Petrology of types 4–6 carbonaceous chondrites

Abstract

We have studied the petrology of three type 4 carbonaceous chondrites, Coolidge, Karoonda, and Pecora Escarpment 82500, which was previously classified as an LL6 chondrite, and compared them to published descriptions of the other known metamorphosed C chondrites, Yamato 6903 and Mulga (west). All except Coolidge have abundant matrix, typically 80 vol %, composed largely of homogeneous olivine, Fa 29–33, and surprisingly heterogeneous plagioclase, mostly An 20–75. Their opaque mineralogy indicates high oxygen fugacities during metamorphism: metallic Fe,Ni and troilite are rare, and magnetite and pentlandite are abundant with accessory pyrrhotite, chalcopyrite, and pyrite. Possibly because of a lower matrix abundance, Coolidge has more reduced minerals: metallic Fe,Ni and olivine with Fa 14. The properties of all C4–6 chondrites appear to be consistent with an origin by metamorphism of C3‐like precursors by processes analogous to those operating in ordinary chondrite parent bodies or planetesimals. However, type 4–6 material was not well mixed with type 3 material in the CV3 parent body, as it was in the ordinary chondrite parent bodies. Conceivably, C4–6 chondrites come from one or more separate parent bodies.

Silicate darkening in the Kobe CK chondrite: Evidence for shock metamorphism at high temperature

“…Many plagioclase grains also exhibit weak undulatory extinction. Based on these features of olivine, Kobe is assigned to be shock stage S2 (Stoffler et al, 1991), which is comparable to the majority of known CK chondrites (Scott et al, 1992;Rubin, 1992). Based on the shock classification for ordinary chondrites (Stoffler et al,199 l), the assignment of shock stage S2 means that Kobe was very weakly shocked at pressures that are consistent with equilibrium peak shock pressures less than 5-10 GPa.…”

“…If so, the suggestion of Rubin ( 1992) would only be valid for type 6 chondrites. In fact, there seems to be no strong correlation between degree of silicate darkening and shock stage among the CK chondrites (Scott et al, 1992;Rubin, 1992) In this paper we present the results of detailed petrographic and analytical SEM observations of matrix and chondrules of Kobe. We used, in addition to a conventional SEM, an analytical field emission (FE) SEM to obtain especially highmagnification images.…”

“…Unlike other types of carbonaceous chondrites, most CK chondrites have been thermally metamorphosed to various degrees to yield petrologic types from 4 to 6 (e.g., Scott and Taylor, 1985; Kallemeyn et al, 199 1). Therefore, they potentially provide insight into early processes of thermal metamorphism that occurred in the carbonaceous chondrite parent bodies.…”

“…Regarding this question, Rubin (1 992) suggested that the olivines in those chondrites have been annealed to remove strong shock effects, such as planar fractures and deformation features, by post-shock heat during intense shock that produced the silicate darkening. However, Scott et al (1992) argued that the erasing of shock features in olivine requires a high degree of thermal metamorphism that is characteristic of petrologic type 6. If so, the suggestion of Rubin ( 1992) would only be valid for type 6 chondrites.…”

Microinclusion-rich vesicular olivine in the Karoonda CK4 chondrite: transmission electron microscopy

“…Karoonda like carbonaceous (CK) chondrites differ from other types of carbonaceous chondrites, in that most of them have been thermally metamorphosed to petrologic types 4 6 (e.g., Scott and Taylor, 1985;Kallemeyn et al, 1991). A unique feature of the CK chondrites is that most of their silicate minerals appear dark and dusty in transmitted light, which has been referred to as silicate darkening (Kallemeyn et al, 1991;Rubin, 1992).…”