NOBLE GASES AND COSMOGENIC RADIONUCLIDES IN THE GOLD BASIN L4 CHONDRITE SHOWER: THERMAL HISTORY, EXPOSURE HISTORY, AND PRE-ATMOSPHERIC SIZE

Abstract

We measured the concentrations of the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of 10Be, 26Al, and 41Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the 22Ne/21Ne ratio at very high shielding. The 21Ne/10Be and 21Ne/26Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of 21Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron-capture 36Ar in most samples are consistent with measured concentrations of neutron-capture 36Cl and an exposure age of 18 Myr. Large excesses of neutron-capture 36Ar were found in those samples with an excess of 21Ne, providing additional evidence of a first-stage exposure on the parent body. The excess of spallation-produced 21Ne and neutron-capture-produced 36Ar in these samples indicate a first-stage exposure of 35–150 Myr on the parent body. The radiogenic 4He and 40Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.