Unambiguous evidence for fission‐produced nuclides in meteorites has been shown in the abundance pattern of the Xe isotopes from the Pasamonte meteorite. Calculation of the age of the meteorite from fissiogenic Xe136 and uranium contents of the meteorite yields a value of T′ = 2 × 1010 years, which seems far too large. The result of this calculation leads to the conclusion that part of the fissiogenic Xe in Pasamonte must have been produced by other fissionable nuclide(s), such as Pu244, in addition to U238. Assuming that the fissiogenic Xe136 in Pasamonte is produced from spontaneous fission of U238 and extinct Pu244, then Ξ, the time interval between the cessation of nucleosynthesis and formation of the meteorite, is estimated to be ∼300 m.y.

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Isotopic compositions have been measured mass spectrometrically for xenon fractions released from the carbonaceous chondrite Murray in stepwise heating experiments. The isotopic ratios varied quite considerably; for example, the 136Xe/132Xe ratio in the 1000°C fraction was almost identical to the atmospheric ratio (0.330), whereas the ratio in the 1300°C fraction agreed with that in Sucor (0.305). It appears that these variations can best be explained as being due to the fact that reservoirs of two isotopically distinct gases (solar and planetary) exist in the meteorite and that mixtures of these gases in various proportions are being released at different temperatures. The major difference in the isotopic compositions of solar and planetary xenon can be attributed to a mass‐dependent fractionation process, which must have occurred during the early stages of the history of the solar system. The abundance ratios of the xenon isotopes are also altered by the cosmic ray irradiation and neutron capture processes. According to this interpretation, it is unnecessary to assume the existence of the so‐called carbonaceous chondrite fission component. The decay products of extinct radionuclides 129I and 244Pu did not alter the xenon isotopic ratios significantly in the case of the carbonaceous chondrite Murray. The light isotopes 124Xe and 126Xe in the earth's atmosphere appear to be partially of cosmic ray origin.