THE CV AND CK CHONDRITES: A SINGLE PARENT ASTEROID?

The CK chondrites are a highly-oxidized group of carbonaceous chondrites characterized in part by the presence of nickel-rich olivine and Cr₂O₃-rich magnetite. Oxygen isotopes, mineral compositions, and bulk rock compositions of the CK chondrites are similar to those of the oxidized CV chondrites. These similarities have led to the suggestion that the CK and CV chondrites represent a single group of chondrites, derived from the same parent asteroid. However, despite their similarities, the CV and CK chondrites have one significant difference - the degree of metamorphism that they have experienced. CK chondrites are the only group of carbonaceous chondrites to exhibit the complete range of thermal metamorphism, from type 3 (low degrees of metamorphism, unequilibrated) to type 6 (highly metamorphosed, equilibrated). All CV chondrites, however, are classified as petrologic type 3. If the CV and CK chondrites are derived from the same parent body, we would expect them to have a similar thermal history. In that case, the unequilibrated (type 3) CV and CK chondrites should share a common metamorphic sequence. Though the CV chondrites are thought to be less metamorphosed than the type 3 CK chondrites, very little work has been done to determine the petrologic subtypes (from type 3.0 – 3.9) of the CK and CV chondrites. Understanding the degree of metamorphism these samples have experienced is particularly useful for determining if both groups are derived from the same parent asteroid.

This work represents an ongoing study to characterize the petrologic subtypes of the type 3 CK and CV chondrites. Petrologic subtype is determined using recognizable effects of thermal metamorphism, the most pronounced of which is the chemical equilibration of silicate minerals that results from diffusive exchange of cations between chondrite components and matrix material. Our previous work has demonstrated that NiO and Cr₂O₃ content of olivine in type II chondrules can be used as a geochemical indicator of metamorphism, but NiO content in magnetite does not seem to be as reliable. Here we determine the petrologic subtypes of additional CK and CV chondrites in an attempt to more firmly establish if a metamorphic sequence exists between the CK and CV chondrites.