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A detailed analysis of the rare gas content of the chondrite Renazzo is presented. Fractions of different isotopic composition are separated by heating the sample to successively higher temperatures. The abundance and isotopic composition of the gases is similar to that in the carbonaceous chondrite Murray, with a high percentage of so‐called primordial gas. For the light rare gases this primordial component appears to be highly fractionated with respect to the supposed cosmic isotopic ratios. Variations in the isotopic composition of xenon from Renazzo indicate that a minimum of three distinct components are required to describe the situation, and an analysis of this three‐component system is presented in terms of ‘correlation planes.’ In terms of this analysis, three possible origins for the isotopic variations are considered. An I127‐Xe129 formation interval for Renazzo is measured using an irradiated specimen of the meteorite. A minimum value of 66±6 m.y. is indicated.

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Abstract. Thirty-nine samples of natural gases representing varied chemical compositions and geological occurrences were analyzed for their helium, radiogenic argon, and atmospheric argon contents. The total range in the (He/A)r•d ratio was found to be 1.6 to 130 with most samples having values between 6 and 25. This range of values is essentially equal to the production ratio from the uranium, thorium, and potassium in average igneous rocks and a wide variety of sediments. This indicates that all of these natural gases have obtained their radiogenic gases from rather average rock types. This is true in spite of the fact that the gases range in helium content from 37 to 62,000 ppm.A theoretical discussion of the origin of helium and argon in natural gases is given. It can be shown from the ratio of nitrogen to atmospheric argon that most of the nitrogen in these gases cannot come from the entrapment of air. From a consideration of the concentration of atmospheric argon in natural gases it is possible to estimate the proportion of gaseous and aqueous phases assuming diffusive equilibrium.The isotopic composition of the carbon in the methane of these gases was found to be very light. It was shown that for coexisting CH 4-CO 2 pairs the carbon dioxide was always isotopically heavier.