Nanophase Iron Particles Derived From Fayalitic Olivine Decomposition in Chang'E-5 Lunar Soil: Implications for Thermal Effects During Impacts

Surface-correlated nanophase iron particles (npFe(0)) alter the reflectance spectrum characteristics of airless bodies, thus making it an essential aspect of studying space weathering. Vapor deposition has been the only strongly proven npFe(0) formation mechanism owing to the long exposure time of Apollo samples, whereas other formation mechanisms remain questioned. Newly returned younger Chang'E-5 samples provide an opportunity to study the incipient formation mechanism of npFe(0). Here, we combined transmission electron microscopy and electron energy loss spectroscopy to characterize the microscopic features of Chang'E-5 olivine rims. The uppermost layer of these grains exhibits the simultaneous coexistence of npFe(0) with Si-rich material overlying an Mg-rich layer, as well as numerous irregular vesicles containing oxygen-rich (SiO and O-2) components embedded in the npFe(0). These microscopic features collectively suggest subsolidus olivine decomposition during (micro)impact-induced fragmentation or local heating processes, which may be the essential agent to alter the reflectance spectrum of airless bodies.

Automated summary

Surface-correlated nanophase iron particles play an essential role in studying space weathering. The only strongly proven formation mechanism for these particles is vapor deposition, while other mechanisms remain debated. The analysis of the Chang'E-5 olivine rims suggests that subsolidus olivine decomposition may be the key factor in altering the reflectance spectrum of airless bodies.