Mantle ratio

Figure 42 Covariation of Lu/Hf versus Sm/Nd for minerals from mantle peridotite xenoliths. Lines marked are the primitive mantle ratios.

The model is consistent with the isotopic evidence that upper mantle xenon does not have a simple direct relationship to atmospheric xenon. The radiogenic xenon presently seen in the atmosphere was degassed from the upper portion of the solid Earth prior to the establishment of the present upper mantle steady state xenon isotope compositions and concentrations. The lower mantle ratios are established early in Earth history by decay of I and Pu decay produces a relatively small fraction of fissiogenic nuclides (Porcelli and Wasserburg, 1995b). The xenon daughters (now in the upper mantle) of the shortlived parents are supplied from the lower mantle. The MORE Xe/ Xe ratio (when corrected for air contamination) has no radiogenic contributions... 

Several lines of evidence indicate problems with strict application of the layered model. It is particularly difficult to satisfy the requirement for primitive composition of the lower mantle. For example, Zindler and Hart (1986) showed that the isotopic database for the oceanic mantle could be described by mixing between depleted MORE mantle and enriched components that have compositions quite unlike primitive mantle. Similarly, the constancy of Nb/U and Ce/Pb (Hofmann, 1986 Newsom et al., 1986) suggests that there is no current reservoir with primitive mantle ratios. Interestingly, there is a strong correlation between a depleted component, termed FOZO by Hart et al. (1992), and the high He source observed in many hot spots (Hart et al., 1992 Farley et al., 1992 Hanan and Graham, 1996). 

The Fe/Ni value of the core (16.5) is well constrained by the limited variation in chondritic meteorites (17.5 0.5) and the mantle ratio (32), as well as the mass fraction of these elements in the two terrestrial reservoirs. The total content of... 

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