Shock melt in the Cold Bokkeveld CM2 carbonaceous chondrite and the response of C-complex asteroids to hypervelocity impacts

Abstract

Many of the CM carbonaceous chondrites are regolith breccias and so should have abundant evidence for collisional processing. The constituent clasts of these fragmental rocks frequently display compactional petrofabrics; yet, olivine microstructures show that most CMs are unshocked. To better understand the reasons for this contradiction, we have sought other evidence for hypervelocity impact processing of CM chondrites using the Cold Bokkeveld meteorite. We find that this regolith breccia contains rare particles of vesicular shock melt that are close in chemical composition to bulk CM chondrite. Transmission electron microscopy of a melt bead shows that it is composed of silicate glass with inclusions of pentlandite, pyrrhotite, and wüstite. Characterization of shards of another bead by atom probe tomography reveals nanoscale clusters of sulfur that represent sulfide inclusions arrested at an early stage of growth. These glass particles are mineralogically comparable to micrometeoroid impact melt described from the Cb-type asteroid Ryugu and melt that has been experimentally produced by pulsed laser irradiation of CM targets. The glass could have formed by in situ shock-melting, but petrographic evidence is more consistent with an origin as ballistic ejecta from a distal impact. The scarcity of melt in this meteorite, and CM chondrites more broadly, is consistent with the explosive fragmentation of hydrous asteroids following energetic collisions. Cold Bokkeveld's parent body is likely to be a second-generation asteroid that was constructed from the debris of one or more earlier bodies, and only a small proportion of the reaccreted material had been highly shocked and melted.