Cosmogenic effects on chromium isotopes in meteorites

Liu, Jia; Qin, Liping; Xia, Jiuxing; Carlson, Richard W.; Leya, Ingo; Dauphas, Nicolas; He, Yongsheng (2019). Cosmogenic effects on chromium isotopes in meteorites. Geochimica et cosmochimica acta, 251, pp. 73-86. Elsevier Science 10.1016/j.gca.2019.01.032

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The ⁵³Mn-⁵³Cr short-lived radionuclide decay system is a powerful tool to investigate the timescales of early solar systemprocesses. A complication arises, however, from the fact that spallation and thermal/epithermal neutron capture processesinduced by cosmic rays can significantly alter ⁵³Cr/⁵²Cr ratios in solar system objects that have long exposure ages and high Fe/Cr ratios. Quantifying these cosmogenic effects helps constrain the cosmic ray exposure history of extraterrestrial samples.The isotopic shifts produced by cosmic ray irradiation also need to be corrected before the Cr isotope systematics can be usedas a dating tool and as a tracer of nucleosynthetic provenance. To investigate the impact of cosmogenic production on Cr, theCr isotopic compositions of 25 samples from 16 iron meteorites belonging to nine different chemical groups were measured.The measurements show that exposure to cosmic rays can cause large coupled excesses in ɛ⁵³Cr (-0.04 v 0.44 to +268.29 ± 0.14; 2SE)and ɛ⁵⁴Cr (+0.28 ± 0.72 to +1053.78 ± 0.72; 2SE) with a best fit line of ɛ⁵⁴Cr = (3.90 ± 0.03) x ɛ⁵³Cr. The magnitude of Cr isotopeproduction is controlled by various factors including the exposure age, the chemical composition (i.e., Cr concentrationand Ni/Fe ratio) and shielding conditions. Nevertheless, the correlation of ɛ⁵³Cr and ɛ⁵⁴Cr is independent of these factors,which provides an effective method to evaluate the cosmogenic contribution to ɛ⁵³Cr by monitoring the cosmogenic variations in ɛ⁵⁴ in meteoritic irons. The results are compared with modeling results that yield a slightly shallower slope of 3.6 ± 0.2. Modeling results for the olivine in stony meteorites yield a higher slope (~5.4). However, the previous estimated results forlunar samples (stony targets for comic ray irradiation) exhibit an observably shallower slope (~2.62). The reason for the dif-ferent slopes is that the production rates of different cosmogenic Cr isotopes in iron meteorites and lunar samples are in dif-ferent proportions. The differences may not be completely controlled by the higher thermal and epithermal neutron fluenciesin lunar samples than in iron meteorites, but instead may largely reflect different radiation geometry between the two. Morestudies are needed to solve this open question.

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