Xenon and neon from acid-resistant residues of Inman and Tieschitz

Abstract

Xenon and neon from highly oxidized acid residues of the unequilibrated ordinary chondrites Inman and Tieschitz are dominated by Xe-HL and Ne-A2, although Xe-S is also present in the oxidized Inman residue. The isotopic composition of Xe released at lower temperatures from the Inman residues most likely represents mixtures of trapped Xe and Xe-HL at all isotopes except 129Xe and 128Xe where significant and correlated excesses are observed. These are clearly both derived from I, the former from decay of extinct 129I and the latter from neutron capture by 127I, requiring a thermal equivalent fluence of ~3 × 1017 n/cm2, or 2-3 × 1016 n/cm2 in the resonance region, confirmed by artificial irradiation of an aliquot. The 129I127I ratio at Xe closure is about 1 × 10-4, similar to the canonical meteoritic value which, given the short half-life of 129I, suggests closure times similar to other meteorites and subsequent irradiation within the solar system. In Tieschitz there may be some evidence for separation of Xe-L from Xe-H as would be expected if the neutron-deficient end of the Xe-HL spectrum is not produced by the same processes as the neutronrich end and is not completely homogenized with it. Ne-E(H) is measured in oxidized residues from both of these meteorites, but no evidence is found for Ne-E(L). Correlations are observed between cosmogenic 21Ne and Ne-E(H) in the higher temperature fractions of both Inman and Tieschitz. These may not be due to a common host phase, but could instead be due to high temperature reactions between spinel and SiC. © 1991.