Isotopic Compositions of Rare Gases in the Carbonaceous Chondrites Mokoia and Allende


The isotopic compositions have been measured mass spectrometrically for neon, argon, krypton and xenon released from the carbonaceous chondrites Mokoia and Allende in stepwise heating experiments. The isotopic compositions of rare gases released from the meteorites at different temperatures varied quite considerably. The observed minimum and maximum values were, for example, as follows: Ne20 Ne22 = 2.10 (Allende 800°C) and 12.95 (Mokoia 400°C); Ar36 Ar38 = 4.20 (Allende 1600°C) and 6.05 (Allende 800°C); Kr84 Kr86 = 3.140 (Allende 1000°C) and 3.306 (Mokoia 600°C); Xe134 Xe136 = 1.019 (Allende 800°C) and 1.192 (Allende 1400°C). A marked enrichment of Xe129 due to the decay of extinct nuclide I126 was observed in both meteorites. The variations of the isotopic ratios are partly caused by the presence of cosmic-ray spallation and neutron-capture products. In addition, however, a marked trend of mass-dependent variation of the isotopic ratios was observed in this work. The rare gas isotopes released from the meteorites appear to be systematically mass-fractionated relative to the relative abundances of the average carbonaceous chondrite (AVCC). It seems that this phenomenon can be best explained as due to the fact that there exist reservoirs of two isotopically distinct gases in the meteorites and mixtures of these gases are being released at each temperature fraction. The two different gases being considered here are most likely the so-called solar and planetary rare gases, whose isotopic compositions are quite different from each other.



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