A mantle source formed after the lunar magma ocean crystallisation for the 3000 Ma-old lunar basalts

There is no satisfactory model explaining the incompatible element-enriched chemical characteristics observed in few approximately 3000 Ma-old, low-titanium (low-Ti) lunar basalts. Using in situ Pb-Pb systematics by SIMS, we determined crystallisation ages and initial Pb isotope ratios of the meteorites North West Africa (NWA) 4734, LaPaz Icefield (LAP) 02224, LAP 02205, NWA 4734, Northeast Africa (NEA) 003 and a newly found meteorite, NWA 14178. Our investigation shows that these samples all belong to a unique chemical and age group, clearly distinct from any other magmatic episode and chemical group known from the Moon. The Pb isotope ratios suggest these basalts originated from a KREEP-free source. Monte Carlo simulations based on a three-stage model suggest a mantle source formed from a depleted mantle at 4260 Ma – that is, after the solidification of the LMO. This mantle source may have formed as a result of deep, crust-breaking impact.