Xenon in CO₂ Well Gas Revisited

We have analyzed the rare gases in a CO. well gas from Harding County, New Mexico, by mass spectrometry. We have reconfirmed the monoisotopic excess of '9Xe measured previously. The excesses of fissiogenic Xe isotopes have relative proportions which coincide with the spontaneous fission X e spectrum of '88U. No '44pu-derived, fissiogenic Xe was detected. The uncertainties in the fission Xe component place an upper limit of 20% on the '"Pu contribution to fissiogenic 88Xe. Using this upper limit and the excess of 'gXe, we calculate an upper limit on the formation interval of the earth of 76 m.y. The trapped Ne, Ar, Kr., and Xe are dominated by a component which is similar to that found in mantle- derived terrestrial samples. We also report for the first time an excess of aHe, which is evidence for primordial aHe in the source region of the well gas. The CO. well gases from the Bueyeros field in New Mexico have played a key role in the search for ancient Xe components in the earth. In 1963, Butler et al. (1963) reported a small but significant ?9Xe excess in one such CO. well. They attributed the excess løXe to the decay of 17-m.y. 19I very early in the history of the earth. Butler et al. (1963) also found excesses of fissiogenic X e isotopes in these samples. Although the uncer- tainties in the fissiogenic excesses were relatively large ( 30%), the relative proportions were consistent with those expected from the spontaneous fission of '3øU. Two years later both the løXe and the fissiogenic Xe excesses were confirmed in a number of Bueyeros field CO. well samples by Wasserburg and Mazor (1965), again with relatively large uncertainties. An improvement in experimental precision attended the next reported measurement of Xe in these gases (Boulos and Manuel, 1971). This work confirmed the earlier finding of l9Xe and fissiogenic isotope excesses, but as a result of the increased precision, both of these excesses took on new implications. First, Boulos and Manuel (1971) pinpointed the amount of the løXe excess much better than the earlier work and showed that it was reproducible within narrower limits. Second, the average isotopic composition of the fission excesses corre- sponded to a mixture of '3øU and '44pu spontaneous fission Xe. The latter finding was somewhat obscure, since the uncer- tainties were still lame enough to accommodate either 'øU or '44Pu as the sole source of these excesses. Nevertheless, this work demonstrated for the first time that the existence of '44Pu fission Xe in a CO. well gas was a realistic possibility. Hennecke and Manuel (1975b) subsequently confirmed these results. The precision of their measurements was, however, essentially the same as that of Boulos and Manuel ( 1971 ). Thus they were unable to specify the relative amounts of '44Pu- and 'øU-derived Xe in a statistically sighificant way. In the Berkeley mass spectrometry laboratory we have re- cently completed a new analysis of the rare gases in a sample of CO. well gas. Our primary motivation has been to measure the fissiogenic Xe excesses with greater precision than has been achieved in earlier work and thereby to determine unambig- uously the relative proportions of '44ptl and 'søU fission Xe in CO. well gas.