USING METEORITES FROM NORTHWEST AFRICA TO DERIVE A PETROLOGIC SEQUENCE FOR THE UNEQUILIBRATED CK CHONDRITES

The CK chondrites are a group of highly oxidized carbonaceous chondrites characterized by nickel-rich olivine, rounded magnetite blebs, and sulfides. They are the only group of carbonaceous chondrites to exhibit the full range of thermal metamorphism, from petrologic type 3 (least metamorphosed) to type 6 (most metamorphosed). In this way they are very similar to the ordinary chondrites, which have been extensively studied. Of the 230 known CK chondrites, only eighteen are classified a petrologic type 3. Because of their small numbers and relatively recent discovery (the earliest in 1993), the type 3 CK chondrites have not been extensively studied. However, as the number of meteorites recovered from Northwest Africa has increased, our understanding of various meteorite groups has significantly improved. This is especially true for the CK chondrites, as all but five type 3 CKs are from Northwest Africa.

Unlike the ordinary chondrites, the petrologic subtypes (3.0 – 3.9) of the unequilibrated CK chondrites have not been established. Understanding the degree of metamorphism these samples have experienced is particularly useful for determining if the CKs are derived from the same asteroid as the CV chondrites. Petrologic subtype is determined using recognizable effects of thermal metamorphism. The most pronounced change that occurs is the chemical equilibration of silicate minerals that results from diffusive exchange of cations between chondrite components and matrix material. Several geochemical criteria, which are used to determine petrologic subtype of the ordinary chondrites, are also applicable to the CK chondrites, including compositional homogeneity of olivine (e.g. CaO, FeO, and Cr₂O₃ abundances) and nickel content of sulfides. Compositional variability of magnetite, particularly NiO, may also serve as an indicator of metamorphism.

Here I report electron microprobe-measured geochemical data of magnetite and olivine in two Northwest African CK chondrites (NWA 1559 and NWA 5956) and CK chondrite Hart (from Hart, Texas). Results suggest that the samples form a progressive metamorphic sequence from Hart to NWA 1559 to NWA 5956. However, the absence of magnetite in NWA 5956 may suggest that it has been incorrectly classified.