Lower Mantle

The mantle (upper and lower mantle) and The core (outer and inner core). 

The most abundant of all minerals in the interior of the earth is (Mg,Fe)Si03 perovskite. It constitutes greater than seventy percent of the lower mantle, so it is of great importance to geophysics. At room temperature the hardness of MgSi03 is VHN = 1800kg/mm2 and its Chin-Gilman parameter is 0.01. 

Holzapel C., Rubie D.C., Frost D.J., and Langenhorst F. (2005) Fe-Mg interdiffusion in (Mg,Fe)Si03 perovskite and lower mantle reequilibration. Science 309, 1707-1710. 

Hager BH, Clayton RW, Richards MA, Comer RP, Dziewonski AM (1985) Lower mantle heterogeneity, dynamic topography and the geoid. Nature 313 541-545... 

Hoemle K, Behncke B, Schmincke H-U (1996) The geochemistry of basalt from the Iblean Hills (Sicily) and the Island of Linosa (Strait of Sicily) evidence for a plume from the lower mantle. Goldschmidt Conf, J Conf Abstr 1 264 www.the-conference.com/JConfAbs/l/264.html Hofmann AW (1997) Mantle geochemistry the message from oceanic volcanism. Nature 385 219-229... 

He found that both the melting curves (Figure 2.1a) and the densities (Figure 2.1b) for xenon and krypton are well above the estimated temperature and density of the lower mantle. One might then suppose that Xe and Kr are in solid form in the lower mantle. If this is indeed the case, implications on noble gas degassing from the mantle and hence on the evolution of the atmosphere would be far-reaching (cf. Section 6.9). However, the formation of solid Xe or Kr seems to be unlikely because of its... 

Table 6.3 summarizes our best (subjective) evaluation of the noble isotopic state of the mantle. In this table we distinguish between MORB-source and OIB-source mantle, following the common view that these represent the upper and lower mantle, respectively. The noble gas data themselves provide no information about the location (upper or lower) of their respective mantle sources, of course. They do, however, allow a clear and consistent distinction between a more- and less-degassed (upper and lower, respectively) mantle the degassed distinction is also quite compatible with (although more extreme) the depleted distinction, which emerges from isotopic data for Pb, Sr, and Nd, for example. 

The imbalance between heat flow and 4He flux can also be seen from the consideration of a uranium inventory in the Earth. O Nions and Oxburgh (1983) pointed out that even though a reasonable geochemical model with 5ppb of U (K/U = 104, Th/U = 3.8) for the upper mantle can approximately account for the observed helium flux, it will yield only 3% of the observed heat flux at ridges. This result indicates that the remaining 97% of the heat flow must come from somewhere other than the upper mantle, namely either from the lower mantle or from the core or from both, whereas little extraneous 4He flux is required. This led O Nions and Oxburgh to conclude that 4He flux from the lower mantle is essentially inhibited. 

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. 

D = 10 8cm2/s, which is the value for olivine at 1300°C at an atmospheric pressure (Hart, 1984 Trull et al., 1991) and d = 30km, we have l 1013 years. Hence, we may safely assume that diffusive helium transportation from the lower mantle is negligible even if integrated over the age of the Earth. 

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. (1999) Geophysically constrained mantle mass flow and the 40Ar budget A degassed lower mantle Earth Planet. Sci. Lett., 166, 149-62. 

Niedermann, S., Bach, W., Erzinger, J. (1997) Noble gas evidence for a lower mantle in MORBs from the southern East Pacific Rise Decoupling of helium and neon isotope systematics. Geochim. Cosmochim. Acta, 61, 2697-715. 

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). 

The a - P — y phase transitions involving (Mg,Fe)2Si04 are isochemical. However, below 670 km where the Lower Mantle begins, disproportionation... 

Inversion of phase relationships induced by spin-pairing in Fe2+ ions provides one mechanism for possibly enriching this transition element in the Lower Mantle. Other, more general mechanisms influencing element fractionations, are the effects of pressure on relative sizes, crystal field stabilization energies, bond-types and oxidation states of the cations. 

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. 

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