking slabs is the dominant driving force. Slabs can penetrate deep into the lower mantle and with the induced return flow we would expect the mantle to mix efficiently. This leaves us with an interesting dilemma. If the mantle convects as a whole, how can it preserve the large-scale and long-hved heterogeneity seen in the geochemistry of oceanic basalts ... 

Richter E. M. and McKenzie D. P. (1981) On some consequences and possible causes of layered mantle convection. J. Geophys. Res. 86, 6133-6142. 

Van Keken P. E. and Ballentine C. J. (1998) Whole-mantle versus layered mantle convection and the role of a high-viscosity lower mantle in terrestrial volatile evolution. Earth Planet. Sci. Lett. 156, 19-32. 

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...

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