Ancient age of the South Pole-Aitken basin evidenced by the oldest returned lunar basalt

The absolute age of the Moon’s largest impact basin, the South Pole-Aitken (SPA) basin, remains unconfidently constrained due to the absence of datable materials directly linked to its formation. Basaltic magma can erupt on the Moon’s surface only after an impact event has sufficiently thinned the thick lunar crust. Therefore, identifying the oldest basalts in the SPA basin could provide key constraints on the its formation age. We report the oldest basalt yet returned, dated at 4.341±0.003 Ga, from the SPA basin sampled by the Chang’e 6 mission. Eruption of this high-alumina basaltic magma requires an extremely thin crust, which must have occurred within an impact basin exceeding 800 km in diameter. Only the SPA basin satisfies both crater scale and basaltic eruption age. Consequently, this basalt age yields a tight lower age limit for the SPA basin. Accounting for impact melt solidification timescale, the SPA basin formed at 4.37±0.03 Ga. Furthermore, trace element and Pb isotopic compositions of this basalt suggest a relatively fertile mantle source barely melted by the SPA-forming impact. This fertile source indicates that lunar interior may have remained partially molten before impact, thereby preventing it from extensive impact-induced melting, a mechanism supported by numerical simulations. Our results suggest that the SPA-forming impact may have limited effects on mantle structure and composition in the Moon.