11:00 AM - 1:00 PM
[PPS08-P03] Oxygen isotopic compositions of olivine, low-Ca pyroxene and spinel in the Hayabusa-2 returned sample from Ryugu
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.