Noble gas measurements in the L/LL5 chondrite Knyahinya |
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| Authors: | Bernard LAVIELLE Seraphin TO ERIC GILABERT |
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| Institution: | Bernard LAVIELLE,Seraphin TOÉ,ERIC GILABERT |
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| Abstract: | Abstract— The L/LL5 chondrite Knyahinya had an approximately spherical shape, and as it experienced a single stage exposure history, it represents a very interesting object to study depth profiles of cosmic-ray-produced nuclide concentrations. Such data are required to improve and to validate model calculations of production rates. We report Ne, Ar, Kr and Xe isotopic abundances in five bulk samples. The adopted procedure of noble gas extraction included two pyrolysis steps at 450 °C and 650 °C, respectively, followed by a combustion step in pure O2 at 650 °C before melting the sample. This procedure allows for the separation of a significant fraction of the trapped Kr and Xe, leading to an enrichment of the cosmic-ray produced component, which is released in the melting step. The isotopic composition of the trapped Xe component measured in the combustion step is found to be identical with the OC-Xe composition (Lavielle and Marti, 1992) and supports the suggestion that ordinary chondrites formed in a homogeneous trapped noble gas reservoir. Cosmic-ray produced Kr and Xe components and depth profiles were measured, including for the first time a 81Kr profile. The calculated exposure age of 39.5 ± 1.0 Ma, based on the 81Kr-Kr method, is found to be in excellent agreement with previous determinations. The concentrations of trapped and fissiogenic noble gas components are clearly lower than those generally observed in type 5 ordinary chondrites and may suggest diffusion losses before a meter-sized object was exposed to the cosmic radiation. |
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