Elevated ilmenite in lunar nearside cumulates revealed by extremely high-Ti glass beads augmented large-scale volcanism

The cause of mare asymmetry between the lunar nearside and farside remains a long-lasting conundrum. Due to scarcity of directly sourced deep samples, the formation mechanism of this phenomenon has not been well constrained. Here we characterized a class of rare impact glass beads with extremely high TiO2 (16–25 wt%) and FeO (25–35 wt%) in the Chang’e-5 regolith. Petrological modeling indicates that the precursor rock of these glass beads cannot be formed through normal magmatic evolution, but rather has a deeper mantle origin. Phase diagram calculations show that the precursor rock has a typical mineral assemblage similar to ilmenite-bearing cumulates. Based on different residual anorthite proportions, the restored Lunar Magma Ocean (LMO) original crystalline mineral assemblage is predicted to contain ~15–20 vol% ilmenite, significantly higher than the average predicted levels. Therefore, the nearside ilmenite-bearing cumulates may have higher ilmenite abundance than farside, causing increased partial melting and more abundant volcanism. Lunar mare asymmetry could be explained by a higher nearside ilmenite content which would cause increased partial melting and volcanism relative to the farside, according to observations and petrological modelling of high-Ti glass impact beads in Chang’e-5 samples.