Secondary impact-driven melting on the CBa parent body: Investigation of the effects of impact melt deformation on the composition and microstructure of metal in Gujba

The CB chondrites are a metal-rich group of carbonaceous chondrites that likely formed in an impact-melt vapour plume resulting from a collision between two planetesimals. Many samples, such as the CBa type meteorite Gujba, show evidence of secondary impact processes that occurred after formation, such as silicate-dominated impact melt veins. We investigated the texture, crystallographic microstructure, and chemical composition of Fe,Ni metal particles in Gujba to distinguish primary features from those caused by secondary impact melting. We developed a qualitative classification relating the morphology of primary metal particles in Gujba to the extent that they were deformed by secondary impact melt. Electron backscatter diffraction analyses showed that less deformed Fe,Ni metal particles preserve a prior crystallographic structure, partially overprinted by recrystallisation textures. Fe,Ni metal inside melt veins has been melted and recrystallised. Unmelted metal particles retain similar compositions to each other, but metal entrained in impact melt, and melted Fe,Ni metal differ slightly. Conditions on the CB parent body after secondary impact resulted in no significant equilibration between particles with primary compositions, likely as the effects of impact heating were relatively rapid. Metal-melt particles in larger systems from other planetary bodies may show more evidence of modification during impact melting.