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Cosmic ray exposure and pre-atmospheric size of the Gebel Kamil iron meteorite

Ott, U.; Merchel, S.; Herrmann, S.; Pavetich, S.; Rugel, G.; Faestermann, T.; Fimiani, L.; Gomez-Guzman, J. M.; Hain, K.; Korschinek, G.; Ludwig, P.; D’Orazio, M.; Folco, L.


Cosmogenic He Ne and, Ar as well as the radionuclides 10Be, 26Al, 36Cl, 41Ca, 53Mn and 60Fe have been determined on samples from the Gebel Kamil ungrouped Ni-rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry (AMS), respectively. The meteorite is associated with the ~45 m diameter Kamil crater in southern Egypt. Samples originate from an individual large fragment (“Individual”) as well as shrapnel. Concentrations of all cosmogenic nuclides – stable and radioactive - are a factor ~4x lower in the shrapnel samples than in the Individual. Assuming negligible 36Cl decay during terrestrial residence (indicated by the young crater age <5000 a; Folco et al. 2011), data are consistent with a simple exposure history and a 36Cl-36Ar cosmic ray exposure age (CRE) of ~(366 +- 18) Ma (systematic errors not included). Both noble gases and radionuclides point to a pre-atmospheric radius >85 cm, i.e. a pre-atmospheric mass >20 tons, with a preferred radius of 115-120 cm (50-60 tons). The analyzed samples came from a depth of ~20 cm (Individual) and ~50-80 cm (shrapnel). The size of the Gebel Kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere (Folco et al. 2010, 2011).

Keywords: accelerator mass spectrometry; AMS; cosmogenic nuclides; cosmochemsitry; meteorite

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