Isotopic anomalies of noble gases in meteorites and their origins—IV. C3 (Ornans) carbonaceous chondrites

Abstract

The C3O chondrites Kainsaz, Lancé and Ornans were studied by an acid dissolution technique, to characterize the noble-gas components in 3 mineral fractions: HF, HCl-solubles (99% of the meteorite), chromite and carbon (0.3–0.9%), and ‘phase Q’, a poorly characterized trace mineral (0.05–0.4%) containing most of the Ar, Kr, Xe. For all fractions, gas contents decline in the order Kainsaz > Lancé > Ornans; this trend parallels volatile contents but not heterogeneity of olivine composition or degree of metamorphism and seems to reflect progressively higher condensation temperatures from the solar nebula. Solubles contain nearly unfractionated Xe, and show 136 Ar 132 Xe ratios up to 850. Hence the high Ar Xe ratios (200–400) of bulk C3O chondrites must be due to an HF-soluble mineral (possibly magnetite). Phase Q contains ordinary planetary gases and a Ne component of 20 Ne 22 Ne = 10.3 ± 0.4. Chromite and carbon contain Ne of 20 Ne 22 Ne = 8.6 ± 0.1 and ‘CCF’ xenon (a peculiar component of possibly fissiogenic origin, enriched in the heavy isotopes but accompanied by a component enriched in the light isotopes). In all primitive chondrites, both the amount and the chemical separability of CCFXe parallel the abundance of promordial noble gases and other volatiles, such as C, N, Tl, Bi and In. The close correlation of CCFXe with various properties of undoubtedly local origin (volatile content, petrologic type, presence of ferrichromite and carbon, etc.) is more consistent with a local than with an extrasolar origin of this component. A volatile superheavy element seems to be the most plausible source, but the evidence is not conclusive.