Trapped noble gases in unequilibrated ordinary chondrites

Abstract

The abundances and isotopic compositions of noble gases were determined in bulk samples and acid-resistant residues of eight unequilibrated (type 3.0–3.8) ordinary chondrites (two LLs; four Ls, with two from a paired fall; two Hs) including the most primitive one, Semarkona. HL-Xe was found in all cases except Dhajala (H 3.8). The H-part of HL-Xe is isotopically indistinguishable from that in Allende; the same we find true for the ratio H-Xe/L-Xe. Thus, our data do not confirm previous reports of variations of this ratio among ordinary chondrites nor do they confirm that in ordinary chondrites this ratio is any different from that in Allende. Constraints on the origin of HL-Xe as well as the trapping mechanism are discussed. Also present in all meteorites except Dhajala is an HF/HCl soluble component with a high Ar/Xe ratio. The abundances of both isotopically ordinary Xe and HL-Xe correlate with petrological subtype. Combustion of a Dhajala acid-resistant residue yielded Ne with abundance ratios 20Ne 22Ne = 10.1 ±0.2 and 21Ne 22Ne ≤ 0.04 for that part of the Ne (“Q-Ne”) which is associated with the bulk of the (ordinary) heavy noble gases. The noble gases released in this experiment were associated with the combustion of only a minor portion of C; the highest 132Xe C atom ratio of 1.3 × 10 −8 was found for the first combustion step and is only 10 times lower than the solar ratio. In Semarkona (LL 3.0) there is evidence for the presence of Ne-E; in ALH A 77278 about half the Ne is of solar origin (SEP-Ne).