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

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 source of 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.