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[PPS08-10] Host mineral phases of solar wind helium and their occurrence in Northwest Africa 801 meteorite
Keywords:Solar wind, Noble gases, Ion imaging
Solar-gas-rich chondrites are regolith breccias that preserved an irradiation history of solar wind to the surfaces of the meteorite parent bodies [1]. Northwest Africa 801 (NWA 801) CR2 chondrite has abundant solar wind noble gases. The clastic matrix in NWA 801 contains solar wind noble gases (~10-2 cm3 STP g-1) [2]. Host mineral phases and their occurrence may be reflected trap processes of solar wind on a parent body and subsequent gardening process by brecciation. However, an occurrence of host mineral phases of solar wind noble gases in carbonaceous chondrites has not been identified.
Laser ionization mass nanoscope (LIMAS) was developed for nanoscale (surface and depth direction ~10 nm) multi-isotope analysis at the same time, including noble gases [3]. LIMAS demonstrated the implanted helium energy and fluence from three-dimensional images of helium isotope [4, 5]. We have developed a large-area ion imaging system by LIMAS to investigate the occurrence of solar wind host, which enabled us to perform helium imaging of up to 200 × 100 µm square [6].
In this study, we conducted a coordinated study using large-area imaging of helium isotope by LIMAS and detailed petrographic observations of NWA 801 meteorite to identify host mineral phases of solar wind helium.
The analyzed area is the matrix adjacent to chondrules. The matrix is composed of olivine, pyroxene, iron sulfide, and iron hydroxide grains (Fig. 1a). The large-area ion image shows that many constituent grains of the matrix preserve solar wind helium, while helium cannot be detected from olivine and Fe-Ni metal from chondrule (Figs. 1b, c). Dark areas of low concentration of helium are distributed patchily in the matrix. This occurrence implies lithology formed by impact gardening. The iron sulfide and iron hydroxide grains in the matrix preserve substantial amounts of helium. Moreover, helium in the iron hydroxide grain is distributed rim of the grains (Fig. 1d). This observation indicates that the solar wind helium was retained after terrestrial weathering, which was a hydroxyl reaction of metallic iron.
The helium-enriched layer shown as green to yellow color in figure 1d had wider than 100 nm in width. This width is comparable with the primary beam diameter of the helium imaging. This observation suggested that solar wind helium was implanted into the grain surface.
If the helium in the iron sulfide and the iron hydroxide grains were incorporated on the surfaces of the parent bodies, olivine and pyroxene grains would capture solar wind helium by the same process. However, the iron sulfide and iron hydroxide grains in the matrix of the NWA 801 meteorite contain more abundant helium than that in the olivine and pyroxene grains. This inconsistency indicates that the retentivity of helium in the metallic phase may differ from the silicate phase in the matrix.
[1] Wieler (2002) Rev. Mineral. Geochem. 47. [2] Obase et al. (2021) GCA 312, 75–105. [3] Nagata et al. (2019) Appl. Phys. Express 12: 085005. [4] Bajo et al. (2015) GJ 49, 559–566. [5] Bajo et al. (2016) SIA 48, 1190–1193. [6] Wada et al. (2021) Annual Meeting of the GSJ, 68, 143
Laser ionization mass nanoscope (LIMAS) was developed for nanoscale (surface and depth direction ~10 nm) multi-isotope analysis at the same time, including noble gases [3]. LIMAS demonstrated the implanted helium energy and fluence from three-dimensional images of helium isotope [4, 5]. We have developed a large-area ion imaging system by LIMAS to investigate the occurrence of solar wind host, which enabled us to perform helium imaging of up to 200 × 100 µm square [6].
In this study, we conducted a coordinated study using large-area imaging of helium isotope by LIMAS and detailed petrographic observations of NWA 801 meteorite to identify host mineral phases of solar wind helium.
The analyzed area is the matrix adjacent to chondrules. The matrix is composed of olivine, pyroxene, iron sulfide, and iron hydroxide grains (Fig. 1a). The large-area ion image shows that many constituent grains of the matrix preserve solar wind helium, while helium cannot be detected from olivine and Fe-Ni metal from chondrule (Figs. 1b, c). Dark areas of low concentration of helium are distributed patchily in the matrix. This occurrence implies lithology formed by impact gardening. The iron sulfide and iron hydroxide grains in the matrix preserve substantial amounts of helium. Moreover, helium in the iron hydroxide grain is distributed rim of the grains (Fig. 1d). This observation indicates that the solar wind helium was retained after terrestrial weathering, which was a hydroxyl reaction of metallic iron.
The helium-enriched layer shown as green to yellow color in figure 1d had wider than 100 nm in width. This width is comparable with the primary beam diameter of the helium imaging. This observation suggested that solar wind helium was implanted into the grain surface.
If the helium in the iron sulfide and the iron hydroxide grains were incorporated on the surfaces of the parent bodies, olivine and pyroxene grains would capture solar wind helium by the same process. However, the iron sulfide and iron hydroxide grains in the matrix of the NWA 801 meteorite contain more abundant helium than that in the olivine and pyroxene grains. This inconsistency indicates that the retentivity of helium in the metallic phase may differ from the silicate phase in the matrix.
[1] Wieler (2002) Rev. Mineral. Geochem. 47. [2] Obase et al. (2021) GCA 312, 75–105. [3] Nagata et al. (2019) Appl. Phys. Express 12: 085005. [4] Bajo et al. (2015) GJ 49, 559–566. [5] Bajo et al. (2016) SIA 48, 1190–1193. [6] Wada et al. (2021) Annual Meeting of the GSJ, 68, 143