Mantle Noble Gas Abundance

In Sections 6.6 and 6.7, we consider how the data for mantle-derived sources described in Sections 6.1-6.5 can be used to constrain the noble gas state of the mantle. 

Although it would be difficult to resolve a noble gas inventory in the mantle solely on the basis of these analytical data, correlation among noble gas contents in mantle-derived samples offer interesting information about mantle noble gas evolution in the mantle. 

These observations suggest that the heavier noble gases, Ne (except for in MORB), Ar, Kr, and Xe had behaved concomitantly throughout the whole geological processes until they were finally trapped in the glassy margins of the OIB and MORB samples. 

Th are plotted against 4He (normalized to 22Ne). The straight line corresponds to a constant production ratio of the nucleogenic 21Ne to 4He in the mantle (e.g., 

These results then lead us to some important questions, which are directly related to a fundamental issue in mantle geochemical dynamics. When and how did the failure of the open system condition take place Did the loss or the gain of He occur in the samples or dining magma production-transportation processes Were they pertinent in the mantle sources We may safely rule out the first possibility because there seems to be no plausible process to input a significant amount of He into a solid material. Unlike solid materials, both addition and loss of He would be possible in melt. However, it is very difficult to understand why the failure of a closed system 

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For a bulk silicate Earth concentration of 21 ppb U (O Nions et at, 1981) and Th/U = 3.8 (e.g.. Doe and Zartman, 1979), a total of 1.02 X 10 atoms " Heg is produced over 4.5 Ga. Assigning this reservoir an Iceland value of He/" He = 37/ a and an initial value of He/ He = 120/ a (see Section 4.11.2.2), 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 (see Section 4.11.2.3), a concentration in a closed system lower mantle of 7 X 10 atoms Ne g is obtained. A benchmark for comparison is the atmospheric Ne abundance divided by the mass of the upper mantle (1 X 10 g) of 1.8 X 10 atoms Ne g which is much higher, and might be taken to indicate that the atmosphere source reservoir was more gas-rich than any deep isolated reservoir. However, such an undegassed reservoir has a He concentration that is still 40 times greater than that of the MORE source (see Section 4.11.2.7), and so is still relatively gas-rich. 

Dymond, X, Hogan, L. (1973) Noble gas abundance patterns in deep-sea basalts - primordial gases from the mantle. Earth Planet. Sci. Lett., 20, 131-9. 

Noble gases and nitrogen in martian meteorites reveal several interior components having isotopic compositions different from those of the atmosphere. Xenon, krypton, and probably argon in the mantle components have solar isotopic compositions, rather than those measured in chondrites. However, ratios of these noble gas abundances are strongly fractionated relative to solar abundances. This decoupling of elemental and isotopic fractionation is not understood. The interior ratio in martian meteorites is similar to chondrites. 

Of the possible carbon sources listed above, only magmatic carbon can be eliminated from our interpretation. Using noble gas abundances and isotopes to assess the possibility of mantle-source volatiles in North Sea hydrocarbon accumulations. Hooker et a . (1985) and Turner et al. (1993) concluded that deep-source contributions are negligible, even in fields close to deep graben faults. This situation can be contrasted with the sediments and basalts of the... 

Kumagai H, Kaneoka 1 (1998) Variations in noble gas abundances and isotope ratios in a single MORB pillow. Geohys Res Lett 25 3891-3894 Kunz J (1999) Is there solar argon in the Earth s mantle Nature 399 649-650... 

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