[P-PS08] Formation and evolution of planetary materials in the Solar System

[PPS08-P12] Oxygen isotope ratios of many olivine particles in Ryugu returned samples

*Miyabi Ueda¹, Tomoki Nakamura¹, Takayuki Ushikubo², Megumi Matsumoto¹, Daisuke Nakashima¹, Hideto Yoshida³, Takashi Mikouchi³ (1.Tohoku University, 2.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 3.Tokyo University)

Keywords:Hayabusa2, Ryugu, Oxygen isotope ratio, Olivine

Although it has been known that protoplanetary disks consist of two distinct regions (Warren et al. 2011), isotopic ratios of samples recovered from the asteroid Ryugu suggest that there is a third region in the disk and that Ryugu was formed in that region (Hopp et al. 2022; Paquet et al. 2023; Spilzer et al. 2023). CAI and chondrules were found in Ryugu samples (T. Nakamura et al. 2022), but the low abundance of these high-temperature materials suggests that the third region was located outside the other two regions. The presence of carbonate water in the Ryugu samples suggests that the third region was located outside the CO₂ snow line (~10au) at cryogenic temperatures (< -200°C). On the other hand, the Ryugu samples were found to contain many less-altered clasts, with the mineral assemblage different from that of the more-altered major lithology (T. Nakamura et al. 2022). Oxygen isotopic ratios of two CAIs and three chondrule-like materials in the less-altered clasts were similar to those of the Sun, suggesting that these objects were transportedfrom the solar neighborhood (Nakashima et al. 2023).
In this study, we measured the major element compositions and oxygen isotope ratios of 43 olivine particles (mostly contained in less-altered clasts) in the Ryugu samples. Most olivines were fine-grained (< 10 μm), unrounded, irregularly shaped, and Mg-rich . All olivines showed oxygen isotopic compositions along the PCM line (Ushikubo et al. 2012), with 23 olivine particles rich in ¹⁶O (Δ¹⁷O< -13‰) and 20 olivine particles poor in ¹⁶O (Δ¹⁷O> -13‰). ¹⁶O-rich olivine grains generally show oxygen composition close to the sun, but three grains show intermediate compostions between sun and ¹⁶O-poor olivines. The ratio of ¹⁶O-rich olivine particles to ¹⁶O-poor olivine particles was high (¹⁶O-rich olivine particles/¹⁶O-poor olivine particles = 1). This result is in agreement with previous studies (six olivines: Kawasaki et al. 2022; seven olivines: E. Nakamura et al. 2022). Olivine particles rich in ¹⁶O tend to be enriched in Mn and low in Cr, while those poor in ¹⁶O tend to be rich in Cr and low in Mn. On the other hand, there were many olivine particles with low concentrations of both Mn and Cr.
Based on oxygen isotope ratios and trace element abundances, the ¹⁶O-rich olivine particles are considered to be similar to AOA olivine, which is a high-temperature condensate, while the ¹⁶O-poor olivine particles are considered to be similar to chondrule olivine. The olivine particles we measured occur in the same less-altered clasts as the two CAIs measured by Nakashima et al. (2023). Therefore, our results suggest a high AOA/CAI ratio in Ryugu samples. Our results also suggest a high AOA/chondrule ratio in the Ryugu samples, as also pointed out by Kawasaki et al. (2022). The high AOA/CAI ratios are especially important because the AOA/CAI ratios condensed at high-temperature regions close to the Sun are considered to be ~5 based on the molar fraction of solar abundance. The AOA/CAI ratio of Ryugu samples is close to this value, while that of carbonaceous chondrites is low (e.g., CV AOA/CAI ratio ~1; Ebel et al. 2015). This suggests that AOA and CAI condensed in the solar neighborhood are preserved in Ryugu with a high ratio at the time of condensation, while carbonaceous chondrites with a lower AOA/CAI ratio have undergone a process that decreases the AOA/CAI ratio in their formation region.