3:45 PM - 4:00 PM
[PPS10-02] Irradiation experiments on CM chondrites: To estimate surface textures of the returned samples by Hayabusa 2
Keywords:irradiation experiment, CM chondrites, TEM
Samples and methods: We performed irradiation of 4 keV He+ ions on Murchison CM chondrites at Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency (TARRI, JAEA). The fluences are 5 × 1016 and 5 × 1017 He+/cm2, which correspond to ~102- and ~103-year irradiation at 1.1 AU (the averaged orbital radius of Ryugu). Reflectance spectra of the irradiated surface were measured at JASCO Co. Ltd. by using JASCO V-670 absorption spectrometer with an integrating sphere. The irradiated samples were observed by field-emission scanning electron microscope (FE-SEM) at JAEA and Kyushu University. We observed the samples by using 2 or 3 kV acceleration voltage to avoid structural changes during observation. Thin samples were prepared by using scanning electron microscope-focused ion beam sample preparation machine and low acceleration voltage Ar milling machine at Kyushu University. They were observed by transmission electron microscope (TEM) at Kyushu University.
Results and discussion: Reflectance spectrum of the sample irradiated by a fluence of 5 × 1016 He+ does not show remarkable difference from the spectra of an un-irradiated sample. By contrast, a broad absorption from 0.7 to 1.4 μm, related to the absorption by Fe-rich serpentine group minerals, is disappeared in the case of the sample irradiated with 5 × 1017 He+. These data suggest that 1000-year equivalent solar wind irradiation gives an effect on the shape of reflectance spectra, which is similar to the effect by dehydration . There is no remarkable difference in surface morphology of the sample irradiated by a fluence of 5 × 1016 He+ from those of un-irradiated sample. On the other hand, the sample irradiated with 5 × 1017 He+ shows blistering on both matrix and chondrules. The surface of fine-grained matrix has a ~30-nm thick amorphous layer. In the amorphous layer, a small amount of nanoparticles is observed. Their 0.2-nm lattice fringes suggest that they are nanophase Fe0. In the case of the sample irradiated with 1017 He+ has ~60-nm amorphous rim containing abundant bubbles (blistering), which is especially remarkable in cronstedtite-tochilinite intergrowth. Just below the amorphous layer, both cronstedtite and tochilinite show sharp lattice fringes. The amorphous rim contains abundant nanoparticles is observed. They also show 0.2-nm lattice fringes, suggestive of nanophase Fe0. This result is consistent with .
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