Detailed petrogenesis of the unsampled Oceanus Procellarum: The case of the Chang'e-5 mare basalts

Qi He, Yiheng Li, Ioannis Baziotis, Yuqi Qian, Long Xiao, Zaicong Wang, Wen Zhang, Biji Luo, Clive R. Neal, James M.D. Day, Fabin Pan, Zhenbing She, Xiang Wu, Zhaochu Hu, Keqing Zong, Lu Wang,

Icarus,Volume 383,2022,115082

ISSN 0019-1035

https://doi.org/10.1016/j.icarus.2022.115082

https://www.sciencedirect.com/science/article/pii/S0019103522001919

Abstract: Lunar mare basalts provide a probe to study the magmatic and thermal evolution of the Moon. The Chang'e-5 (CE-5) mission returned samples from a young and hitherto unsampled mare terrain, providing fresh opportunities to understand lunar volcanic history. A detailed petrologic survey was conducted in this study on basalt fragments and glasses from the returned CE-5 soil samples. Relatively large-sized (100–400 μm) basaltic fragments were hand-picked and examined for texture, mineral assemblage and mineral chemistries. Basaltic fragments exhibit dominantly subophitic textures and are phenocryst-free, with low to intermediate-Ti (2.1–5.5 wt%) and low Mg# (Mg/(Mg + Fe) × 100, 19–47, with an average whole-rock Mg# of 33) consistent with olivine-melt equilibrium calculation (Mg# = 34). A range of highly evolved basaltic materials have been identified, in which abundant fayalitic olivine, symplectitic intergrowths, and Si + K-rich mesostasis co-exist were found resulting from late-stage silicate liquid immiscibility. Basaltic glass compositions largely overlap with basaltic fragment compositions suggesting they are locally derived. The CE-5 basalts have a relatively limited range of eruption temperatures of 1150–1230 °C. Based on their petrographic and geochemical characteristics, some CE-5 mare basalts are highly evolved and some of the resultant basaltic melt products underwent high crystallization. Thermodynamic modeling using MELTS suggests highly evolved basaltic magma was produced by a low-pressure and simple fractional crystallization under reduced conditions. This may have occurred at the surface in the inflated Em4/P58 flow with a thickness of ~50 m. The low degree of partial melting mantle sourceof the parental melts is the late-stage lunar magma ocean cumulates in a similar manner to some evolved low-Ti mare basalt meteorites, although the source of CE-5 basalts may have been slightly more Ti-rich.