Petrology and mineralogy of CK chondrites: Implications for the metamorphism of the CK chondrite parent body

Abstract

The petrology and mineralogy of four CK chondrites, Karoonda (CK4), Maralinga (CK4), Yamato(Y)-693(CK4), and Elephant Moraine (EET) 87507 (CK5) were investigated in detail to estimate the origin of their quite heterogeneous plagioclases and the metamorphic history of CK chondrite parent body. EPMA analyses and SEM observations revealed that plagioclases in chondrules, CAIs, and matrices in CK chondrites have different compositional variations and that plagioclases in matrices display distinct reverse zoning in regard to An content. Cores of matrix plagioclases (An_∿An_) may have formed by recrystallization of primary plagioclases. Rims of matrix plagioclases were probably formed during metamorphism, because the reverse zoning can be observed, regardless of degree of recrystallization. The occurrences and compositions of pyroxenes are different among less recrystallized CK chondrites and recrystallized CK chondrites. Low-Ca pyroxenes in recrystallized matrices are enclosed in matrix plagioclases and different in CaO and Al_2O_3 contents from those in chondrules. Compositions of low-Ca pyroxenes in matrix plagioclases suggest that they include Fe^. They were probably formed during metamorphism under high oxygen fugacities. Highly oxidized metamorphic conditions are supported by the existence of Fe^ in spinels and high NiO contents in olivines. Olivine-spinel geothermometry and pyroxene compositions suggest that Maralinga, Y-693,EET87507,and some fragments in Karoonda were heated to the temperatures as high as those of type 5 to 6 ordinary chondrites (750 to 850℃). Slow cooling below∿400℃ of these meteorites was estimated by sulfide mineral assemblages. Because Maralinga has many different properties from other CK chondrites investigated, it is an anomalous CK chondrite