The Kobe CK carbonaceous chondrite: petrography, mineralogy and metamorphism

Abstract

A petrographic and mineralogical study of Kobe indicates that it can be classified as a CK4 carbonaceous chondrite. CK chondrites differ from other groups of carbonaceous chondrites, because all normal CK chondrites have been thermally metamorphosed. Olivine and plagioclase are the two most abundant minerals in Kobe. Olivine is very homogeneous in composition, which is consistent with the metamorphic feature of Kobe. However, plagioclase shows large grain to grain compositional variations. The matrix and chondrule mesostases exhibit pronounced silicate darkening. They contain unusual olivine that contains numerous small vesicles (< 0.1 3 μm in diameter) and inclusions (< 0.1 10 μm) of magnetite, pentlandite and minor plagioclase, pyroxene and chlorapatite. The vesicular olivine is particularly abundant in regions that exhibit a high degree of darkening. The vesicular olivine is clearly the principal cause of the silicate darkening. Plagioclase in matrix and chondrule mesostases also contains numerous small vesicles and inclusions of magnetite, pyroxene and minor pentlandite and olivine. The texture and mineralogy of the vesicular olivine and plagioclase suggest that they recrystallized from melts that were produced from olivine and plagioclase in the matrix by shock. We suggest that the shock event occurred at a relatively mild pressure and a high temperature, probably at the state where the meteorite underwent thermal metamorphism.

Structural state of plagioclase from the Kobe CK chondrite: Implications for the thermal history of the CK parent body

“…The textures and chemical compositions of constituent minerals of the Kobe meteorite are described in detail by Tomeoka et al (2005). Consistent with this earlier study, plagioclase in the PTS analyzed in the present study occurs mainly in the matrix as irregularly shaped grains that are typically 50 150 μm in size.…”

“…Figure 2 shows a histogram of the An mol% of all plagioclases randomly analyzed in this study. The data extend over a wide range from An 23.4 − 90.4 , as reported previously by Tomeoka et al (2005); however, approximately 70% of analyses fall in the range An 50 − 65 . Accordingly, as the crystals to be analyzed in terms of structural state fall in the range An 54.7 − 63.3 , they can be considered to be representative of plagioclase from the Kobe meteorite.…”

“…As noted by Tomeoka et al (2005), plagioclase in the PTS of the present study varies considerably in chemical composition, even within individual grains, and in part contains numerous tiny inclusions (several microns or less in size) of magnetite and pentlandite. Chemically homogeneous and inclusion free areas (30 50 µm in size) within individual plagioclase grains were removed from the PTS and mounted on glass fibers of ~ 3 μm diameter for X ray diffraction analysis using a 114 mm diameter Gandolfi camera (Gandolfi, 1967).…”

“…Kallemeyn et al (1991) applied some of the petrographic criteria used in studies of ordinary chondrite to carbonaceous chondrites, including: 1) homogeneity of olivine composition, 2) mean diameter of plagioclase, 3) presence or absence of primary glass, 4) chondrule delineation, and 5) coarseness of ground mass grains, and suggested that CK chondrites form a metamorphic sequence, as observed in ordinary Structural state of plagioclase from the Kobe CK chondrite chondrites. Tomeoka et al (2005) analyzed the Kobe meteorite and found that olivine crystals have homogeneous compositions (standard deviation of Fa content = 0.4), grain sizes of plagioclase typically range from 50 to 150 μm, primary glass is absent, chondrules are moderately defined to well defined, and the grain size of matrix olivine is typically 5 30 μm. Based on the latter two criteria, the authors concluded that a CK4 classification is most appropriate for the Kobe meteorite, although the grain size of plagioclase is consistent with a type 6 classification (Kallemeyn et al, 1991).…”

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

“…A scanning electron microscope (SEM) study of the Kobe CK4 chondrite (Tomeoka et al, 2001(Tomeoka et al, , 2005 revealed that it contains an abundance of unusual olivine in both the matrix and chondrules, and this contains numerous small vesicles and inclusions of magnetite, pentlandite, and a variety of other minerals. In the Kobe matrix, the vesicular olivine fills interstices of nonvesicular olivine grains forming a complex network of veinlets.…”