The noble gases in five ordinary chondrites from Grove Mountains in Antarctica

Abstract

Here we present the abundances and isotopic compositions of the noble gases He, Ne, Ar, Kr, and Xe in five bulk ordinary chondrites from Antarctica, namely Grove Mountains (GRV)054472, GRV053690, GRV022403, GRV023149, and GRV052243. The cosmic ray exposure (CRE) ages of the three H chondrites GRV054472 (H3), GRV053690 (H4), and GRV022403 (H4) are 3.8 ​± ​0.3 ​Ma, 5.9 ​± ​0.4 ​Ma, and 16.6 ​± ​1.5 ​Ma, respectively. This finding is consistent with peaks in the exposure age histogram for H chondrites at 4 ​Ma and 7.2 ​Ma, respectively and also with the broad age cluster at 15–20 ​Ma, which is especially pronounced for H4 chondrites. The two L6 chondrites GRV023149 and GRV052243 are most likely paired with consistent exposure ages of 17.4 ​± ​1.1 ​Ma and 17.6 ​± ​1.5 ​Ma that both fall into the peak at 15 ​Ma for L6 chondrites. The two meteorites GRV054472 and GRV053690 show significant 3He or 3H diffusive losses of about 10% and 20–30%, respectively. The 4He gas retention ages for the three H chondrites are in the range 1.09–3.18 ​Ga, showing different degrees of degassing either at a thermal event on the parent body or during cosmic ray exposure after the ejection from the parent body. The low gas retention ages for the two L6 chondrites based on (U–Th)/He systematics are 259 ​Ma (mean age of two aliquots) and 320 ​Ma or 364 ​Ma (mean age of two aliquots) and 307 ​Ma based on K–Ar system. These ages indicate significant gas loss during the catastrophic break-up of the L-chondrite parent body. Trapped Kr in GRV054472 and GRV053690-2 is almost pure Kr-Q. In addition, other meteorites also contain Q as trapped Kr, but have discernable cosmogenic Kr. The excess 82Kr observed in GRV052243 and GRV022403 is due to the neutron capture on 81Br. Radiogenic 129Xe∗ from the decay of 129I was observed in all studied chondrites. Measured Xe isotopes in all the studied chondrites predominantly shows Q component with minor contributions from the terrestrial atmosphere. For the samples GRV053690-2 and GRV023149-2, measured Xe falls on a mixing line with Xe-SW and Xe-HL, although the later component contributes very little.