CV AND CK CHONDRITES: ONE PARENT BODY OR TWO?

Despite the close chemical and mineralogical relationship between the CK and CV chondrites, the long-standing view has been that these meteorites represent two different asteroidal parent bodies. Recently, it has been suggested that CV and CK chondrites originate from the same parent body. If this is correct, the CV and CK chondrites should represent a continuous metamorphic sequence (from petrologic type 3 to type 6) in which nickel diffuses from magnetite into olivine. If a metamorphic sequence is present, we should observe a decrease in the Ni content of magnetite along with an increase in Ni content of olivine from type 3 CV chondrites to type 6 CK chondrites. To determine if a continuous metamorphic sequence exists between the two groups, magnetite and olivine compositions in four CK chondrites were measured using a JEOL JXA 8200 electron microprobe at Washington University in St. Louis. At least 10 magnetite grains and 15 olivine grains per sample were analyzed to ensure compositions were accurately characterized. Measured NiO and FeO abundances were compared to existing data for CV chondrites to identify trends indicative of a metamorphic sequence.

It does appear that NiO in magnetite is decreasing, as expected, during metamorphism of the CK chondrites. However, NiO content in the CV chondrite Allende is lower than that of the CK chondrites, which is contradictory to what is expected from a continuous CV – CK chondrite metamorphic sequence. NiO content in olivine does increase from the CV to the CK chondrites, though it does not appear to increase within the equilibrated CK chondrites (as petrologic type increases). NiO content in olivine and magnetite do not provide a clear answer to the question of whether the CV and CK chondrite are from the same parent body. Olivine may indicate this possible, but magnetite does not. It is unclear why this is the case. It may be possible that the source of Ni in the CV chondrites is metal rather than magnetite, but this seems unlikely since magnetite dominates metal in the CV chondrites. It is also possible that Ni content in the CV chondrites may have been modified by alteration, which is expected since both groups represent complex alteration histories. Additional samples will need to be analyzed to confirm the presence of these trends.