Petrogenesis of Chang'E‐5 young mare low‐Ti basalts

Abstract

The regolith samples returned by the Chang'E‐5 mission (CE‐5) contain the youngest radiometrically dated mare basaltic clasts, which provide an opportunity to elucidate the magmatic activities on the Moon during the late Eratosthenian. In this study, detailed petrographic observations and comprehensive geochemical analyses were performed on the CE‐5 basaltic clasts. The major element concentrations in individual plagioclase grain of the CE‐5 basalts may vary slightly from core to rim, whereas pyroxene has clear chemical zonation. The crystallization sequence of the CE‐5 mare basalts was determined using petrographic and geochemical relations in the basaltic clasts. In addition, both fractional crystallization (FC) and assimilation and fractional crystallization models were applied to simulate the chemical evolution of melt equilibrated with plagioclase in CE‐5 basalts. Our results reveal that the melt had a TiO2 content of ~3 wt% and an Mg# of ~45 at the onset of plagioclase crystallization, suggesting a low‐Ti parental melt of the CE‐5 basalts. The relatively high FeO content (>14.5 wt%) in melt equilibrated with plagioclase could have resulted in extensive crystallization of ilmenite, unlike in Apollo low‐Ti basalts. Furthermore, our calculations showed that the geochemical evolution of CE‐5 basaltic melt could not have occurred in a closed system. On the contrary, the CE‐5 basalts could have assimilated mineral, rock, and glass fragments that have higher concentrations of KREEP elements (potassium, rare earth elements, and phosphorus) in the regolith during magma flow on the Moon's surface. The presence of the KREEP signature in the CE‐5 basalts is consistent with literature remote sensing data obtained from the CE‐5 landing site. These KREEP‐bearing fragments could originate from KREEP basaltic melts that may have been emplaced at the landing site earlier than the CE‐5 basalts.