Impact-induced submicroscopic metallic iron particles identified in “dusty olivine” from brecciated lunar meteorite Bechar 012
Yutong Ma, Zhuang Guo, Aicheng Zhang, Jingjing Niu, Shan Qin
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 27 January 2026
“Nanophase iron particles are ubiquitous in lunar soils and are largely attributed to space weathering; however, no nanophase iron particles formed in lunar crystalline minerals via impact events have been reported. Their growth, migration, spatial distribution, and interactions with host minerals under impact remain poorly understood. Here, we report the discovery of impact-induced dusty olivine clasts (containing nanoscale to submicron iron metal particles) in the lunar breccia meteorite Bechar 012. These clasts encapsulate a comprehensive record of iron particle formation and distribution within their crystalline host. Their well-preserved state provides a clear snapshot of the microscopic mineral processes often obscured in more heavily processed soils or breccias, making Bechar 012 an ideal natural sample for study. Crystallographic orientation analysis suggests that these dusty olivine grains undergo plastic deformation, with iron metal particles concentrated within the deformed regions, indicating a correlation between deformation microstructures and iron metal particle formation. We propose a three-step model for the formation of dusty olivine and the iron metal particles therein: (1) impact-induced plastic deformation of olivine; (2) sub-solidus olivine decomposition (Fe2SiO4 = 2Fe + SiO + 3/2O2) within partially amorphous zones, leading to the nucleation of nanosized iron metal particles, with SiO and O2 diffusing away through disordered pathways (e.g., partially amorphous zones, dislocations and pores) in the deformed olivine; and (3) capture of particles by migrating dislocations, followed by aggregation and oriented attachment (OA)-driven growth along the olivine lattice at dislocations and subgrain boundaries, resulting in the formation of submicron-sized iron metal particles. These processes indicate an equilibrium shock pressure below 16 GPa, with temperatures between 1000 °C and 1650 °C. These results confirm the pivotal role of impact-induced olivine deformation in facilitating the formation, migration, and growth of iron metal particles and highlight the significance of OA in their coarsening. The discovery of impact-induced iron metal particles in the lunar meteorite indicates that these particles can be broadly formed within crystalline minerals, rather than being limited to the amorphous rims of lunar regolith minerals and glassy impactites, while also offering a potential explanation for the lunar magnetic anomalies.”
