Discovery of a highly shocked alkali suite clast in the Chang'e-5 lunar soils

Lunar soils contain various types of components and thus are crucial for unraveling the geological and impact histories of the Moon. Little is known about the amount and source regions of the non-mare components preserved in the Chang'e-5 (CE5) lunar soils. A study of systematic identification and characterization of the non-mare components in the CE5 lunar soils was carried out in this work. A non-mare clast, displaying contrast to mare basalts, was firstly identified in the scooped CE5 lunar soils using nondestructive micro-CT investigating. Further detailed petrographic observations and analyses of mineral chemistry reveal that this non-mare clast is a highlands alkali suite (HAS). This HAS clast is mainly composed of plagioclase (46.5), pigeonite (30.9), K-Si-rich mesostasis (17.0), K-felspar (4.12), and silica (0.87) in area%, with minor ilmenite, merrillite, apatite, baddeleyite, and troilite. This clast contains high-pressure polymorph of silica, seifertite and stishovite, and is characterized by high K₂O (1.88 wt%), and low TiO₂ (0.63 wt%) and low FeO (8.36 wt%) contents. Pb-Pb dating of baddeleyite yields a date of 3913 ± 7 Ma, probably representing a plutonic event or an impact event happened on the Moon. Another ejection event is required to transport the alkali suite to the landing site of the CE5 mission postdated the emplacement of the CE5 mare basalt.