Presentation Information
[PPS12-14]Formation and evolution history of the CY chondrite parent body inferred from shock-metamorphic textures
*Yuto Takaki1, Masaaki Miyahara1, Akira Yamaguchi2, Naotaka Tomioka3 (1.Hiroshima University, 2.National Institute of Polar Research, 3.Kochi Institute for Core Sample Research)
Keywords:
CY Chondrite,Shock metamorphism,Agglutinate
CY chondrites are carbonaceous chondrites that exhibit similar petrographic and chemical characteristics to CI and CM chondrites which are considered to have the most solar-like compositions [1]. A key feature of CY chondrites is that they experienced short-duration thermal metamorphism following aqueous alteration, and impact heating has been proposed as the most plausible heat source for this thermal event [1]. Recent studies have reported similarities between the reflectance spectra of CY chondrites and those of the C-type asteroid Ryugu, from which samples were returned by the Hayabusa2 mission [1]. In addition, Ryugu grains contain microfaults and slickenside-like textures interpreted as impact-related features [2,3]. Furthermore, impact-generated agglutinates have been reported from the Orgueil CI chondrite [4]. This suggests that CI/CY chondrites may have experienced more significant impact processing than previously recognized.
In this study, we aim to constrain the formation and evolutionary history of the CY chondrite parent body through detailed observations and analyses of shock metamorphic textures. The examined samples include CY chondrites (Y 980115, Y 86029, Y 86737, Y 980134, Y 86720, Y 86789, Y 82162, and B 7904) and CY-like CM chondrites (Y 791198 and A 881655). Microtextural observations were conducted using an FE-SEM combined with EDS and EPMA analyses. In addition, focused ion beam (FIB) ultrathin sections of shock-related textures were prepared for transmission electron microscopy (TEM) observations.
The results reveal the presence of three types of lithologies, within single samples: (1) regions that experienced little to no shock metamorphism, (2) agglutinates interpreted to have formed by shock-induced melting, and (3) domains characterized by pervasive subparallel fractures. The agglutinates consist of euhedral olivine crystals with normal zoning, Fe–Ni sulfide particles, vesicles indicative of degassing, and mesostasis glass. Agglutinates were identified in four samples, while subparallel fractures were observed in two samples; both features occur together in the strongly brecciated samples Y 980115 and Y 980134. However, the areal ratio of shock-metamorphosed regions is limited, reaching at most ~2 % for subparallel fractures and ~0.3 % for agglutinates.
Recent shock recovery experiments on CI/CY chondrites suggest that subparallel fracturing and shock-induced melting become prominent at pressures of approximately 4 GPa and 10 GPa, respectively [5]. If the textures observed in this study are of shock origin, their coexistence implies that lithic fragments recording different shock histories can be incorporated within a single meteorite. These observations support a formation model in which the CY chondrite parent body experienced impact-induced fragmentation followed by reaccumulation.
References
[1] King et al. (2019) Geochemistry, 79: 125531. [2] Tomioka et al. (2023) Nature Astronomy, 7: 669–677. [3] Miyahara et al. (2024) Meteoritics & Planetary Science 59: 3181–3192. [4] Zolensky et al. (2022) Meteoritics & Planetary Science 57: 1902-1923. [5] Nakahashi et al. (2025) Earth and Planetary Science Letters, 668: 119559.
In this study, we aim to constrain the formation and evolutionary history of the CY chondrite parent body through detailed observations and analyses of shock metamorphic textures. The examined samples include CY chondrites (Y 980115, Y 86029, Y 86737, Y 980134, Y 86720, Y 86789, Y 82162, and B 7904) and CY-like CM chondrites (Y 791198 and A 881655). Microtextural observations were conducted using an FE-SEM combined with EDS and EPMA analyses. In addition, focused ion beam (FIB) ultrathin sections of shock-related textures were prepared for transmission electron microscopy (TEM) observations.
The results reveal the presence of three types of lithologies, within single samples: (1) regions that experienced little to no shock metamorphism, (2) agglutinates interpreted to have formed by shock-induced melting, and (3) domains characterized by pervasive subparallel fractures. The agglutinates consist of euhedral olivine crystals with normal zoning, Fe–Ni sulfide particles, vesicles indicative of degassing, and mesostasis glass. Agglutinates were identified in four samples, while subparallel fractures were observed in two samples; both features occur together in the strongly brecciated samples Y 980115 and Y 980134. However, the areal ratio of shock-metamorphosed regions is limited, reaching at most ~2 % for subparallel fractures and ~0.3 % for agglutinates.
Recent shock recovery experiments on CI/CY chondrites suggest that subparallel fracturing and shock-induced melting become prominent at pressures of approximately 4 GPa and 10 GPa, respectively [5]. If the textures observed in this study are of shock origin, their coexistence implies that lithic fragments recording different shock histories can be incorporated within a single meteorite. These observations support a formation model in which the CY chondrite parent body experienced impact-induced fragmentation followed by reaccumulation.
References
[1] King et al. (2019) Geochemistry, 79: 125531. [2] Tomioka et al. (2023) Nature Astronomy, 7: 669–677. [3] Miyahara et al. (2024) Meteoritics & Planetary Science 59: 3181–3192. [4] Zolensky et al. (2022) Meteoritics & Planetary Science 57: 1902-1923. [5] Nakahashi et al. (2025) Earth and Planetary Science Letters, 668: 119559.