Xe and Kr analyses of silicate inclusions from iron meteorites

Abstract

The Xe and Kr contents of silicate inclusions from the iron meteorites Copiapo, Four Corners, Linwood, Pine River, Weekeroo Station and Woodbine (Xe¹³²= 3–40, Kr⁸⁴= 5–100 × 10⁹ atoms/g;Kr⁸⁴Xe¹³² = 2) are comparable to chondritic values. The isotopic compositions show trapped gas of both chondritic composition (dominant in Pine River) and atmospheric composition (dominant in Linwood). Large spallation effects (Kr_(sp) ⁸⁴ = 4 × 10⁹, Xe_(sp) ¹²⁶ = 2 × 10⁸ atoms/g) occur in Weekeroo Station and Four Corners. The spallation Xe and Kr spectra in Weekeroo differ from those reported for stone meteorites. A re-analysis of literature data shows that systematic variations also exist among stone meteorite spallation spectra which can be qualitatively understood in terms of target element abundance and shielding variations. Very large neutron capture effects on Br and I occur in several meteorites (Kr⁸⁰/Kr⁸³/Kr⁸⁶ = 100/2/3 × 10¹⁰ atoms/g in Linwood). The (Kr⁸⁰Kr⁸²)n≅ 2·8 ratios from neutron capture on Br for Linwood and Copiapo are distinct from that found in stone meteorites. All samples have pronounced Xe¹²⁹ excesses (3–50 × 10⁹ atoms/g) which apparently indicate differences in formation times from chondrites of less than about 10⁸ yr; however, the presence of trapped Xe¹³² in silicates which were enclosed in molten Fe-Ni and cooled slowly prove that they were not entirely outgassed; thus, some of the Xe¹²⁹ excess may also be trapped. No discernable fission Xe was observed.