Though primitive asteroids did not reach temperature high enough to induce melting, many of these asteroids experienced some degree of metamorphism. The effects of metamorphism have been most extensively studied in the ordinary chondrites because their availability and abundance. However, more recent studies have recognized the effects of metamorphism in three carbonaceous chondrites groups (CV, CK, and CO) (Kallemeyn et al., 1991; Guimon et al., 1995; Chizmadia et al., 2002). The CK chondrites are unique because they are the only group of carbonaceous chondrites to exhibit the full range of thermal metamorphism, from petrologic type 3 (minimally metamorphosed) to petrologic type 6 (extensively metamorphosed) (Van Schmus and Wood, 1967; Kallemeyn et al., 1991; Geiger et al., 1993). Despite this unique characteristic, the CK chondrites, until recently, were one of the least studied groups of carbonaceous chondrites. For that reason, Dunn\u2018s recent work has focused on characterizing the petrography and mineralogy of the least metamorphosed CK chondrites in order to address questions regarding the thermal evolution of the CK chondrite parent asteroid.<\/p>\n
Though initially classified as two groups, Greenwood et al. (2010) argued that the CV and CK chondrites originated from a single, thermally-stratified (i.e., onion shell) asteroid. In the onion shell model, an asteroid experiences the highest degree of metamorphism in its center but becomes progressively less metamorphosed toward the surface, as heat escapes (Wood, 2003; Treiloff et al., 2003). If this model for the CV-CK parent body is correct, the CV and CK chondrites must represent a single metamorphic sequence, from least metamorphosed material (CV \u00e0 CK3) to the most metamorphosed material (CK 4-6). The absence of such a sequence would suggest either that the CV and CK chondrites were derived from two parent asteroids, or that the CV and CK chondrites originated from the same asteroid, but it was not thermally stratified<\/em>.<\/p>\n The ordinary chondrites, which are derived from thermally stratified asteroids (Wood, 2003; Treiloff et al., 2003) serve as an excellent analogue for the CK chondrites. Geochemical changes that take place during metamorphism have been well characterized in the ordinary chondrites due to their availability and abundance. The most pronounced change that occurs is the chemical equilibration of olivine (Huss et al., 1978, 1981). Using known geochemical changes in the ordinary chondrites, Dunn and Gross (2017) established criteria for determining petrologic type of the unequilibrated CK chondrites. Results, based primarily on chondrule and matrix olivine in eight CK chondrites, suggested that the CK chondrites were less metamorphosed than previously thought. Subsequently, (Dunn et al., in review<\/em>) characterized matrix material in the least metamorphosed CK chondrite (NWA 5343), in order to better qualify lower limits of metamorphism in the CK chondrite parent body.<\/p>\n Any potential model of a single CV-CK asteroid requires that both groups formed under similar parent body conditions, such as temperatures and oxygen fugacities. Magnetite, which forms from oxidation of metal on a parent asteroid (Choi et al., 1997, 2000; Davidson et al., 2014), can be a useful indicator of the physical conditions under which an asteroid parent body formed. Dunn et al. (2016) analyzed magnetite in eight CK and five CV chondrites to address whether both could have formed on the same asteroid. They argued that observed differences in oxygen fugacity of the CV and CK chondrites could not be reconciled using the onion shell model, as this model would require a sudden increase in oxidation state during metamorphism of the parent asteroid. This is problematic, as there is no plausible source of the fluid necessary to drive this increase in oxidation state, and maximum temperatures reached during metamorphism of the CK3 chondrites (~1070K) are below the experimental temperatures necessary to convert CV chondrites to CK chondrites (Geiger and Bischoff, 1991). Using compositions of magnetite and olivine, Dunn and Gross (2017) also established parameters for distinguishing between unequilibrated CK and CV chondrites.<\/p>\n The question of whether the CV and CK chondrites are derived from one or two parent bodies has yet to be resolved. Other single parent body models that do not invoke thermal stratification have been proposed, such as the model of Wasson et al. (2013), in which the CK chondrites are buried and aqueously altered on a shocked CV-like parent body. Dunn will continue to pursue this line of research.<\/p>\n","protected":false},"excerpt":{"rendered":" Though primitive asteroids did not reach temperature high enough to induce melting, many of these asteroids experienced some degree of metamorphism. The effects of metamorphism have been most extensively studied in the ordinary chondrites because their availability and abundance. However, … Continue reading