Noble gases: characteristics

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... 

An outstanding question is how much of the mantle still maintains high volatile concentrations. This involves resolution of the nature of the high He/" He OIB-source region. Most models equate this with undepleted, undegassed mantle, although some models invoke depletion mechanisms. However, none of these has matched the end-member components seen in OIB lithophile isotope correlations. It remains to be demonstrated that a primitive component is present and so can dominate the helium and neon isotope signatures in OIB. The heavy-noble-gas characteristics in OIB must still be documented. It is not known to what extent major volatiles are stored in the deep Earth and associated with these noble gas components. 

None of the acquisition models described above can explain all of the presently observed noble gas characteristics in the terrestrial planets, indicating that subsequent loss processes have caused further modifications. 

For 1.02 x 10 atoms " He/g (see Radiogenic He section), an Iceland value of He/" He = 37 Ra and an initial value of He/ He = 120 Ra (see Mantle Noble Gas Characteristics Section), then the reservoir has 7.6 x 10 ° atoms He/g. The concentration of another noble gas is required for comparison of lower mantle noble gas abundances with the atmosphere. Using He/ Ne = 11, a concentration in a closed system lower mantle of 7x10 atoms Ne/g is obtained. A benchmark for comparison is the atmospheric Ne abundance divided by the mass of the upper mantle (1 x lO g) of 1.8 x 10 atoms Ne/g, which is much higher, and might be taken to indicate that the atmosphere source... 

While some objections can be overcome with model modifications, it must be concluded that the fundamental mechanism of this model, generation of atmospheric and MORE noble gas characteristics by progressive extraction of the atmosphere from the upper mantle as presently sampled by MORE, cannot account for many of the available constraints, especially those from Ne and Xe isotopes. 

Heavy noble gas characteristics in OIB. Variations in Ar and Xe isotopic compositions are not yet clearly documented. Constraining the compositions associated with high He/He ratios is clearly critical for mantle models. 

It is expected that due to the short residence time of Be and Cl in the atmosphere, 10Be and 36C1 measurements on ice cores will directly reveal isotope production variations. Due to dilution in the C02 exchanging system the atmospheric 14C/C-ratio shows a dampened response to 14C production rate variations. In contrast to the noble gas radioisotopes the size of the effective dilution reservoir - atmosphere plus parts of the ocean and biosphere - depends on the characteristic... 

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