A more reduced mantle beneath the lunar South Pole-Aitken basin.

The oxygen fugacity (fO(2)) of the lunar mantle is of pivotal significance in comprehending the formation and evolution of the Moon. However, the fO(2) of the lunar farside mantle remains unknown due to the lack of samples returned from the farside. Here, we determine the oxygen fugacity of 23 basaltic fragments from the Chang'e-6 (CE6) soil, the first farside sample collected from the South Pole-Aitken (SPA) basin. The spinel V oxybarometer and pyroxene Eu oxybarometer yield an average fO(2) of ΔIW -1.93 ± 0.58 (2σ), indicating a more reduced state compared to the nearside Apollo and Chang'e-5 (CE5) basalts, which have an average fO(2) of ΔIW -0.80 ± 0.64 (2σ). Such asymmetry in oxygen fugacity of the lunar mantle can be attributed to two processes: nearside mantle oxidation by a larger amount of Fe sinking into the core triggered by asymmetric crystallisation of the lunar magma ocean, and/or farside mantle reduction caused by S(2) and CO degassing during the SPA massive impact. Nevertheless, the reduced nature of the underlying mantle beneath the SPA basin reveals another aspect of lunar asymmetry.