Metamorphosed calcium‐aluminum‐rich inclusions in CK carbonaceous chondrites

CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CK s are described as calcium‐aluminum‐rich inclusion ( CAI )‐poor objects, the abundance of CAI s in the 18 CK 3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine‐grained CAI s recrystallized as irregular assemblages of plagioclase + Ca‐rich pyroxene ± olivine ± Ca‐poor pyroxene ± magnetite. Coarse‐grained CAI s display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca‐rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAI s investigated. The Al‐Mg isotopic systematics measured in fine‐ and coarse‐grained CAI s from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al‐Mg isotopic signatures obtained in the core of a coarse‐grained CAI ( CG 1 ‐ CAI ) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAI s in CK and CV chondrites confirms that CV s and CK s form a continuous metamorphic series from type 3 to 6.