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Chromium isotopic homogeneity between the Moon, the Earth, and enstatite chondrites

Abstract : Among the elements exhibiting non-mass dependent isotopic variations in meteorites, chromium (Cr) has been central in arguing for an isotopic homogeneity between the Earth and the Moon, thus questioning physical models of Moon formation. However, the Cr isotopic composition of the Moon relies on two samples only, which define an average value that is slightly different from the terrestrial standard. Here, by determining the Cr isotopic composition of 17 lunar, 9 terrestrial and 5 enstatite chondrite samples, we reassess the isotopic similarity between these different planetary bodies, and provide the first robust estimate for the Moon. In average, terrestrial and enstatite samples show similar ε 54 Cr. On the other hand, lunar samples show variables excesses of 53 Cr and 54 Cr compared to terrestrial and enstatite chondrites samples with correlated ε 53 Cr and ε 54 Cr (per 10,000 deviation of the 53 Cr/ 52 Cr and 54 Cr/ 52 Cr ratios normalized to the 50 Cr/ 52 Cr ratio from the NIST SRM 3112a Cr standard). Unlike previous suggestions, we show for the first time that cosmic irradiation can 2 affect significantly the Cr isotopic composition of lunar materials. Moreover, we also suggest that rather than spallation reactions, neutron capture effects are the dominant process controlling the Cr isotope composition of lunar igneous rocks. This is supported by the correlation between ε 53 Cr and ε 54 Cr, and 150 Sm/ 152 Sm ratios. After correction of these effects, the average ε 54 Cr of the Moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an Earth-Moon-Enstatite chondrite system homogeneity. This is compatible with the most recent scenarios of Moon formation suggesting an efficient physical homogenization after a high-energy impact on a fast spinning Earth, and/or with an impactor originating from the same reservoir in the inner proto-planetary disk as the Earth and enstatite chondrites and having similar composition.
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Bérengère Mougel, Frédéric Moynier, Christa Göpel. Chromium isotopic homogeneity between the Moon, the Earth, and enstatite chondrites. Earth and Planetary Science Letters, Elsevier, 2018, 481, pp.1-8. ⟨10.1016/j.epsl.2017.10.018⟩. ⟨insu-02917177⟩

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