Meteoritic evidence for a Ceres-sized water-rich carbonaceous chondrite parent asteroid

Carbonaceous chondrite meteorites record the earliest stages of Solar System geological activities and provide insight into their parent bodies’ histories. Some carbonaceous chondrites are volumetrically dominated by hydrated minerals, providing evidence for low-temperature, low-pressure aqueous alteration¹. Others are dominated by anhydrous minerals and textures that indicate high-temperature metamorphism in the absence of aqueous fluids¹. Evidence of hydrous metamorphism at intermediate pressures and temperatures in carbonaceous chondrite parent bodies has been virtually absent. Here we show that an ungrouped, aqueously altered carbonaceous chondrite fragment (numbered 202) from the Almahata Sitta (AhS) meteorite contains an assemblage of minerals, including amphibole, that reflect fluid-assisted metamorphism at intermediate temperatures and pressures on the parent asteroid. Amphiboles are rare in carbonaceous chondrites, having only been identified previously as a trace component in Allende (CV3_oxA) chondrules². Formation of these minerals would require prolonged metamorphism in a large (about 640–1,800 kilometres in diameter) asteroid that is as yet unknown. Because Allende and AhS 202 represent different asteroidal parent bodies, intermediate conditions may have been more widespread in the early Solar System than is recognized from known carbonaceous chondrite meteorites, which are likely to represent a biased sampling.