3:30 PM - 3:45 PM
[PPS08-19] Helium-rich clast in the MIL 090657 CR chondrite: Evidence for exposure to solar energetic particles from the pre-main-sequence Sun
Keywords:Isotope imaging, CR chondrite, Helium, Solar energetic particles
Noble gases are highly depleted in meteorite minerals due to their volatility and chemical inertness. Ion implantation by the solar wind can induce large amounts of noble gases in a surface layer of the irradiated materials, such as regolith materials of the Moon and asteroids [e.g., 1, 2, 3]. Some meteorites containing large amounts of solar wind noble gases are thought to be derived from parent body surface materials once exposed to the solar wind. Recently, isotope imaging using the Laser Ionization Mass Nanoscope (LIMAS) has revealed the nanometer-scale distribution of solar wind He in solar-gas-rich meteorites [4]. The MIL 090657 CR chondrite is depleted in solar wind noble gases [5]. Despite that, we newly discovered a unique He-rich clast from MIL 090657. Here we discuss the incorporation process of He into this clast based on mineralogy and the nanometer-scale distribution of 4He.
A polished section of MIL 090657 was prepared for this study. The He-rich clast was found by LIMAS isotope imaging of 4He, 24Mg, 32S, and 58Ni from this section. The mineralogy of this clast was analyzed by SEM/EDS and TEM/EDS.
The He-rich clast is ~10 μm in diameter embedded in a fine-grained matrix of MIL 090657. The clast mainly consists of a porous aggregate of nanocrystals. The nanocrystals mainly consist of nanocrystalline iron sulfide (pyrrhotite or troilite). Nanocrystals of magnetite and Ni-rich sulfide are also present in the aggregate. Schreibersite crystals (2 μm in maximum) are embedded in the aggregate. Euhedral pyrrhotite crystals (3 μm in maximum) are present on the periphery of the clast. A high concentration of 4He was detected from the schreibersite crystals (~4×1019 atoms/cm3), while 4He was not detected from the porous aggregate and the euhedral pyrrhotite. 4He imaging using a primary beam with a small spot size (~300 nm) revealed that 4He is uniformly distributed in the 2 μm schreibersite crystal.
The porous texture and mineral composition suggest that the porous aggregate was formed by decomposition of tochilinite [6]. The tochilinite and the euhedral pyrrhotite are likely formed by aqueous alteration on the CR chondrite parent body that occurred at 4–13 Myr after the CAI formation [7]. The fact that 4He is only concentrated in schreibersite indicates that the incorporation of 4He occurred before the aqueous alteration. At this time, the Sun was likely in its pre-main-sequence phase (~10 Myr after the formation [8]).
The penetration depth of the current solar wind 4He (~4 keV on average) in schreibersite is ~20 nm (numerically simulated by the TRIM program [9]). This is inconsistent with the uniform distribution of 4He in the 2 μm schreibersite crystal. The 4He distribution and high concentration suggest that the schreibersite crystals were exposed to a large fluence of solar energetic particles (SEPs) with energy higher than 1 MeV. The He-rich schreibersite is evidence for the active young Sun, as suggested by X-ray observations of pre-main-sequence stars of near solar mass, which produce much more frequent and powerful X-ray flares than the contemporary Sun [10].
References: [1] Wieler (2002) RiMG vol. 47, 21–70. [2] Nagao et al. (2011) Science 333, 1128–1131. [3] Okazaki et al. (2023) Science 379, eabo0431. [4] Wada et al. (2023) JpGU2023, PPS08-12 (abstract). [5] Obase et al. (2021) GCA 312, 75–105. [6] Harries and Langenhorst (2013) M&PS 48, 879–903. [7] Jilly-Rehak et al. (2017) GCA 201, 224–244. [8] Feigelson and Montmerle (1999) ARAA 37, 363–408. [9] Ziegler, http://www.srim.org. [10] Feigelson et al. (2002) ApJ 572, 335–349.
A polished section of MIL 090657 was prepared for this study. The He-rich clast was found by LIMAS isotope imaging of 4He, 24Mg, 32S, and 58Ni from this section. The mineralogy of this clast was analyzed by SEM/EDS and TEM/EDS.
The He-rich clast is ~10 μm in diameter embedded in a fine-grained matrix of MIL 090657. The clast mainly consists of a porous aggregate of nanocrystals. The nanocrystals mainly consist of nanocrystalline iron sulfide (pyrrhotite or troilite). Nanocrystals of magnetite and Ni-rich sulfide are also present in the aggregate. Schreibersite crystals (2 μm in maximum) are embedded in the aggregate. Euhedral pyrrhotite crystals (3 μm in maximum) are present on the periphery of the clast. A high concentration of 4He was detected from the schreibersite crystals (~4×1019 atoms/cm3), while 4He was not detected from the porous aggregate and the euhedral pyrrhotite. 4He imaging using a primary beam with a small spot size (~300 nm) revealed that 4He is uniformly distributed in the 2 μm schreibersite crystal.
The porous texture and mineral composition suggest that the porous aggregate was formed by decomposition of tochilinite [6]. The tochilinite and the euhedral pyrrhotite are likely formed by aqueous alteration on the CR chondrite parent body that occurred at 4–13 Myr after the CAI formation [7]. The fact that 4He is only concentrated in schreibersite indicates that the incorporation of 4He occurred before the aqueous alteration. At this time, the Sun was likely in its pre-main-sequence phase (~10 Myr after the formation [8]).
The penetration depth of the current solar wind 4He (~4 keV on average) in schreibersite is ~20 nm (numerically simulated by the TRIM program [9]). This is inconsistent with the uniform distribution of 4He in the 2 μm schreibersite crystal. The 4He distribution and high concentration suggest that the schreibersite crystals were exposed to a large fluence of solar energetic particles (SEPs) with energy higher than 1 MeV. The He-rich schreibersite is evidence for the active young Sun, as suggested by X-ray observations of pre-main-sequence stars of near solar mass, which produce much more frequent and powerful X-ray flares than the contemporary Sun [10].
References: [1] Wieler (2002) RiMG vol. 47, 21–70. [2] Nagao et al. (2011) Science 333, 1128–1131. [3] Okazaki et al. (2023) Science 379, eabo0431. [4] Wada et al. (2023) JpGU2023, PPS08-12 (abstract). [5] Obase et al. (2021) GCA 312, 75–105. [6] Harries and Langenhorst (2013) M&PS 48, 879–903. [7] Jilly-Rehak et al. (2017) GCA 201, 224–244. [8] Feigelson and Montmerle (1999) ARAA 37, 363–408. [9] Ziegler, http://www.srim.org. [10] Feigelson et al. (2002) ApJ 572, 335–349.