KREEP metasomatism directly evidenced by mantle relicts in a 4.1-Ga Chang’e 6 basalt

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Abstract
The lunar mantle remains enigmatical due to the absence of unequivocally identified mantle materials. The South Pole-Aitken (SPA) basin, as the largest impact structure on the Moon, holds great potential to have excavated lunar mantle materials. The samples returned from the SPA basin by Chang’e 6 (CE6) mission, thus, provide an unprecedented opportunity to search for mantle materials. We report lunar mantle olivine and orthopyroxene hosted in a 4.1-Ga high-alumina CE6 basalt. These mantle crystals with reaction rims are highly magnesian and are in equilibrium with the bulk melt, indicating that they are residues of mantle partial melting occurring at depths of 170–190 km below the Moon's surface. The extreme enrichment of rare earth elements (REEs) in the residual orthopyroxene coupled with a high μ value in the mantle source provides compelling evidence that this farside mantle was metasomatized by potassium, REEs, and phosphorus (KREEP) melt. These mantle mineral residues thus directly demonstrate the presence of KREEP melt on the Moon’s farside. Moreover, the KREEP melt must have migrated downward into the deep mantle before partial melting. This finding suggests that KREEP was likely distributed globally, supporting the existence of a magma ocean on the early Moon.