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

We measured the concentrations of the cosmogenic radionuclides ¹⁴C (half-life = 5.73 x 10³ years) in the bulk and of ¹⁰Be (1.5 x 10⁶ years), ²⁶Al (7.05 x 10⁵ years), ³⁶C1 (3.01 x 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 ²⁶AI⁻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.