Light noble gas records and cosmic ray exposure histories of recent ordinary chondrite falls
Abstract
We measured noble gas concentrations and isotopic ratios (He, Ne, and Ar isotopes) in six recent ordinary chondrite falls: Mangui (L6), Vinales (L6), Ozerki (L6), Tamdakht (H5), Kheneg Ljouad (LL5/6), and Katol (L6). Among them, the three L6 chondrites Mangui, Vinales, and Ozerki fell in only a few months interval; their apparent similar petrographic and mineralogic characteristics might indicate source crater pairing. To test this hypothesis, we have investigated those meteorites for their cosmic ray exposure (CRE) histories, using the cosmogenic noble gases He-3, Ne-21, and Ar-38. We systematically (re)calculated the CRE ages as well as the gas retention ages of these meteorites. The CRE age of the Mangui is, based on noble gases, <1 Ma, which is unusually short for an L chondrite. Indeed, the range of exposure ages for L chondrites is generally distributed between similar to 1 and similar to 60 Ma, with major peaks occurring around similar to 5, similar to 30, and similar to 40 Ma. In addition, the cosmogenic He-3(cos) data of two Mangui duplicates are consistent with a remarkably high loss of helium by diffusion due to heating by solar radiation. Such a short parent body-Earth transfer time (<1 Ma) can be explained by a delivery from an Earth-crossing object. Regarding the other L6 chondrites, Vinales has a nominal CRE age of similar to 9.4 Ma, whereas the Ozerki meteorite has a nominal CRE age of similar to 1.2 Ma, which is consistent with Korochantseva et al. (2019). Based on their CRE ages as well as on their gas retention ages, it appears that none of these three recent L6 chondrite falls are source crater paired, and therefore, all three originate from different meteoroids. The nominal exposure ages of Tamdakht, Kheneg Ljouad, and Katol are similar to 3.2, similar to 11, and similar to 30 Ma, respectively, and are consistent with identified age peaks on the exposure age histogram of H, LL, and L chondrites, respectively. The nominal CRE age of Tamdakht is consistent with previous observations for H chondrites and implies that they are dominated by small impact events occurring in several parent bodies.