Metallic stable isotopic insights into the magmatic evolution of Chang’e-5 samples

The Chang’e-5 (CE-5) mission returned 2.0 Ga old mare basalts from Oceanus Procellarum, providing critical constraints on late-stage lunar volcanism in the Procellarum KREEP Terrane. However, the genesis mechanism of this relatively late volcanism remain highly debated. Metallic stable isotope systems are sensitive to lunar magma ocean (LMO) solidification, magma differentiation and degassing. Thus, we integrate multiple metal isotope systems, including Fe, Mg, Si, Sr, Cr, Ni, Cu, and Zn, to investigate the nature and evolution of the mantle source feeding young lunar volcanism. The CE-5 basalts exhibit significantly lower Mg# (approximately 35 to 14.5) than Apollo basalts and most lunar meteorites, reflecting extensive fractional crystallization. Correspondingly, isotopic fractionation has been observed in the Cr and Fe isotope systems, with the largest effect recorded in the redox-sensitive element Cr (δ53Cr variations of up to ~0.4 ‰ between the least and most evolved basalts). In contrast, isotope systems primarily influenced by olivine crystallization (Mg, Ni), melt polymerization (Si), or plagioclase fractionation (Sr) display little to no isotopic variation, indicating limited sensitivity to late-stage magma evolution. Combined metal isotope constraints indicate that the CE-5 basalts originated from a mantle source isotopically similar to Apollo low-Ti basalts, but their low Mg# and near-source isotopic compositions on the least evolved basalts require lower degrees of partial melting of an “Apollo-like” low-Ti mantle, except for a deeper depth. Incompatible Moderately Volatile Elements (MVEs) exhibit relatively high abundances with isotopic compositions intermediate to, or slightly lighter than, those of typical lunar basalts, indicating limited volatile loss. Together, these observations indicate that young volcanism in Oceanus Procellarum originated from a mantle reservoir that preserved incompatible elements, particularly MVEs, suggesting that the source experienced little prior melting or melt extraction.