Isotopic and compositional constraints on the source of basalt collected from the lunar farside.

Volcanism on the Moon has produced surface basalt deposits, which record lunar interior processes. The Chang'e-6 mission retrieved samples from the South Pole–Aitken basin (SPA) on the Moon's far side. We analyzed basalt fragments collected by Chang'e-6 and found that their composition resembles that of low-titanium basalts previously sampled by the Apollo missions. Uranium-lead (U-Pb) dating indicated a basalt age of 2823.0 ± 5.9 million years and a source 238U/204Pb ratio (μ-value) of 480 ± 56. This high μ-value indicates that the source contained materials that formed during the late stages of lunar magma ocean (LMO) crystallization. The results are consistent with the LMO model, but the Chang'e-6 basalt deviates from the trend in Apollo mission samples of younger basalts having higher μ-values. We suggest that this indicates post-LMO modification of the mantle by the SPA impact. Editor's summary: Orbital observations have shown that parts of the far side of the Moon are covered with volcanic basalt. The Chang'e-6 spacecraft landed in one of those regions, collected samples of the surface, and returned them to Earth. Che et al. analyzed the basalt fragments retrieved by Chang'e-6, using radiometric dating to show that it formed 2.8 billion years ago. The composition of the basalt constrains the properties of the magma source of the volcanism. The authors found both similarities to and differences from samples taken on the near side of the Moon and discuss the implications for geochemical models. —Keith T. Smith [ABSTRACT FROM AUTHOR]