Thermal metamorphism of the unequilibrated CK chondrites: evidence for a single CK-CV chondrite parent body? The CV and CK chondrites share many similarities (oxygen isotopes, mineral compositions,…

Thermal metamorphism of the unequilibrated CK chondrites: evidence for a single CK-CV chondrite parent body? The CV and CK chondrites share many similarities (oxygen isotopes, mineral compositions, and bulk rock compositions) (greenwood ET AL. 2010) and are thus collectively referred to as the CV-CK clan (weisberg ET AL. 2006). Despite their close chemical and mineralogical relationship, the long-standing view has been that each group represents a single asteroidal parent body. However, greenwood ET AL. (2010) recently advocated that the CK chondrites would be better described as a subgroup of the CV chondrites, and they proposed a model in which the CK and CV chondrites were derived from a single, thermally-stratified asteroid. If this is the case, the CV and CK chondrites should represent a continuous metamorphic sequence from petrologic type 3 to type 6. The single parent body hypothesis can be tested by identifying a metamorphic sequence within the type 3 CV and CK chondrites (since a metamorphic sequence already exists in the equilibrated CK chondrites). A likely metamorphic sequence would consist of the cvox chondrites and CK chondrites rather than the reduced CV chondrites, because the CK chondrites and the oxidized CV chondrites formed under similar oxidizing conditions. Based on observations about the degree of metamorphism experienced by each group, the cvox chondrites would represent the most primitive material in this metamorphic sequence (≤3.3) and the CK3 chondrites would represent the more altered material (>3.5). Before the single parent body hypothesis can be tested, IT is necessary determine the degree of metamorphism (petrologic subtype) of the existing CK3 chondrites. The most effective way to determine petrologic subtypes is using thermoluminescence (e.g. sears ET AL. 1980). However, the CK chondrites do not produce any detectable thermoluminescence (guimon ET AL. 2005). Petrologic type must instead be established using recognizable effects of thermal metamorphism. The most pronounced change that occurs is the chemical equilibration of silicate minerals (huss ET AL. 1978, 1981) that results from diffusive exchange of cations (e.g. FE2+, MG2+, CR3+, MN2+, and CA2+) between chondrite components (chondrules or cais) and matrix material. These metamorphic effects are particularly prominent in petrologic type 3 materials, as most mineral compositions have reached equilibrium by type four conditions. To determine the petrologic type, i will examine compositional variability and homogeneity of chondrule olivine (both in type i and type II chondrules and matrix), magnetite, and pentlandite in twelve existing type 3 CK chondrites. Once a preliminary metamorphic sequence has been established for the CK3 chondrites, i will analyze magnetite, olivine, and pentlandite in five oxidized CV chondrites, for which petrologic subtype has already been determined, in order to look for geochemical evidence of a single metamorphic sequence between the cvox and CK chondrites.