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Geochemical Journal
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Trapped noble gas components and exposure history of the enstatite chondrite ALH84206

DAISUKE NAKASHIMA, TOMOKI NAKAMURA
Geochemical Journal, Vol. 40, No. 6, P. 543-555, 2006

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.

KEYWORDS

enstatite chondrite, regolith breccia, solar noble gas, primordial noble gas, cosmic-ray exposure age

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