Elemental and Sr isotopic compositions of plagioclase as an indicator of lunar source-rock type: Insights from Chang'e 5 plagioclase fragments

Plagioclase is the most abundant mineral in lunar crustal rocks, and its elemental and Sr isotopic compositions vary among different lunar surface rock types, implying that plagioclase fragments in lunar regolith can be used to trace source-rock types. In this study, we measured major- and trace-elemental and RbSr isotopic compositions of plagioclase fragments from lunar soil returned by the Chang'e 5 (CE5) mission. Correlation between Sr and An contents allows the CE5 plagioclase fragments to be divided into three groups: normal-Sr (group A), high-Sr (group B), and low-Sr (group C). The similarity of elemental and RbSr isotopic compositions between plagioclase in groups A and B and plagioclase from CE5 basalt clasts indicates that plagioclase from groups A and B originated from the comminution of local CE5 basalt. Only two of the eighty-two analyzed plagioclases (∼2.4%) are classified as group C and have Sr isotopes that are distinct from those of groups A and B plagioclases, indicating an exotic origin. One group C plagioclase has a high 87Sr/86Sr ratio (0.70242) and content of rare earth elements and might have been derived from the Sharp B or Aristarchus craters, which are enriched in the KREEP component. The other group C plagioclase has much lower 87Sr/86Sr (0.69908) and a moderately high content of An (92.3) indicating an Mg-suite source rock and possible derivation from the Pythagoras crater. This study highlights the applicability of using elemental and Sr isotopic compositions of plagioclase fragments to trace the origin of lunar regolith.