09:45 〜 10:05
[PPS08-04] Shock history of ordinary chondrites: 40Ar/39Ar ages and shock features
★Invited Papers
キーワード：隕石、衝撃変成作用、コンドライト、アルゴン年代
We have investigated shock metamorphism and 40Ar/39Ar ages (hereafter Ar/Ar ages) of ordinary chondrites to reveal their collisional history from the early solar system to the present. Chondrites record various degrees of shock metamorphism due to impact events, and its degrees are classified from shock stage (C-S) 1 to 6 (e.g., Stöffler et al. 2018). Since impact heating enhances degassing of radiogenic Ar retained in rocks, Ar/Ar ages are thought to be disturbed or reset by impact and represent an age of the last impact (heating) event. However, the duration of shock heating is sometimes too short to completely degas radiogenic Ar (e.g., Bogard et al. 1995). Therefore, to precisely interpret meteorites' Ar/Ar ages, we have to reveal its shock history based on shock metamorphic features. While several studies intended to disclose chondrites' shock histories based on shock features combined with Ar/Ar age data (e.g., Friedrich et al. 2014, 2017), the number of such studies is still limited and more analyses should be conducted on many meteorites. Here, we report our results focusing on Ar/Ar ages and shock features of ordinary chondrites and discuss their collisional histories.
We prepared 36 ordinary chondrites. We observed polished thin sections of each meteorite by an optical microscope and FE-SEM at National Institute of Polar Research. Chips of each meteorite were irradiated by neutrons at Institute for Integrated Radiation and Nuclear Science, Kyoto University. Ar/Ar analyses were performed at the University of Tokyo. Detailed data reduction is described in Takenouchi et al. (2021). Although we have observed shock features of the above meteorites, the Ar/Ar analysis is ongoing, and we have obtained the Ar/Ar ages of 18 meteorites.
Seagraves (c) (L6, C-S5) shows strong mosaicism of olivine and contains high-pressure minerals such as garnet. This meteorite yielded a partial degassing pattern in Ar/Ar age spectra, indicating degassing event at <1.6-4.2 Ga. Shock melt rock Yamato-790519 (LL) shows Ar/Ar ages of <1.9 Ga and <2.7-3.1 Ga at low-temperature and high-temperature fractions, respectively. The above two meteorites experienced severe shock metamorphism. However, the shock events could not wholly reset the Ar/Ar ages. H chondrites exhibiting wavy extinctions and/or weak mosaicism (C-S3-4) have various Ar/Ar age spectra patterns. For example, Tulia (a) (H3-4, C-S3) shows a partial degassing pattern (<2.0-4.2) similar to Seagraves (c), while Hassayampa (H4, C-S3) and Plainview (1917) (H5 clast, C-S4) have plateau ages at 4.58±0.10 Ga, and 4.46±0.11 Ga with weak partial degassing at low-temperature fractions, respectively. Most meteorites with C-S1-2 show plateau ages around 4.5 Ga, and these ages are indistinguishable from the formation ages of the meteorites. On the other hand, Mills (H6, C-S2) shows a plateau age at 4.21±0.10 Ga, significantly younger than chondrites' formation ages.
Ar/Ar ages of shocked meteorites (>C-S3) were partly reset, while those of unshocked meteorite (C-S2) were completely reset. This result indicates that Ar/Ar ages are not always completely reset by shock metamorphism of <S5 and shock metamorphism of C-S3-4 hardly induces complete Ar extraction due to short heating duration. When Ar is thoroughly degassed by shock heating, such heat may also erase shock metamorphic features and lower the shock stage to below C-S2. Regarding meteorites with C-S3-4, various degrees of degassing indicate that heating effects vary within similar shock stages. If a heating effect is weak enough to preserve a plateau age, such as Hassayampa and Plainview (1917), the age may not indicate an impact age. When we refer to literature data to discuss impact ages, we must pay attention to this point.
Impact events with a strong heating effect, such as the case of Mills, may have occurred until ~4.2 Ga. This result possibly indicates changes in impact conditions around that time. We need to continue further analyses to reveal the history of collisional events in the solar system.
We prepared 36 ordinary chondrites. We observed polished thin sections of each meteorite by an optical microscope and FE-SEM at National Institute of Polar Research. Chips of each meteorite were irradiated by neutrons at Institute for Integrated Radiation and Nuclear Science, Kyoto University. Ar/Ar analyses were performed at the University of Tokyo. Detailed data reduction is described in Takenouchi et al. (2021). Although we have observed shock features of the above meteorites, the Ar/Ar analysis is ongoing, and we have obtained the Ar/Ar ages of 18 meteorites.
Seagraves (c) (L6, C-S5) shows strong mosaicism of olivine and contains high-pressure minerals such as garnet. This meteorite yielded a partial degassing pattern in Ar/Ar age spectra, indicating degassing event at <1.6-4.2 Ga. Shock melt rock Yamato-790519 (LL) shows Ar/Ar ages of <1.9 Ga and <2.7-3.1 Ga at low-temperature and high-temperature fractions, respectively. The above two meteorites experienced severe shock metamorphism. However, the shock events could not wholly reset the Ar/Ar ages. H chondrites exhibiting wavy extinctions and/or weak mosaicism (C-S3-4) have various Ar/Ar age spectra patterns. For example, Tulia (a) (H3-4, C-S3) shows a partial degassing pattern (<2.0-4.2) similar to Seagraves (c), while Hassayampa (H4, C-S3) and Plainview (1917) (H5 clast, C-S4) have plateau ages at 4.58±0.10 Ga, and 4.46±0.11 Ga with weak partial degassing at low-temperature fractions, respectively. Most meteorites with C-S1-2 show plateau ages around 4.5 Ga, and these ages are indistinguishable from the formation ages of the meteorites. On the other hand, Mills (H6, C-S2) shows a plateau age at 4.21±0.10 Ga, significantly younger than chondrites' formation ages.
Ar/Ar ages of shocked meteorites (>C-S3) were partly reset, while those of unshocked meteorite (C-S2) were completely reset. This result indicates that Ar/Ar ages are not always completely reset by shock metamorphism of <S5 and shock metamorphism of C-S3-4 hardly induces complete Ar extraction due to short heating duration. When Ar is thoroughly degassed by shock heating, such heat may also erase shock metamorphic features and lower the shock stage to below C-S2. Regarding meteorites with C-S3-4, various degrees of degassing indicate that heating effects vary within similar shock stages. If a heating effect is weak enough to preserve a plateau age, such as Hassayampa and Plainview (1917), the age may not indicate an impact age. When we refer to literature data to discuss impact ages, we must pay attention to this point.
Impact events with a strong heating effect, such as the case of Mills, may have occurred until ~4.2 Ga. This result possibly indicates changes in impact conditions around that time. We need to continue further analyses to reveal the history of collisional events in the solar system.