Mantle convection

Galer SJG, O Nions RK (1985) Residence time of thorium, uranium and lead in the mantle with implications for mantle convection. Nature 316 778-782. 

EUiott T, Jeff coate AB, Bouman C (2004) The terrestrial Li isotope cycle light-weight constraints on mantle convection. Earth Planet Sci Lett 220 231-245 Ellis AS, Johnson TM, Bullen TD (2002) Chromium isotopes and the fate of hexavalent chromium in the environment. Science 295 2060-2062... 

Both diffusion and convection are modes of mass transfer. Typically, large-scale mass transfer is accomplished by convection, and small-scale mass transfer is accomplished by diffusion. Similarly, large-scale heat transfer in the Earth is through convection (mantle convection), and small-scale heat transfer is through heat conduction (e.g., through the lithosphere). To treat the complicated convection pattern and diffusion requires a large computational effort. Some simple problems can be treated analytically. 

Schubert, G., Turcotte, D.L. and Olsen, P., 2001. Mantle convection in the Earth and Planet. Cambridge University Press, Cambridge, 940 pp. 

The preservation of distinct mantle reservoirs over time in Mars and the limited degassing experienced by the Martian mantle after the initial period, as indicated by the almost quantitative retention of 244Pu-produced fission xenon, show that Mars has been a static planet with no mantle convection since very early in its history. 

Williams DR, Pan V (1992) Internally heated mantle convection and the thermal and degassing history of the earth. J Geophys Res 97 8937-8950... 

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. 

The mode of the mantle convection, layered or mantle-wide, is one of the most fundamental problems in current earth science. As we discussed earlier, 4He-heat systematics appears to suggest that the lower mantle (apart from the exact locale) is essentially isolated from the upper mantle by a barrier that impedes He migration between the layers. Other noble gas characteristics, for example much higher 40Ar/36Ar and 129Xe/130Xe in the upper mantle than in the lower mantle, also appear... 

Davies, G. F. (1984) Geophysical and isotopic constraints on mantle convection An interim synthesis. J. Geophys. Res., 89, 6017-40. 

Morgan, W. J. (1972) Plate motions and deep mantle convection. Geol. Soc. Amer. Memoir, 132, 7-22. 

The chemical composition of the Lower Mantle below 670 km is essentially unknown. It has often been assumed to be the same as the Upper Mantle with the seismic discontinuity at 670 km representing a phase change to denser polymorphs rather than a chemical boundary (Liu and Bassett, 1986). However, some models of the Earth s interior suggest that the Mantle is stratified with the Upper Mantle and Lower Mantle convecting separately, leading to compositional density differences between these two regions. There is a commonly held view that the Lower Mantle has a higher Fe/(Mg+Fe) ratio than the Upper Mantle (Liu and Bassett, 1986 Jeanloz and Knittle, 1989). 

Einally, a well-mixed compositionally uniform mantle is also supported by geophysical evidence (i.e., tomography), showing that slabs penetrate into the lower mantle, which would support whole mantle convection (Van der Hilst et al, 1997 see Chapter 2.02). 

Bina C. R. and Liu M. (1995) A note on the sensitivity of mantle convection models to composition-dependent phase relations. Geophys. Res. Lett. 22, 2565—2568. 

Liu M. (1994) Asymmetric phase effects and mantle convection patterns. Science 264, 1904-1907. 

With the exception of Davies, who favored whole-mantle convection all along, the above authors concluded that it was only the upper mantle above the 660 km seismic discontinuity that was needed to balance the continental crust. The corollary conclusion was that the deeper mantle must be in an essentially primitive, nearly undepleted state, and consequently convection in the mantle had to occur in two layers with only little exchange between these layers. These conclusions were strongly reinforced by noble gas data, especially He/ He ratios and, more recently, neon isotope data. These indicated that hotspots such as Hawaii are derived from a deep-mantle source with a more primordial, high He/" He ratio, whereas MORBs are derived from a more degassed, upper-mantle reservoir with lower He/ He ratios. The noble-gas aspects are treated in Chapter 2.06. In the present context, two points must be mentioned. Essentially all quantitative evolution models dealing with the noble gas evidence concluded that, although plumes carry... 

Albarede F. (1998) Time-dependent models of U-Th-He and K-Ar evolution and the layering of mantle convection. Chem. Geol. 145, 413 -429. 

Albarede F. and Van der Hilst R. D. (1999) New mantle convection model may reconcile conflicting evidence. EOS. Trans. Am. Geophys. Union 80(45), 535-539. 

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

Davies G. F. (1998) Plates, plumes, mantle convection and mantle evolution. In The Earth s Mantle-composition, Structure, and Evolution (ed. I. Jackson). Cambridge University Press, Cambridge, pp. 228-258. 

Davies G. F. (2002) Stirring geochemistry in mantle convection models with stiff plates and slabs. Geochim. Cosmochim. Acta 66, 3125-3142. 

Klein E. M., Langmuir C. H., Zindler A., Staudigel H., and Hamelin B. (1988) Isotope evidence of a mantle convection boundary at the Australian-Antarctic discordance. Nature 333, 623-629. 

Phipps Morgan 1., Morgan W. 1., and Zhang Y.-S. (1995) Observational hints for a plume-fed, suboceanic astheno-sphere and its role in mantle convection. J. Geophys. Res. 100, 12753-12767. 

Tackley P. J. (2000) Mantle convection and plate tectonics toward an integrated physical and chemical theory. Science 288, 2002-2007. 

Fertilization may alternatively result from the fractional solidification of partial melts incompletely drained from residual mantle, upon cooling of melting domains due to mantle convection or tectonic upwelling. In this situation, the refertilized peridotites (sometimes associated with replacive pyroxenites—see Section 2.04.4.2.2) may dehne a layering interpreted in terms of high-porosity compaction waves or porous-flow channels (Obata and Nagahara, 1987 Van der Wal and Bodinier, 1996 Garrido and Bodinier, 1999). 

Moores E., Kellogg L. H., and Dilek Y. (2000) Tethyan ophiolites, mantle convection, and tectonic historical contingency a resolution of the ophiolite conundrum . In Ophiolites and Oceanic Crust New Insights from Field Studies and the Ocean Drilling Program, Special Paper (eds. Y. Dilek, E. M. Moores, D. Elthon, and A. Nicolas). Geological Society of America, Boulder, CO, USA, vol. 349, pp. 3-12. 

Polve M. and Allegre C. J. (1980) Orogenic Iherzolite complexes studied by Rb- Sr a clue to understand the mantle convection processes Earth Planet. Sci. Lett. 51, 71-93. 

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