South Pole–Aitken massive impact 4.25 billion years ago revealed by Chang'e-6 samples

Abstract

As the largest and oldest well-preserved impact structure on the Moon, the South Pole-Aitken (SPA) basin on the lunar farside is critical for understanding early solar system dynamics and lunar history, but accurately determining its age remains challenging. Crater-counting chronology and Apollo sample studies propose various SPA-forming ages, which require validation by <i>in</i> <i>situ</i> sampling of the SPA basin. Here, we present the petrology, geochemistry and chronology of norite clasts from the SPA basin that were returned by Chang'e-6. These norites have highly anorthite-rich, rare-earth element-poor plagioclase and magnesium-rich pyroxene, in contrast to Mg-suite norites that were returned from the lunar nearside. Abundant Fe-Ni metals with meteoritic Ni/Co ratios, depletion of volatile elements and variable grain sizes and cooling rates strongly indicate that the norites were crystallized from an impact melt sheet. Precise Pb-Pb ages of zirconium-bearing minerals in the norites yield two distinct impact events at 3.87 and 4.25 Ga. The former represents an impact-resetting event within the basin. The latter finding is most consistent with the age of the SPA impact, providing an initial 4.25-Ga anchor for the older end of the lunar crater chronology and refining the timeline for early lunar evolution.