Isotopic variations in terrestrial xenon

Abstract

Isotopic composition and amounts of xenon extracted from eclogite, from old granite, and from CO_2-well gases have been measured. The Xe content of Bavarian eclogite is close to that of the earth (assumed to be outgassed), and this Xe exhibits no isotope anomalies. Xenon from several old granite samples from the central Transvaal contains substantial amounts of fissionogenic Xe from spontaneous fission of U^(238). Isotopic composition of this Xe is in agreement with values reported by Wetherill. The amount of fission Xe in samples from three related granites was very uniform, suggesting that U-Xe dating would be a valuable adjunct to K-Ar dating for old samples. Rough calculations of the U-Xe ages of these granites are in agreement with Rb-Sr ages obtained by Allsopp. Xenon from the CO_2-well gases contained a component from spontaneous fission. The He^4/Xe^(136)_F ratio for this gas is very close to that expected for gas evolved from rocks with a Th/U ratio of 3. In addition, there was a small component of excess Xe^(129), i.e., Xe^(129*)/Xe^(136)_F = 0.92 ± 0.42. Arguments are given to show that this residual Xe^(129) is not due to fast or slow neutron absorption in Te^(126). The excess Xe^(129), if real appears to be due to decay, soon after formation of the earth, of now extinct 17-m.y. I^(129). The effect has far-reaching implications: (1) it implies that the outgassing of the interior of the earth is not complete and that even the decay products of some short-lived activities have not been homogenized; (2) calculations based on an extremely simple model (but one for which results are fairly representative) give a rough indication of the time interval by which the earth postdates the meteorites. It is clear that further experiments with well gases should be made to see whether the effect is reproducible.

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