Rare gases and fossil particle tracks in the Kenna ureilite

Fossil particle tracks and spallation-produced He and Ne in the Kenna ureilite indicate that it existed in space as a small object for 23 m.y. In our study of Kenna, we found no evidence of trapped He or Ne. Large amounts of heavy rare gases occur in Kenna in concentrations typical of ureilites. In a step-wise release of gases, the isotopic compositions of Kr and Xe were found to be constant above 600°C, revealing the presence of a single retentively sited component. The Xe isotopic abundances are characterized by 124:126:128:129:130:131:132:134:136 = 0.471:0.414:8.280:103.61: 16.296:81.92:100:37.70:31.23. This isotopic composition is distinct from AVCC (average carbonaceous chondritic), but similar to compositions known for some time in certain temperature fractions of Renazzo, Murray and Murchison. Kenna-type Xe appears to be one of the several components found in carbonaceous chondrites.

Binz et al. (Geochim. Cosmochim. Acta 39, 1576–1579, 1975) have recently found that many volatile trace elements are strongly depleted in ureilites. Thus, the relatively large amounts of heavy rare gases present in ureilites did not result from a mixture of a volatile-rich component with the ureilite host. It appears that some material rich in carbon and heavy rare gases was incorporated into a differentiated ureilite host. All current hypotheses which purport to explain the origin of trapped gases in meteorites encounter difficulty in accounting for trapped gases in ureilites in a straightforward manner.