METAMORPHISM OF CK CARBONACEOUS CHONDRITES: SO, WHAT’S YOUR PETROLOGIC TYPE?

The CK chondrites are a group of highly-oxidized carbonaceous chondrites characterized by the presence of magnetite, Ni-bearing sulfides, and NiO-rich olivine. The CK chondrites are also the only carbonaceous chondrite group to exhibit the full range of thermal metamorphism (from petrologic type 3 to type 6). Though the majority of CK chondrites are equilibrated (types 4-6), thirty unequilibrated (type 3) CK chondrites have been discovered since 1990. Unequilibrated chondrites are particularly sensitive to the effects of metamorphism, and are thus divided into petrologic subtypes 3.0 – 3.9. Unequilibrated CK chondrites are thought to be metamorphosed to conditions associated with petrologic subtype 3.7 or higher. However, no criteria for establishing petrologic subtypes in the CK chondrites have been established. In this study, we examine the mineralogies and textures of seven unequilibrated and two equilibrated (type 4) CK chondrites to better characterize the extent of metamorphism experienced by the unequilibrated CK chondrites.

Though there are many geochemical indicators of metamorphism, homogeneity of chondrule olivine is one of the most useful. In unequilibrated chondrites, Fa content of olivine becomes increasing homogenous with petrologic type, eventually converging on equilibrated values (~30-32 mol% Fa in the CK chondrites). Though olivine homogeneity in most samples is consistent with petrologic types 3.8 or 3.9, the unequilibrated CK chondrites exhibit varying degrees of homogeneity. NWA 5343 is the least homogeneous, with matrix olivine ranging from Fa_34.9-Fa_37.9 and chondrule olivine from Fa_0.3-Fa_43.4. Percent mean deviation (PMD) of NWA 5343 is 17.1. When compared to the ordinary chondrites, this degree of heterogeneity and disequilibrium is more consistent with that of petrologic type 3.6 or 3.7. In contrast, the most homogeneous unequilibrated CK chondrite (DaG 431) has a PMD of 6.3. Abundances of CaO, Al₂O₃, MnO, CaO, and NiO in chondrule olivine also suggest that there are varying degrees of metamorphism within the CK chondrites.