Petrological, chemical, and chronological study of breccias in the Chang'e-5 soil

Abstract

We carried out a petrological, mineralogical, and geochemical study of fragmental and regolith breccia clasts separated from two Chang'e-5 (CE-5) soil samples, CE5C0000YJYX03501GP and CE5C0400, which provide an opportunity to investigate the compositional change of regolith at the landing site through time. Fragmental breccia CE-5-B3 contains a diverse range of basaltic clasts and basaltic mineral fragments, and some rare Mg-suite-like minerals. Regolith breccias CE-5-B006, CE-5-B007, CE-5-B010-08, CE-5-B010-09, CE-5-B011-07, and CE-5-B016-03 contain mare basaltic fragments, mare vitrophyric clasts, rare Mg-rich fragments possibly derived from the Mg-suite rocks, and impact-derived glass spherules. Pb-isotope data obtained for baddeleyite grains found both inside some of the basaltic clasts identified in breccia fragments and in the breccia matrices yield Pb/Pb dates similar to the 2 Ga crystallization age of the CE-5 basalt fragments, extracted directly from the soil sample. Seventy-four Pb isotope analyses of Ca-phosphate grains also indicate that the majority of these grains have Pb/Pb dates of 2 Ga, suggesting that they originate from the CE-5 basalts. In addition, a Pb–Pb isochron drawn through analyses of four Ca-phosphates in breccia CE5-B006 yielded an intercept corresponding to a date of 3871 ± 46 Ma, which is the best possible estimate of the formation age of these four grains. Electron probe microanalysis shows that the breccias contain components similar to CE-5 mare basalt fragments extracted directly from the soil sample, implying that the fragmental and regolith breccia fragments are mostly composed of material sourced from the underlying basalts. The general absence of impact melt breccia clasts, along with the general lack of Fe–Ni metal and absence of added meteoritic debris all suggest that the regolith at the CE-5 landing site is immature and dominated by material mixed together by small local impact cratering events. Trace element analyses show that the glass beads in the regolith breccias have a Th abundance of 4.06–5.28 μg g⁻¹. This is similar to the Th content of the regolith above the Em4 unit at the landing site as measured from orbit, as well as the estimated bulk Th content of CE-5 basalts, suggesting that Th of the local regolith is predominantly sourced from the underlying mare basalts, without significant Th addition from Th-rich exotic clasts sourced from evolved lunar lithologies.