Noble gases have been measured in one bulk sample, six light inclusions, and one black xenolith from four specimens of the carbonaceous chondrite Allende which cover a range in their 60Co content of about a factor of ten. The release pattern obtained upon heating the samples at different temperature steps shows: 1. 1.) The parent nuclides of radiogenic 4He are finely dispersed throughout the inclusions in host minerals with grain sizes smaller than the typical range of α-particles within them; 2. 2.) Neon from the inclusions contains a component deficient in 20,21Ne which is most conspicuous at low temperatures. The apparent excess 22Ne* is attributed to spallation reactions on sodium although the reasons for excluding NeE are weak; 3. 3.) 36Ar/38Ar ratios up to 90 are observed at release temperatures around 800°C. For inclusions from the same specimens the excess 36Ar* is proportional to the chlorine content, for different specimens the chlorine-normalized amounts of 36Ar* tend to correlate with 60Co; 4. 4.) 80,82Kr are found enriched about 80 fold, the ratio of the excess amounts 80Kr* 82Kr* is 2.68 ± 0.06; 5. 5.) Xenon is dominated by 129Xe, the overabundances are accompanied by much smaller excesses of 128Xe. The ratio of the excess amounts 128Xe* 129Xe* is constant for inclusions from the same specimens while for different specimens there is a range of a factor of four. The abundance anomalies in Ar and Kr as well as that of 128Xe can be explained as being due to the capture of neutrons by Cl, Br, and I, respectively, during the exposure of the meteoroid to the cosmic radiation. There is no compelling evidence for a contribution from an early neutron irradiation nor, in the case of 36Ar, for a contribution from extinct 36Cl. The ratio of the specific excesses 36Ar* 128Xe* as well as the 80Kr* 82Kr*ratio indicate no more than ~20 percent of the neutrons to have been thermal neutrons.
ASJC Scopus subject areas
- Geochemistry and Petrology