Yamato 793321 CM chondrite: Dehydrated regolith material of a hydrous asteroid

Based on the reflectance spectra, surface materials of many C, G, B, and F-type asteroids are altered by heating and consist of low-crystalline dehydrated phyllosilicates, similar to heated hydrous carbonaceous chondrites. However, meteorites of this kind studied so far were not samples from the asteroid regolith, but from the asteroid interior. They lack both of two properties characteristic for regolith materials: lithologies with impact-induced brecciation and noble gases from implanted solar winds. Thus no meteorite samples have ever been identified as a real sample of C, G, B, and F-type asteroid regolith. Here I report the discovery of a meteorite having mineralogical and isotopic signatures expected for regolith samples of these asteroids through the investigation of seven CM carbonaceous chondrites. The meteorite is the Yamato (Y-)793321 CM carbonaceous chondrite. It exhibits a variety of petrologic features that are characteristic of active regolith material: chondrules are crushed, matrix is comminuted, and coarse silicate crystals show deformation and characteristic planner fractures due to repeated impacts. Synchrotron X-ray diffraction analysis showed that fine-grained matrix is dominated by amorphous silicates with minor amounts of olivine and magnetite, which was produced by dehydration of preexisting hydrous phases probably serpentine during impact-induced heating at 500 °C. In addition, isotopic analysis revealed that the meteorite is enriched in implanted solar noble gases, suggesting that it was once located at surface layers of a parent asteroid. All results, taken together, demonstrate that Y-793321 is a regolith breccia of a hydrous asteroid, which underwent impacts, heating, dehydration, and solar-wind implantation at the surface layers of the asteroid. The reflectance spectrum of this meteorite matches well with those of asteroids C, G, B, and F, confirming that it was derived from these asteroid surfaces.