Cosmogenic effects on Cu isotopes in IVB iron meteorites

We measured Cu isotope compositions of 12 out of the 14 known IVB iron meteorites. Our results show that IVB iron meteorites display a very large range of delta Cu-65 values (-5.84% < delta Cu-65 < -0.24 parts per thousand; defined as per mil deviation of the Cu-65/Cu-63 ratio from the NIST-976 standard). These Cu isotopic data display clear correlations with W, Pt, and Os isotope ratios, which are very sensitive to secondary neutron capture due to galactic cosmic ray (GCR) irradiation. This demonstrates that delta Cu-65 in IVB irons is majorly modified by neutron capture by the reaction Ni-62(n,gamma) Ni-63 followed by beta decay to 63 Cu. Using correlations with Pt and Os neutron dosimeters, we calculated a pre-exposure delta Cu-65 of -0.3 +/- 0.8% (95% conf.) of IVB irons that agrees well with the Cu isotopic compositions of other iron meteorite groups and falls within the range of chondrites. This shows that the volatile depletion of the IVB parent body is not due to evaporation that should have enriched IVB irons in the heavy Cu isotopes. (C) 2016 Elsevier Ltd. All rights reserved.