[P-PS08] 太陽系物質進化

[PPS08-P03] リュウグウ試料中のオリビン，パイロキシン，スピネルの酸素同位体組成とその起源

*川崎 教行¹、永島 一秀²、坂本 直哉¹、馬上 謙一¹、和田 壮平¹、圦本 尚義¹、The Hayabusa2-initial-analysis chemistry team、The Hayabusa2-initial-analysis core (1.北海道大学、2.ハワイ大学)

Mineralogy, petrology, and chemistry of the Hayabusa-2 returned samples from Ryugu indicate their kinships with the CI (Ivuna-like) carbonaceous chondrites (Nakamura et al., 2022; Noguchi et al., 2022; Yokoyama et al., 2022). Similar to the rare occurrences of anhydrous silicates in the CI chondrites, the Ryugu samples contain small amounts of primary phases such as olivine, low-Ca pyroxene, and spinel (Nakamura et al., 2022). In this study, we measured O-isotopic compositions of these minerals in the polished section C0002-C1001 prepared from a sample from the 2nd touchdown site on Ryugu to decipher their origin. Those minerals in the Ivuna CI chondrite were also measured for comparison. In situ O-isotope measurements were conducted using a SIMS instrument (Cameca ims-1280HR) at Hokkaido University.
Olivine, low-Ca pyroxene, and spinel grains are observed in the “less-altered lithologies” of the polished sections of C0002 (Figs. 1a, 1b) and Ivuna, where calcite is also observed rather than dolomite and breunnerite, consistent with the previous observation (Nakamura et al., 2022). Most of olivine grains in C0002 and Ivuna show MgO-rich compositions with Fo_>~98. Figures 1c and 1d show O-isotopic compositions of olivine, low-Ca pyroxene, and spinel in C0002 and Ivuna. They are distributed along the Primitive Chondrule Mineral (PCM) line (Ushikubo et al., 2012) with Δ¹⁷O ranging between ~ −24‰ and −2‰ for C0002 and ~ −24‰ and 0‰ for Ivuna. Among the 6 olivine grains measured in C0002, 3 are ¹⁶O-poor (Δ¹⁷O ~ −5‰) and 3 are ¹⁶O-rich (Δ¹⁷O ~ −24‰). The low-Ca pyroxene is ¹⁶O-poor (Δ¹⁷O ~ −5‰). For Ivuna, among the 30 grains measured (28 olivines and 2 low-Ca pyroxenes), 23 grains, including all low-Ca pyroxene grains, are ¹⁶O-poor (Δ¹⁷O ~ −7‰ to 0‰), while 7 olivine grains are ¹⁶O-rich (Δ¹⁷O ~ −24‰). These proportions are similar to that for ferromagnesian 81P/Wild2 particles, for which 20 of 26 grains are ¹⁶O-poor (Defouilloy et al., 2017 and references therein). All measured Mg-Al spinel grains are ¹⁶O-rich (Δ¹⁷O ~ −25‰).
The ¹⁶O-rich olivine and spinel grains in C0002 (Δ¹⁷O = −24.4 ± 1.6, 2SD) and Ivuna (Δ¹⁷O = −24.6 ± 1.1‰) have identical Δ¹⁷O values. They are also identical to those of olivine and spinel in Ca-Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) from carbonaceous chondrites, −23 to −24‰ (e.g., Makide et al., 2009; Ushikubo et al., 2017; Kawasaki et al., 2018; Fukuda et al., 2021), indicating that the ¹⁶O-rich olivine and spinel grains in C0002 and Ivuna are originated from CAIs, AOAs, and/or related ¹⁶O-rich objects.
The ¹⁶O-poor olivine and low-Ca pyroxene in C0002 and Ivuna range between Δ¹⁷O ~ −7‰ and 0‰ and in Ivuna FeO-rich olivine grains (Fo_<~90) tend to have ¹⁶O-poor compositions. These are consistent with the O-isotope data for olivine and low-Ca pyroxene in chondrules in carbonaceous chondrites (e.g., Tenner et al., 2018 and references therein) and ¹⁶O-poor ferromagnesian 81P/Wild2 particles (Defouilloy et al., 2017 and references therein), suggesting that the ¹⁶O-poor olivine and low-Ca pyroxene grains in C0002 and Ivuna are originated from chondrules, and/or related objects formed in the similar conditions to chondrules.

The Hayabusa2-initial-analysis chemistry team: T. Yokoyama, K. Nagashima, Y. Abe, J. Aléon, C. M. O'D. Alexander, S. Amari, Y. Amelin, K. Bajo, M. Bizzarro, A. Bouvier, R. W. Carlson, M. Chaussidon, B-G. Choi, N. Dauphas, A. M. Davis, T. D. Rocco, W. Fujiya, R. Fukai, I. Gautam, M. K. Haba, Y. Hibiya, H. Hidaka, H. Homma, P. Hoppe, G. R. Huss, K. Ichida, T. Iizuka, T. R. Ireland, A. Ishikawa, M. Ito, S. Itoh, N. Kawasaki, N. T. Kita, K. Kitajima, T. Kleine, S. Komatani, A. N. Krot, M-C. Liu, Y. Masuda, K. D. McKeegan, M. Morita, K. Motomura, F. Moynier, I. Nakai, A. Nguyen, L. Nittler, M. Onose, A. Pack, C. Park, L. Piani, L. Qin, S. S. Russell, N. Sakamoto, M. Schönbächler, L. Tafla, H. Tang, K. Terada, Y. Terada, T. Usui, S. Wada, M. Wadhwa, R. J. Walker, K. Yamashita, Q-Z. Yin, S. Yoneda, E. D. Young, H. Yui, A-C. Zhang, H. Yurimoto.
The Hayabusa2-initial-analysis core: S. Tachibana, T. Nakamura, H. Naraoka, T. Noguchi, R. Okazaki, K. Sakamoto, H. Yabuta, H. Yurimoto, Y. Tsuda, S. Watanabe.

Figure 1. Combined X-ray elemental maps with (a) Mg (red), Ca (green), and Al (blue) and (b) Mg (red), Si (green), and Fe (blue) of “less-altered lithology” in the polished section C0002-C1001. O-isotopic compositions of olivine, low-Ca pyroxene, and spinel in (c) C0002 and (d) Ivuna. Errors correspond to 2σ. Al-Sp, Mg-Al spinel; Bru, breunnerite; Cal, calcite; Cr-Sp, Cr-rich spinel; Dol, dolomite; Ol, olivine; Px, pyroxene.