Noble gases in “phase Q”: Closed-system etching of an Allende residue

Abstract

We analyzed for the first time in nearly pure form the noble gases in “phase Q,” the elusive major carrier of the planetary gases in carbonaceous chondrites. Noble gases were extracted from an HF/ HCl residue of Allende by oxidation with HNO 3, using the closed-system stepped etching technique. 10–15% of the primordial He and Ne were released, in addition to more than 90% of the Ar-Kr-Xe. All five noble gases liberated from “Q” have uniform isotopic composition throughout the extraction: 3He 4He = (1.59 ± .04) × 10 −4 ; 20Ne 22Ne = 10.70 ± .15 ; 36Ar 38Ar = 5.32 ± .02 . Kr-Q and Xe-Q differ from AVCC. Relative to solar wind values, He, Ne, and Ar in “Q” are depleted in the light isotopes as well as the light elements. Xe-Q may be (nearly) unfractionated solar Xe blended with some fission Xe. Ureilites and several ordinary chondrites contain noble gases with isotopic compositions and elemental abundances very similar or identical to those of Q-gases. This component thus seems to have been widespread in the solar nebula. A close relation between noble gases in ureilites and Q-gases is strongly suggested, particularly by the near match of their Ne and Xe isotopic compositions but also by their rather similar He and Ne abundances.