Rare gases in stony meteorites

Abstract

The contents and isotopic composition of He, Ne and Ar in 81 stony meteorites have been determined. The 3He exposure ages, and the radiogenic 4He and K-Ar ages confirm the conclusions of previous publications. The exposure ages of diogenites are clustered around 20 × 10 6 yr. The H-chondrites Benoni, Cee Vee, Grassland, Heredia, Hessle and Menow, the L-chondrite Bishunpur and the carbonaceous chondrites Alais and Essebi contain He and Ne as primordial components. A dark inclusion in the amphoterite Krähenberg has an extremely high K-content of 12000 ppm. The contents of 84Kr and 132Xe and the 129Xe: 132Xe ratio have also been determined in 92 stony meteorites. Not only the 84Kr and 132Xe contents but also the content of the 129Xe-excess are proportional to the primordial 36Ar content over a larger range. The primordial 36Ar contents are related to the petrological classes of Van Schmus and Wood. The 129Xe: 132Xe ratios of all ordinary chondrites lie between 1 and 3 and all enstatite chondrites have ratios between 3 and 7. If one assumes the I- 129Xe relation from the work by Reynolds and co-workers to be valid, one has to conclude that all ordinary chondrites had initially about the same Xe:I ratio and that most of them lost a large fraction of their primordial rare gases after the 129I decay while the high temperature fraction retained all of the 129Xe. This imposes quite narrow constraints on the model on I- 129Xe dating.