Composition and origin of the Chang'E-5 regolith breccias

Abstract

Numerous studies of basalt clasts in regolith samples returned by the Chang'E-5 (CE-5) mission have provided constraints on the timing and nature of the youngest magmatism on the Moon. However, there have been far fewer studies of breccias, one of the main constituents of regolith. Here, we present a comprehensive investigation of the mineralogy, petrology, and U-Pb geochronology of two CE-5 regolith breccia samples, which are composed of lithic clasts, agglutinates, glass particles, and mineral fragments. In contrast to the high level of maturity of CE-5 regolith, the regolith breccias are immature, as judged by their low agglutinate (~11 vol%) and moderate to low matrix contents (~49 vol%). The CE-5 regolith breccias comprise mainly mare (~90 vol%) and non-mare (~10 vol%) materials. A low-Ti mare component of late Imbrian to early Eratosthenian age is identified, in addition to the predominant late Eratosthenian basalts in mare components. Non-mare components include Mg-suite norite, highland impact melt clasts, glass particles, and minor fragmented minerals. The glass particles in the CE-5 regolith breccias are compositionally variable and can be divided into five types, that is, basaltic (mare), KREEP-rich, feldspathic (highland), Si-poor, and Si-K-rich glasses. Among these glasses, most (65%) are compositionally exotic to the site. The diverse provenance of these “exotic” materials in the CE-5 breccias is consistent with the multiple ages of Zr-bearing phases at 3.97–3.92 Ga, ~3.2 Ga, 2.93–2.40 Ga, and ~2.0 Ga, in which early Eratosthenian ages are reported for the first time from returned lunar samples. The contrast in the level of maturity and in glass composition between CE-5 regolith and regolith breccias can be reconciled if CE-5 regolith breccias represent an ancient soil and were excavated from a buried stratigraphic sequence by later impacts. The duration of exposure of this old soil was short (<250 Myr), and its maturation was interrupted by late Eratosthenian basaltic magmatism.