Chondrules in CK carbonaceous chondrites and thermal history of the CV-CK parent body

CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. It is commonly reported than similar to 15 vol% of CK4-6 samples are composed of chondrules. The modal abundance of chondrules estimated here for 18 CK3-6 (including five CK3s) ranges from zero (totally recrystallized) to 50.5%. Although almost all chemically re-equilibrated with the host matrix, we recognized in CK3s and Tanezrouft (Tnz) 057 (CK4) up to 85% of chondrules as former type I chondrules. Mean diameters of chondrules range from 0.22 to 1.05mm for Karoonda (CK4) and Tnz 057 (CK4), respectively. Up to similar to 60% of chondrules in CK3-4 are surrounded by igneous rims (from similar to 20m to 2mm width). Zoned olivines were found in unequilibrated chondrules from DaG 431 (CK3-an), NWA 4724 (CK3.8), NWA 4423 (CK3.9), and Tnz 057 (CK4). We modeled Fe/Mg interdiffusion profiles measured in zoned olivines to evaluate the peak metamorphic temperatures and time scales of the CK parent body metamorphism, and proposed a two-stage diffusion process in order to account for the position of inflection points situated within chondrules. Time scales inferred from Fe/Mg interdiffusion in olivine from unequilibrated chondrules are on the order of tens to a hundred thousand years (from 50 to 70,000years for peak metamorphic temperatures of 1140 and 920K, respectively). These durations are longer than what is commonly accepted for shock metamorphism and shorter than what is required for nuclide decay. Using the concept of a continuous CV-CK metamorphic series, which is reinforced by this study, we estimated peak metamorphic temperatures <850K for CV, 850-920K for CK3, and 920-1140K for CK4-6 chondrites considering a duration of 70,000years.