We present Cd isotope data on Apollo 12 (A12) mare soils and Apollo 16 (A16) highland soils to understand the volatile element distribution and processes of mass-dependent stable isotope fractionation at the lunar surface. Further, thermal neutron capture (n.c.) effects, reflected by an enrichment in 114Cd due to neutron absorption by 113Cd, record the history of near-surface exposure to galactic cosmic rays.
Our results show that Cd concentrations are on average lower in A12 than A16 soils. Covariations between ε112/110Cd and the maturity index are observed for A12 soils but not for A16 soils. Likewise, the positive covariation between n.c. effects and maturity index of A12 soils is lacking for A16 soils. In ε112/110Cd-Cd space, A12 and A16 soils define distinct negative correlations indicative of a Cd isotope dichotomy. These correlations reflect either Rayleigh fractionation due to evaporative Cd loss or binary mixing between mare basalt, highland feldspathic crust and Procellarum KREEP components. The Apollo 12 correlation intersects the BSE value corresponding also to the composition of the immature and KREEP-rich soil 12033. The ε112/110Cd-n.c. effect correlation in A12 soils constrains further the compositions of the KREEP and mare components to be, respectively, isotopically light and heavy. A corollary of a KREEP composition comparable to the bulk Earth value is that the Moon and Earth share a similar Cd isotope signature. This, in turn, would imply Moon formation from a well-mixed vapor disk without significant isotope fractionation imparted by volatile loss during and following the giant impact.