Metamorphosed calcium-aluminum-rich inclusions in CK carbonaceous chondrites
Résumé
CK chondrites are the only group of carbonaceous chondrites with petrologic
types ranging from 3 to 6. Although CKs are described as calcium-aluminum-rich inclusion
(CAI)-poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero
to approximately 16.4%. During thermal metamorphism, some of the fine-grained CAIs
recrystallized as irregular assemblages of plagioclase + Ca-rich pyroxene ! olivine ! Ca-poor pyroxene ! magnetite. Coarse-grained CAIs 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 CAIs investigated. The Al-Mg isotopic systematics measured in fine- and coarsegrained
CAIs 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
(CG1-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 CAIs
in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic
series from type 3 to 6.