Exposure age, terrestrial age and pre‐atmospheric radius of the Chinguetti mesosiderite: Not part of a much larger mass

Abstract

Abstract— We measured the concentrations of the cosmogenic radionuclides 14C (half‐life = 5.73 × 103 years) in the bulk and of 10Be (1.5 × 106 years), 26Al (7.05 × 105 years), 36C1 (3.01 × 105 years) and the light noble gases in metal and stone fractions of the Chinguetti meteorite to investigate the controversial claim that the 4.5 kg mesosiderite is part of a much larger mass in the Mauritanian desert. Based on the 36C1‐36Ar, 10Be‐21Ne and 26Al‐21Ne pairs in the metal fraction, we derive an average cosmic‐ray exposure age of 66 ± 7 Ma. Chinguetti is now the third out of 20 mesosiderites with an exposure age between 60 and 70 Ma. This may be the first hint of a major impact on the parent body of the mesosiderites, which show ages ranging from 10 to 300 Ma (Terribilini et al., 2000). From the 14C‐10Be pair we derive a terrestrial age of 18 ± 1 ka, which seems too recent to be consistent with the original description of the main mass having a heavily wind‐eroded base, overhung by the upper part of the meteorite. Finally, from the radionuclide concentrations in combination with Monte‐Carlo based calculations, we conclude that our sample of Chinguetti was irradiated at a depth of ∼15 cm in an object not larger than 80 cm in radius. This is the most compelling evidence against the reports that the Chinguetti mesosiderite is a small fragment of a mass 100 m long and 40 m high.