Noble gases in unequilibrated ordinary chondrites

Abstract

The rare gases were determined mass-spectrometrically in eleven unequilibrated ordinary chondrites. The amounts of primordial Ar, Kr and Xe are systematically greater than in ordinary chondrites proper. Primordial Ne was detected in only two of the meteorites. The amounts of the heavy rare gases Ar, Kr and Xe are approximately inversely proportional to the degree of equilibration of Fe in olivine, and are directly proportional to C contents. The primordial rare-gas contents of different pieces of Bishunpur, Krymka and Mezö-Madaras vary substantially, indicating that these meteorites are inhomogeneous in this respect. The heavy gases Ar, Kr and Xe are “planetary” in the sense of the model of Signer and Suess. The average primordial Ar 86:Ar 38 ratio of nine cases is 5.35 ± 0.16. Except for Xe 129, the isotopic composition of Xe agrees with that of “average carbonaceous chondrites”. Carbonaceous chondrites, unequilibrated ordinary chondrites, and ordinary chondrites proper form a sequence of decreasing contents of primordial rare gases of the “planetary” type. This is consistent with a recent theory by Larimer and Anders on the formation of these meteorites. We infer from our results, however, that neon does not belong to the “planetary” component.