Long-term reduced lunar mantle revealed by Chang'e-5 basalt

The redox state of a planetary mantle affects its thermal evolution. The redox evolution of lunar mantle, however, remains unclear due to limited oxygen fugacity (fO₂) constraints from young lunar samples. Here, we report vanadium (V) oxybarometers on olivine and spinel conducted on 27 Chang'e-5 basalt fragments from 2.0 billion years ago. These fragments yield an average fO₂ of ΔIW -0.84 ± 0.65 (2σ), which closely aligns with the Apollo samples from 3.6-3.0 billion years ago. This temporal uniformity indicates the lunar mantle remained reduced. This observation reveals that the processes responsible for oxidizing mantles of Earth and Mars either did not occur or had negligible oxidizing effects on the Moon. The long-term reduced mantle may lead to a distinctive volatile degassing pathway for the Moon. It could also make the lunar mantle more difficult to melt, preventing internal heat dissipation and consequently resulting in a slow cooling rate.