Presentation Information
[PPS12-11]Mineralogical Study of Shock Melt in the Sample from Asteroid Ryugu
*Taiga Takase1, Megumi Matsumoto1, Tomoki Nakamura1, Yoshino Fukuda1, Rieto Kawada1, Seima Ishida1, Shohei Yamashita2, Yoshio Takahashi3 (1.Tohoku University, 2.High Energy Accelerator Research Organization (KEK) , 3.The University of Tokyo)
Keywords:
Ryugu,Carbonaceous Asteroid,Shock melting,TEM/STEM-EDS,STXM,SEM-EDS
Observations by the Hayabusa 2 spacecraft and initial analyses of the returned samples revealed that asteroid Ryugu is a rubble-pile body formed via large-scale disruption due to collisional events and subsequent reaccumulation (e.g., Watanabe et al. 2019; Nakamura et al. 2022). These disruptive events might have occurred more than once (Walsh et al. 2024). In contrast, most Ryugu samples show no evidence of shock events. Previous studies reported shock-related microfaults and dislocations in calcite grains from Ryugu samples, which formed by shock events with shock induced pressures lower than ~5 GPa (Tomioka et al. 2022, 2023, 2025; Miyahara et al. 2024; Nakahashi et al. 2025). In this presentation, we report the mineralogical characteristics of newly identified shock melts in a Ryugu sample and estimate the temperature and pressure condition of the shock event which formed the shock melt.
The shock melt was identified within a ~150 μm–sized particle (C0213-036) picked up from a Ryugu aggregate sample (C0213). This particle was first analyzed by synchrotron X-ray diffraction (XRD) at BL-3A of the KEK Photon Factory. Subsequently, the particle was cut and polished using a Xe plasma focused ion beam system (pFIB; TESCAN S8000X), and the polished section was observed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Two FIB sections were prepared from the shock melt on the polished section using a Ga focused ion beam system (FIB; FEI Quanta 200i 3D). These sections were analyzed by scanning transmission X-ray microscopy (STXM) at BL-19A of the KEK Photon Factory to obtain C K-edge and Fe L-edge X-ray absorption near-edge structure (XANES) spectra. After STXM analysis, microstructural observations and elemental analyses were performed using (scanning) transmission electron microscopy (TEM/STEM-EDS; JEOL JEM-2100F).
In XRD analysis, saponite, serpentine, olivine, magnetite, pyrrhotite, and pentlandite were identified as major crystalline phases, and (001) basal reflections of the phyllosilicates were weak and broad. These are consistent with the less altered lithology of Ryugu samples, suggesting a relatively low degree of aqueous alteration (Nakamura et al. 2022; Mikouchi et al. 2022).
The shock melt is a size of ~25 μm and located inside of C0213-036. Melt layers formed on surfaces of Ryugu samples by space weathering are generally small (thicknesses of <10 μm; Noguchi et al. 2023) compared to the shock melt observed in this study.
The shock melt is mainly composed of nanosized Fe sulfides embedded in a silicate matrix. Small amounts of organic matter, olivine, Fe–Ni phosphide, chromite, and taenite as well as spherical vesicles with diameters ranging from ~100 nm to ~3 μm are present in the matrix. This suggests that the matrix was once melted. Selected-area electron diffraction of the silicate part in the matrix shows a halo pattern, indicating that it formed via quenching of melt of precursor phyllosilicates. The (Mg+Fe)/Si ratio of the silicate matrix is approximately 1.1.
In pulse-heating experiments on the Orgueil meteorite which has a similar mineralogical composition with Ryugu samples, it has been reported that vesicular melt was formed at temperatures >1000 °C (Rudraswami et al. 2025). The shock melt in this study might have also experienced such high temperatures. In addition, shock experiments on the Orgueil meteorite using a propellant gun showed that shock melt with nanosized Fe sulfides, melt, and vesicles formed at high pressure conditions (natural shock pressures of ~10 GPa), but not at low pressure conditions (~4 GPa; Nakahashi et al. 2025). These suggest that the shock melt in this study might have experienced shock pressures close to 10 GPa.
The shock melt was identified within a ~150 μm–sized particle (C0213-036) picked up from a Ryugu aggregate sample (C0213). This particle was first analyzed by synchrotron X-ray diffraction (XRD) at BL-3A of the KEK Photon Factory. Subsequently, the particle was cut and polished using a Xe plasma focused ion beam system (pFIB; TESCAN S8000X), and the polished section was observed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Two FIB sections were prepared from the shock melt on the polished section using a Ga focused ion beam system (FIB; FEI Quanta 200i 3D). These sections were analyzed by scanning transmission X-ray microscopy (STXM) at BL-19A of the KEK Photon Factory to obtain C K-edge and Fe L-edge X-ray absorption near-edge structure (XANES) spectra. After STXM analysis, microstructural observations and elemental analyses were performed using (scanning) transmission electron microscopy (TEM/STEM-EDS; JEOL JEM-2100F).
In XRD analysis, saponite, serpentine, olivine, magnetite, pyrrhotite, and pentlandite were identified as major crystalline phases, and (001) basal reflections of the phyllosilicates were weak and broad. These are consistent with the less altered lithology of Ryugu samples, suggesting a relatively low degree of aqueous alteration (Nakamura et al. 2022; Mikouchi et al. 2022).
The shock melt is a size of ~25 μm and located inside of C0213-036. Melt layers formed on surfaces of Ryugu samples by space weathering are generally small (thicknesses of <10 μm; Noguchi et al. 2023) compared to the shock melt observed in this study.
The shock melt is mainly composed of nanosized Fe sulfides embedded in a silicate matrix. Small amounts of organic matter, olivine, Fe–Ni phosphide, chromite, and taenite as well as spherical vesicles with diameters ranging from ~100 nm to ~3 μm are present in the matrix. This suggests that the matrix was once melted. Selected-area electron diffraction of the silicate part in the matrix shows a halo pattern, indicating that it formed via quenching of melt of precursor phyllosilicates. The (Mg+Fe)/Si ratio of the silicate matrix is approximately 1.1.
In pulse-heating experiments on the Orgueil meteorite which has a similar mineralogical composition with Ryugu samples, it has been reported that vesicular melt was formed at temperatures >1000 °C (Rudraswami et al. 2025). The shock melt in this study might have also experienced such high temperatures. In addition, shock experiments on the Orgueil meteorite using a propellant gun showed that shock melt with nanosized Fe sulfides, melt, and vesicles formed at high pressure conditions (natural shock pressures of ~10 GPa), but not at low pressure conditions (~4 GPa; Nakahashi et al. 2025). These suggest that the shock melt in this study might have experienced shock pressures close to 10 GPa.