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Introduction:Pioneering studies of Apollo lunar soils have revealed a geochemical dichotomy in the lunar rego-lith reflecting the dominance of mare and highland lithologies, withvariableadmixtures of Procellarum KREEP Ter-rane,and differences in their exposure to galactic cosmic rays [1]. Cadmium, with a half-mass condensation temper-ature of 652K [2], exhibits a stronger depletion in the bulk Moon and larger stable isotope fractionation inlunar samples [3,4,5] compared to the Earth. In contrast to other volatile elements(e.g., K, Zn, Ga), cadmium has the unique potential to trace thermal neutron capture (n.c.) effects which are reflected by an enrichment in 114Cd due to neutron absorption by 113Cd (cross-section: 2.104 barns). Thus, a Cd isotope study of lunar soils provides a means to understand the origin of volatile elements depletionand processes of stable isotope fractionation at the lunar surface as well as gather information on the formation, evolution and composition of the Moon. Samples and Methods: Lunar soils from Apollo 12 (n=5) and Apollo 16 (n=7) missions were selected to cover a large range of maturity and include the most immature trench samples 12032 and 12033 which have been reported to contain abundant K-and P-rich glass [6].The samples were acid-digested at the University of Colognewhile chemical separation of Cd and isotope measurements, performed by TIMS (ThermoFisher Triton) using a 106Cd-108Cd double spike for instrumental mass bias correction [7], were done at the Max Planck Institute for Chemistry in Mainz. The isotopic compositions are expressed as e112/110Cd relative to NIST SRM3108 Cd [8]. The amounts of Cd analyzed varied from ~2to 20 ng (blanksof 8to 20 pg were negligible)–with an analytical uncertainty on e112/110Cd ranging from ~±1 to ±0.2, respectively. Since measurement of 113Cd by TIMS is compromised by odd-even isotope effects[7,9], the n.c.effect was derived from the 114Cd/112Cd ratio,normalizedto 110Cd/112Cd to remove the natural stable isotope fractionationeffects.Results and discussion: The Cd concentrations are on average lower in Apollo 12 than Apollo 16 soilsand are associated to large Cd stable isotope variations. The e112/110Cd (~0 to +110) of Apollo 12 soils co-varywith the maturity index, whereas Apollo 16 soils show a more restricted range (+60 to +90) with no clear relationship with soil maturity. Likewise, n.c.effects are positively correlated with the maturity index for Apollo 12 soils. In contrast,no clear corre-lation is observed for Apollo 16 soils which are relatively uniform, except for two soils collected at Stone mountains which exhibit the highest n.c.effects. There is a clear Cd isotope dichotomy in e112/110Cd-Cd space where Apollo 12 and Apollo 16 soils define two distinct negative correlations. These can be interpreted in terms of either Rayleigh fractionation due to evaporative Cd lossimparted by micrometeorite impacts, or binary mixing involving distinct components. Given that the isotope fractionation factor inferred from the slopes of the two regressions lines falls below the theoretical kinetic isotope fractionation factor (a112/110Cd=0.9910), the two correlations are best explained by mixing involving at least threecomponents –mare basalt, highland feldspatic crust and Procellarum KREEP.Im-portantly, the Apollo 12 correlation intersects the Bulk Silicate Earth value corresponding also to the isotope compo-sition of the most immatureand KREEP-richsoil 12033.The remarkable positive correlation between e112/110Cdand the n.c.effect in Apollo 12 soils constrainsthe isotopic compositions of the mare and KREEP componentsto be, respectively, isotopically heavy and light.A Cd isotopic composition ofKREEPcomparable tothe bulk Earthvalue may, in turn, suggestthat the Moon and Earth share a similar Cd isotope signature which bears important implications on the lunar and terrestrial volatile elementsdepletion and models of formationof the Moon. In particular, a Cd isotope similarity in the Earth-Moon system would imply Moon formation from a well-mixed vapor disk without significant Cd isotope fractionation imparted by volatile loss during and following the giant impact.Acknowledgments: We thank the NASA teamfor providing the samples and the Deutsche Forschungsgemein-schaft for funding through the SPP 1833.References:[1] Lucey, P. et al. (2006) Rev. Mineral. Geochemi.60:83-219. [2] Lodders,K.(2003) Astrophys. J.591:1220–1247. [3] Sands, D.G. et al. (2001)Earth Planet. Sci. Lett.186:335–346. [4] Schediwy, S. et al. (2006) Earth Planet. Sci. Lett.243: 326–335 [5] Wombacher, F. et al. (2008) Geochim. Cosmochim. Acta72:646–667.[6] Meyer, C. et al. (1971) Proceedings of the Lunar Conf. 1:393-411. [7] Schmitt, A.D et al. (2009) J. Anal. At. Spectrom.24:1079–1088. [8] Abouchami, W. et al. (2013) Geostand.Geoanal.Res. 37:5–17. [9] Göpel, C. and Manhes,G. (2007)Geochim. Cosmochim. Acta71:A618.6146.pdf85thAnnualMeetingofTheMeteoriticalSociety2022(LPIContrib.No.2695)
