5:15 PM - 7:15 PM
[PPS07-P08] Development of 4He imaging technique to reveal microstructure of asteroid regolith
Keywords:SIMS, Noble gas, Isotope imaging
In situ analysis can clarify the location and concentration of solar wind (SW) noble gases, and their distribution can be compared with petrographic observations. In situ 4He isotope imaging using laser ionization mass nanoscope (LIMAS) with sub-micrometer spatial resolution showed that SW-4He was implanted on the surface of individual mineral grains distributed in the matrix of the gas-rich CR chondrite Northwest Africa 801 (NWA 801) [1], demonstrating that there is variation in SW-4He fluence between individual mineral grains [2]. In this study, the imaging area was extended compared to [3]. The previous imaging was limited by the He ionization region, which ionized only ±20 μm from the axial of the focused fs laser of ~20 μm diameter. The focal point of the focused fs laser is adjusted to be consistently aligned with the irradiation position of the primary ion beam. The ionization area can be expanded in a direction perpendicular to the focal point of the fs laser by moving the focusing lens of the fs laser. We performed large area 4He isotope imaging to observe the relationship between the micro-distribution of SW-4He and the petrography of NWA 801.
We examined a polished section of the NWA 801 meteorite. BSE images of the sample were obtained using a FE-SEM system to observe the petrography prior to isotope imaging. We used large area He isotope imaging. Based on the 4He imaging for 300 × 300 μm2 of 4He implanted Si wafer, the 4He ionization region was expanded from ~40 μm to ~150 μm. The isotope imaging of 100 × 150 μm2 was performed on the NWA 801 section. The 4He distribution represents that high concentration 4He is mainly contained within individual mineral grains, which is consistent with [2]. The individual mineral grains are concentrated within clastic regions composed of microbreccia-like structure. On the other hand, regions composed of a relatively homogeneous fine-grained matrix did not contain 4He.
[1] Obase et al. (2021) Geochim. Cosmochim. Acta. 312, 75. [2] Wada et al. (2025) Meteorit. Planet. Sci., under review. [3] Bajo et al. (2024) Anal. Chem. 96, 5143.
We examined a polished section of the NWA 801 meteorite. BSE images of the sample were obtained using a FE-SEM system to observe the petrography prior to isotope imaging. We used large area He isotope imaging. Based on the 4He imaging for 300 × 300 μm2 of 4He implanted Si wafer, the 4He ionization region was expanded from ~40 μm to ~150 μm. The isotope imaging of 100 × 150 μm2 was performed on the NWA 801 section. The 4He distribution represents that high concentration 4He is mainly contained within individual mineral grains, which is consistent with [2]. The individual mineral grains are concentrated within clastic regions composed of microbreccia-like structure. On the other hand, regions composed of a relatively homogeneous fine-grained matrix did not contain 4He.
[1] Obase et al. (2021) Geochim. Cosmochim. Acta. 312, 75. [2] Wada et al. (2025) Meteorit. Planet. Sci., under review. [3] Bajo et al. (2024) Anal. Chem. 96, 5143.