  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. However, the 54Cr isotope
composition of the Moon relies on 2 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 re-assess 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 eps_54Cr. On the other
hand, lunar samples show variables excesses of 53Cr and 54Cr compared to
terrestrial and enstatite chondrites samples with correlated eps_53Cr and
eps_54Cr (per 10,000 deviation of the 53Cr/52Cr and 54Cr/52Cr ratios normalized
to the 50Cr/52Cr ratio from Cr standard). Unlike previous suggestions, we show
for the first time that cosmic irradiation can 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 eps_53Cr and eps_54Cr, and 150Sm/152Sm
ratios. After correction of these effects, the average eps_54Cr 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.
