9:15 AM - 9:30 AM
[PPS08-12] Imaging of solar wind helium in the Fe-Ni metal and metal hydroxide from Northwest Africa 801
Keywords:Solar wind, Noble gases, Ion imaging
Gas-rich meteorites incorporate large amounts of solar wind (SW) noble gases. [1]. Fe-Ni metal is one of the meteoritic minerals with the highest retentivity for SW noble gases [2]. SW noble gases in the gas-rich meteorites record the SW inventory of the past Sun. An analytical technique for the in situ SW noble gases with the fine probe has not been established due to the low spatial resolution for noble gas analysis. The laser ionization mass nanoscope (LIMAS) was recently developed for nanoscale noble gas analysis [3]. Here we study the distribution of the SW-He contents in Fe-Ni metal and metal hydroxide grains from the CR2 chondrite Northwest Africa 801 (NWA 801), which is one of the gas-rich meteorites [4]. As a running standard for the metals, we prepared 4He-implanted Fe-Ni alloy, of which 4He fluence was 3 × 1015 cm-2 and energy of 20 keV, to determine a relative sensitivity factor (RSF) for 4He in the metal.
We performed 4He ion imaging by LIMAS using a spatial resolution of 250 nm (2σ definition). Fe-Ni metal in NWA 801 exists in the interior of chondrules, and the rim of chondrules. Small metal grains are isolated in the matrix (< 50 µm). Some Fe-Ni metal grains are partially or wholly replaced by metal hydroxide due to terrestrial weathering. 4He signals have not been detected from the metal grains of chondrules. On the other hand, the isolated metal and metal hydroxide grains contained abundant 4He. The 4He was distributed at the edge of the grains within the spatial resolution used. The 4He profile steeply decreases and reaches in the baseline.
The 4He imaging in the grains suggests that SW implantation originated the distribution. The 4He concentration in the edge of Fe-Ni metal is calculated to be 7 × 1020 atoms/cm3 using the RSF. Because the concentration is averaged per spatial resolution, the peak concentration at the edge would be more than 2,000 times higher than the bulk 4He concentration of NWA 801 [4]. The SW He into the Fe-Ni grain is calculated to be 1 × 1016 cm−2 from the imaging. The presence of 4He at the edge of the metal hydroxide grain indicates that the SW He was not removed completely from the original location, even if the Fe-Ni metal grain has been oxidized by terrestrial weathering.
References: [1] Wieler (2002) Reviews in Mineralogy and Geochemistry 47. [2] Becker and Pepin (1991) EPSL 103: 55–68 [3] Bajo et al. (2016) SIA 48, 1190–1193. [4] Obase et al. (2021) GCA 312: 75–105.
We performed 4He ion imaging by LIMAS using a spatial resolution of 250 nm (2σ definition). Fe-Ni metal in NWA 801 exists in the interior of chondrules, and the rim of chondrules. Small metal grains are isolated in the matrix (< 50 µm). Some Fe-Ni metal grains are partially or wholly replaced by metal hydroxide due to terrestrial weathering. 4He signals have not been detected from the metal grains of chondrules. On the other hand, the isolated metal and metal hydroxide grains contained abundant 4He. The 4He was distributed at the edge of the grains within the spatial resolution used. The 4He profile steeply decreases and reaches in the baseline.
The 4He imaging in the grains suggests that SW implantation originated the distribution. The 4He concentration in the edge of Fe-Ni metal is calculated to be 7 × 1020 atoms/cm3 using the RSF. Because the concentration is averaged per spatial resolution, the peak concentration at the edge would be more than 2,000 times higher than the bulk 4He concentration of NWA 801 [4]. The SW He into the Fe-Ni grain is calculated to be 1 × 1016 cm−2 from the imaging. The presence of 4He at the edge of the metal hydroxide grain indicates that the SW He was not removed completely from the original location, even if the Fe-Ni metal grain has been oxidized by terrestrial weathering.
References: [1] Wieler (2002) Reviews in Mineralogy and Geochemistry 47. [2] Becker and Pepin (1991) EPSL 103: 55–68 [3] Bajo et al. (2016) SIA 48, 1190–1193. [4] Obase et al. (2021) GCA 312: 75–105.