Cosmogenic radionuclides in L5 and LL5 chondrites from Queen Alexandra Range, Antarctica: Identification of a large L/LL5 chondrite shower with a preatmospheric mass of approximately 50,000 kg

摘要

The collection of approximately 3300 meteorites from the Queen Alexandra Range (QUE) area, Antarctica, is dominated by more than 2000 chondrites classified as either L5 or LL5. Based on concentrations of the cosmogenic radionuclides ~(10)Be, ~(26)Al, ~(36)Cl, and ~(41)Ca in the metal and stone fraction of 16 QUE L5 or LL5 chondrites, we conclude that 13 meteorites belong to a single meteorite shower, QUE 90201, with a large preatmospheric size and a terrestrial age of 125kyr. Members of this shower have properties typical of L (e.g., pyroxene composition) and LL chondrites (e.g., metal abundance and composition), as well as properties intermediate between the L and LL groups (e.g., olivine composition), and is thus best described as an L/LL5 chondrite. Based on comparison with model calculations, the measured radionuclide concentrations in the metal and stone fractions of QUE 90201 indicate irradiation in an object with a preatmospheric radius of approximately 150cm, representing one of the largest chondrites known so far. Based on the abundance of small L5 and LL5 chondrites at QUE and their distinct mass distribution, we conclude that the QUE 90201 shower includes up to 2000 fragments with a total recovered mass of 60-70kg, <1% of the preatmospheric mass of approximately 50,000kg. The mass distribution of the QUE 90201 shower suggests that the meteoroid experienced catastrophic atmospheric fragmentation(s), either because it was a fragile object or it had a high entry velocity.