Three-dimensional description of the Kobe meteorite by micro X-ray CT method: Possibility of three-dimensional curation of meteorite samples.

Abstract

The Kobe meteorite was imaged by a micro X-ray CT scanner to examine the possibility of three-dimensional curation of meteorite samples without destruction. Materials including major minerals in meteorites were also imaged to obtain a quantitative relation between CT value, which is numerical expression of CT contrast, and X-ray linear attenuation coefficient (LAC) of the materials under polychromatic X-ray beams. CT values of the samples are normalized by that of an olivine crystal from San Carlos, U.S.A. (SCO = San Carlos olivine: Fo91.8), which was imaged under the same condition as a standard material (normalized CT value = CT value(sample)/CT value(SCO) × 100 [OU]: OU = olivine unit). The normalized CT values almost equal to similarly normalized LAC values calculated for the samples at <150–250 OU. We can estimate materials by the normalized CT value-LAC relation even if we use polychromatic X-ray beams. Successive 380 CT slice images of apiece of the Kobe meteorite (Kobe C-5-1; 1.144 g and 7–9 mm) were taken to obtain the three-dimensional structure. The pixel size of 31.6 μm × 31.6 μm and the slice width of 25.3 μm determine the spatial resolution for the images of about 60–90 μm. Thin sections were made to compare the CT images with images by an optical microscope, SEM and X-ray fluorescent microscope. In the CT images, we can distinguish plagioclase, ferromagnesian silicates (mainly olivine), magnetite-pentlandite and molybdenite. The CT value for the ferromagnesian silicates is approximately the same as that expected from olivine of FO∼69 in the meteorite. Orthopyroxene and clinopyroxene were not distinguished from olivine due to their small grain sizes and LAC values similar to olivine. Magnetite and pentlandite cannot be distinguished from each other due to their similar LAC values. The CT values of magnetite-pentlandite aggregates are smaller than those expected from their LAC values because the aggregates have inclusions of silicate minerals and Ca phosphates. The CT value of plagioclase is slightly larger than that expected due to small inclusions too. Chondrules with rims with different contrasts can be recognized while chondrules without rims cannot. The two-dimensional shapes of some magnetite-pentlandite aggregates are circular like opaque chondrules under an optical microscope. However, they are not in spherical shape three-dimensionally. Their sub-rounded shapes might be formed by partial melting of these aggregates probably at the same time as a local melting of plagioclase and some olivine. The sample was cut into seven plates for different analysis at different laboratories. Three-dimensional CT images were made for the plates to understand their internal structures. The present results suggest that curation of meteorites is possible by micro X-ray CT.