Noble gases, cosmic ray exposure and radiogenic ages in selected ordinary chondrites

Abstract

Noble gas isotopic ratios and concentrations provide powerful constrains on the nature of trapped component presence in the meteorites. Concentrations and isotopic composition of the noble gases (He, Ne, Ar, Kr and Xe) in eleven bulk ordinary chondrites (OCs), Mahadevpur (H4/5), Didwana-Rajod (H5), Monahans (1998) (H5), Portales Valley (H6), Itawa Bhopji (L3-5), Jodiya (L5), Jalangi (L5/6), Kaprada (L5/6), Devri-Khera (L6), Katol (L6) and Bruderheim (L6) are presented. The aim of the study is to examine the exposure history and radiogenic ages of the recent meteoritic falls. The results of stepwise heating analyses indicate that noble gases are mixture of trapped and cosmic ray produced. Neon isotopes are enriched from galactic cosmic ray (GCR) produce gas, but many samples show a Q-type contribution. Argon isotopes show 40Ar excesses (40Ar/36Ar up to (42.6 ± 0.2) × 104), which is the decay product of 40K. Measured Kr and Xe isotopes in all the studied OCs predominantly shows Q component. Minor contribution from primordial noble gases of HL component is also observed in stepwise heating extractions in few cases. Solar gases are absent in the specimens studied here. The cosmic-ray exposure (CRE) ages of the OCs range from 5.2 ± 0.9 Ma to 36.3 ± 6.7 Ma. These ages are within the range typically observed for the respective meteorite types of OCs. However, the CRE ages of many meteorites are inconsistent with peaks in the exposure age histogram for respective sub classes of OCs, H and L types. The nominal radiogenic age obtained from 40Ar ranges between 0.87 ± 0.09 Ga to 4.57 ± 0.46 Ga in the meteorites with exception of two meteorites. The 4He gas retention ages for the OCs are in the range 0.27 ± 0.03 Ga to 4.40 ± 0.50 Ga, showing different degree of degassing either at a thermal event on the parent body. Radiogenic 129Xe from the decay of 129I was observed in all the studied samples of OCs.