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

Abstract

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.