Compositional and petrographic similarities of CV and CK chondrites: A single group with variations in textures and volatile concentrations attributable to impact heating, crushing and oxidation

Abstract

Greenwood et al. (2010) gathered data on O-isotopic and elemental compositions and reevaluated literature data for CV and CK chondrites. They concluded that these two chondrite groups originated on the same parent asteroid, with CK chondrites being metamorphosed CV chondrites (which are otherwise missing types 4 through 6). To test this interpretation we have gathered new instrumental neutron-activation-analysis (INAA) data for CV and CK chondrites and reexamined their petrographic features. The new INAA data like the older data show scatter attributable to weathering effects, but we conclude that the refractory lithophile abundances are the same in CV and CK, in agreement with the Greenwood et al. interpretation. Several volatile elements are significantly lower in CK than in CV chondrites. Among the elements we determine, the greatest difference between CV and CK is found for Br, for which the CV/CK ratio is ∼4; As and Sb are about 20% lower in CK than CV and smaller differences are observed for Zn, Ga and Se. It seems likely that volatiles were lost during impact-heating events that also provided the heat responsible for metamorphic recrystallization. Within statistical uncertainty, chondrules in CV and CK chondrites are the same size and have similar textural distributions. A significant petrographic difference between CK and CV chondrites cited by Kallemeyn et al. (1991) was the much higher percentage of igneous rims around CV chondrules. However, we now recognize that many chondrules in CK3.8 NWA 1559 have igneous rims and in CK4 chondrites, igneous rims are recognizable by their associated sulfide-rich rings; there are no quantifiable CV–CK differences in igneous-rim abundances. We used Ca and Al maps to show that CK chondrites have CAI abundances similar to those of CV chondrites. It thus appears that there are no resolvable pre-metamorphic petrographic differences between CV and CK chondrites. We recommend that the “CK” designation be abandoned and that the CV group be acknowledged as spanning the range of petrologic types 3–6. We suggest that CK3 chondrites be designated CV3OxK. Most CK4 chondrites are highly fragmental; collisional crushing appears to be much more common than in CV chondrites. It seems likely that CK chondrites formed from reduced CV3 materials after the latter was impacted, buried, aqueously altered and annealed.