ABSTRACT

Noble gases in the Allan Hills (ALH) 84206 EH3 chondrite were measured with stepwise heating of a large sample (98.9 mg) and with total extraction of small samples (0.9-2.3 mg). Light noble gases show solar and cosmogenic components. The elemental ratios of solar gases in the samples suggest no significant loss of solar ³⁶Ar. Some of small samples are richer in cosmogenic ²¹Ne than the large sample. Assuming that the excesses are due to a parent body exposure to cosmic rays, we obtain a parent body exposure age longer than 51 Ma and a meteoroid exposure age shorter than 39 Ma. Heavy noble gases are dominated by elementally fractionated Q-gases and atmospheric noble gases. The presence of fractionated Q-gases combined with no loss of solar ³⁶Ar suggests that solar wind exposure occurred after Q-gas fractionation. Kr isotopic ratios show the presence of neutron-induced ⁸⁰Kr and ⁸²Kr from Br. The minimum radius of the ALH84206 meteoroid was calculated as 27 cm from the abundances of neutron-induced Kr, assuming that these were produced during exposure of the meteoroid.