New geochemical analyses of basalt clasts returned by China’s Chang’e-6 mission from the Moon’s far side suggest that the giant impact that formed the South Pole–Aitken (SPA) basin drove significant volatile loss deep within the Moon. Published in PNAS, the work provides the first high-precision isotopic evidence from Chang’e-6 samples that a basin-forming impact can modify the Moon’s interior.
Since its formation, the Moon has been shaped by countless asteroid impacts – but how the largest of these events altered its deep interior has remained uncertain. Chang’e-6 material from the SPA basin offered an opportunity to examine that question directly.
A team led by Tian Hengci at the Institute of Geology and Geophysics, Chinese Academy of Sciences, reported unusually heavy potassium (K) isotopic compositions in basalt clasts collected from the SPA basin. These signatures differ markedly from those measured in Apollo samples and lunar meteorites, which are widely taken to represent the composition of the lunar mantle.
Potassium is a moderately volatile element, meaning its isotopes are particularly sensitive to high-temperature processes such as evaporation during impacts. To quantify this effect, the researchers carried out high-precision K isotope measurements on four basalt clasts using sapphire collision-cell multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). The Chang’e-6 basalts yielded δ⁴¹K values consistently higher than those of Apollo mare basalts, with an average offset of roughly 0.16 percent.
The team systematically evaluated other mechanisms that could potentially modify potassium isotopes, including long-term cosmic-ray exposure, magmatic differentiation, and contamination by meteoritic material. Each was found to have only a minor influence, insufficient to explain the observed enrichment in heavy K isotopes. The most plausible explanation, the authors conclude, is potassium evaporation driven by the SPA basin-forming impact.
Numerical simulations supported this interpretation, indicating that the impact generated extreme temperatures capable of driving volatile loss from deep crustal and possibly mantle materials, while also inducing large-scale mantle convection. Such depletion could have contributed to the Moon’s long-recognized near–far side asymmetry in volcanic activity.
As the authors write, “our results thus provide robust evidence for significant impact-induced modification of the lunar mantle, and demonstrate that large-scale impacts may have played a key role in creating lunar asymmetry.”
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