Xenon isotopic ratios

Measurements of xenon in high He/" He OIB samples have often found atmospheric isotopic ratios (e.g., Allegre et al, 1983) that appear to be due to overwhelming air xenon contamination (Harrison effll., 1999 Patterson efal., 1990) rather than reflecting mantle xenon with an air composition. Although Samoan samples with intermediate (9-20Ra) helium isotope ratios have been found with xenon isotopic ratios distinct from those of the atmosphere (Poreda and Farley, 1992), the xenon in these samples may have been derived largely from the MORB source. Harrison et al. 

This increase can be determined by measuring either the cesium or xenon isotopic ratios. These results combined with the low flux fission yields can then be used to calculate the cross section of Xe135. The ideal experiment would be one in which the flux remains constant during the entire irradiation so that the amount of Xe136 present at any time is not dependent on flux variations. This approach has been used by Fickel and Tomlinson 28), who arrived at a value of 3.2 X 106 barns (1 barn = 10-24 cm2) for the neutron cross section of Xe136. Cross sections of other fission products have been measured 12, 63, 76) although the conditions in these experiments were such that the effect of flux variations could not be evaluated. 

Finally, we note that deviations from atmospheric-like krypton isotope ratios have not been found for any back-arc basin however, there are two reports of anomalous xenon isotope variations for the Mariana Trough. Ikeda et al. (1998) found coupled " Xe/ °Xe and Xe/ °Xe deviations from air—similar to those found in MORE, whereas Sano et al. (1998a) reported only Xe excesses (relative to Xe). These signatures reinforce the idea that the mantle wedge is the principal source of volatiles in the Mariana Trough particularly where circumstances limit the amount of atmosphere-derived contributions. At all other localities, only atmospheric-like xenon isotope ratios have been found. 

The one-time presence of superheavy elements in meteorites has been inferred from the anomalous xenon effect. The ratios of xenon isotopes measured in some meteorites are very different from those found in terrestrial xenon. This was thought to be due to the former presence of Pu which had produced fission-product xenon as it decayed out. This theory was confirmed in part recently when xenon isotope ratios from Pu were measured and were found to be identical with ratios from one group of meteorites, the achondrites. On the other hand, the chondrite meteorites still do not fit. It had been shown that an element heavier than Cf was needed to give the required ratio, ° and Anders and Heymann and Dakowski suggested independently that extinct superheavy elements could be the cause. Rao indicated that excess Kr could be explained similarly. Schramm has calculated that if fission of a superheavy element is the cause, its half-life would lie in the surprisingly narrow range of 1.6 x lO" —6.8 x 10 yr. 

Krypton and Xenon from Huclear Power Plants. Both xenon and krypton are products of the fission of uranium and plutonium. These gases are present in the spent fuel rods from nuclear power plants in the ratio 1 Kr 4 Xe. Recovered krypton contains ca 6% of the radioactive isotope Kr-85, with a 10.7 year half-life, but all radioactive xenon isotopes have short half-Hves. 

The anomalous micro-composition of the Martian atmosphere with regard to nitrogen, argon, neon, krypton and xenon has also been compared with trapped gases for the Martian meteorite collection (12 in total). The isotope ratios for... 

A major objection that cannot be satisfied through simple modification of the model is that atmospheric and MORB xenon are not complementary that is, extraction of the xenon now seen in the atmosphere from the upper mantle will not leave a mantle reservoir with the xenon isotope characteristics presently observed there. Due to the greater half-life of the ratio of... 

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... 

Nonradiogenic argon and xenon isotope concentrations of such a lower-mantle reservoir cannot be directly calculated without assuming either specific lower-mantle Ar/Ne and Xe/Ne ratios or argon and xenon isotopic compositions. Since modification of the noble gases in the atmosphere has occurred, the relative abundances of the interior of the Earth are unlikely to match those now observed in the atmosphere. For example, a closed-system lower mantle with "Xr/ Ar 3,000 (see Section 4.11.2.4) and 270 ppm K has " Ar= 5.7 X lO atoms g and so... 

Figure 7 The relationship between the xenon isotope compositions of the atmosphere, initial atmosphere, and the upper mantle as sampled by MORE. The line connecting the initial atmosphere (fractionated U-Xe) and the present atmosphere has a slope equal to the ratio of plutonium-derived Xe to radiogenic Xe ( Xe/ Xe) in the atmosphere. Any xenon that remained in the solid reservoir from where this was degassed must have a greater value for this ratio and so he in the shaded region (Ozima et al, 1985). While measured MORBs do so, upper-mantle compositions that have been corrected for U-derived Xe (based on MORE data of Kunz et al, 1998 and on CO2 weU gas data of Phinney et al, 1978) do not, indicating that upper-mantle xenon is not the residual left from atmosphere degassing.

Figure 8 The xenon isotope compositions of U-Xe and solar wind xenon, both nonfractionated and fractionated to match the light xenon isotopes, are compared to the value of the atmosphere. It is clear that fractionated solar wind xenon cannot serve as the non-radiogenic composition of the atmosphere since it has a higher Xe/ °Xe ratio. In order for the upper mantle to have the same Xe/ Xe ratio as the atmosphere, the nonradiogenic composition of the atmosphere must lie on the dotted lines, implying that there is very little plutonium-derived Xe. This is contrary to the inferred Pu budget of the Earth.

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