9:45 AM - 10:00 AM
[PPS10-10] Three-dimensional structure of matrix of the Ivuna meteorite using micro X-ray CT and FIB serial sectioning
Keywords:CI meteorite, aqueous alteration
We made detailed observation of fragments in a thin section of the Ivuna meteorite and obtained elemental maps with FE-SEM/EDX (JEOL JSM7001F/Oxford Instruments X-MaxN 150mm2). Based on the result, a cube ~25 µm in size was sampled from one of the fragments using SEM/FIB (FEI Helios NanoLab G3). Then, the 3D structure of the cube sample was imaged by SIXM (Scanning Imaging X-ray Microscopy)  at BL47XU of SPring-8, Japan with the pixel size ~100 nm. After the CT imaging, we obtained serial BSE images of some portion of the sample with higher resolution by serial sectioning using FIB and observation with FE-SEM (FEI Helios NanoLab G3). From these 3D images, structures in the matrix and mineral grains were extracted using image analysis.
The SEM/EDX study of the thin section showed that the fragment examined in 3D has Mg-rich matrix mainly consisting of coarse and elongated phyllosilicate aggregates (~50 nm in width and ~500 nm in length). Mineral grains of magnetite, pyrrhotite and Ni bearing sulfates were also observed, but any carbonates were not. These features correspond to lithology Ⅱ (carbonates are absent but sulfates are dominant) of  and lithology CGA (Coarse-grained phyllosilicate aggregate) of . We found that the matrix consists of objects a few mm in size, which are aggregates of Mg-rich phyllosilicates covered with Fe-rich phyllosilicate and seem to be spherical in 2D. Hereafter, we call this object PC (phyllosilicate composite).
In the CT images, rod-shaped crystals of magnetite (~1×5 µm) and cavities in irregular and hexagonal-plate shapes were observed as well as PC. Cavities in hexagonal shape (~5×1 µm) should be empty crystals formed by leaching of original pyrrhotite or carbonate. In the serial SEM images, PCs and magnetite crystals was clearly recognized, but cavities not because spattered substances by FIB were redeposited in the cavities. We found that PCs are irregular ellipsoids in 3D, and the directions of elongations of PCs and magnetite crystals are random, suggesting that no remarkable movement of a fluid was occurred during aqueous alteration. Matrix texture consisting of PCs might be a result of strong aqueous alteration of fine aggregates of sub-micron grains of minerals and/or amorphous silicate.
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