Cosmogenic isotopic evidence for multi-stage accumulation history of the Chang’e 5 regolith

The Chang’e-5 (CE-5) mission returned the youngest known lunar mare materials (~2.0 Ga), providing critical constraints on late-stage lunar volcanism and enabling recalibration of lunar crater chronology. However, the returned samples from regolith and lithic fragments recorded the combined effects of impact gardening, cosmic-ray exposure, and multi-stage ejecta deposition. Thus, we investigate the stratigraphy, exposure history, and material sources of the CE-5 regolith core using cosmogenic isotopic signatures of Sm, Hf, and Cr. These isotope systems respond to distinct cosmic-ray processes, including neutron-capture and spallation, and thus provide complementary constraints on burial depth, regolith reworking, and exposure duration. Measurements of ε¹⁴⁹Sm–ε¹⁵⁰Sm, ε¹⁷⁸Hf–ε¹⁸⁰Hf, and ε⁵³Cr–ε⁵⁴Cr reveal that mass-independent Sm and Hf isotopic variations are dominated by neutron capture processes, whereas mass-independent Cr isotopic variations primarily reflect spallation reactions from Fe. The regolith core does not exhibit monotonic depth-dependent trends expected for static irradiation, indicating substantial post-depositional reworking. The rapid decrease of cosmogenic ⁵³Cr in the subsurface of the CE-5 drill core, together with elevated cosmogenic ¹⁵⁰Sm and ¹⁷⁸Hf in both surface and subsurface samples, indicates that the CE-5 regolith comprises two distinct intervals: an surface layer dominated by reworked, previously irradiated ejecta derived from older regolith, and a subsurface layer largely sourced from relatively fresh Eratosthenian mare basalts excavated by an ancient impact. Numerical modeling incorporating cosmic-ray production rates and regolith gardening further constrains the formation of the subsurface regolith to an impact event at ~250 Ma. Our results reveal a complex, multi-stage accumulation history at the CE-5 landing site and underscore the need to integrate multiple cosmogenic isotope systems to constrain the material sources of the CE-5 basalts.