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Average of the Earths accessible reservoirs

Fig. 20.5 Abundances of highly volatile elements in average carbonaceous chondrites and the Earth's accessible reservoirs (EARs). After Kramers (2003).

somewhat low and the water is slightly D-enriched compared with what is needed to reproduce the terrestrial value (Fig. 11.4); however, several CV chondrites show an enhanced H2O content and D/H ratios below the terrestrial value. On the other hand, both C1 and CV chondrites contain Q noble gases with 130Xe/84Kr ratios exceeding the terrestrial value by a factor > 10. This complicates the solution of the "missing Xe" problem (Section 20.3).

Ordinary chondrites have a relatively low abundance of water with variable D/H ratios (Section 11.4). Also, the N in O-chondrites is somewhat heavier than the terrestrial N. For the C-N duo, enstatite chondrite material appears to be a good candidate. These meteorites incorporate appreciable amounts of relatively light C and N (Fig. 11.4) and subsolar noble gases, which would also fit terrestrial initial values. Also important is that their O-isotope composition is identical to the terrestrial value (Fig. 10.8(b)); therefore the formation of E-chondrites could have taken place not far from the terrestrial feeding zone. However, water is virtually absent in E-chondrites.

Thus the above speculations merely indicate that a volatile-rich chondrite-type source for major atmosphere components is realistic. This is not in conflict with a solar-implanted origin for light noble gases in the mantle: as discussed in Section

20.1 an amount of SW implantation that had only a negligible effect on the terrestrial major-volatile-element inventory could still deliver a major contribution of the light noble gases.

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