11:00 〜 13:00
[PPS08-P05] Carbon-isotope compositions of dolomite in Ryugu samples returned by the Hayabusa2 mission
キーワード：はやぶさ２、リュウグウ、炭酸塩鉱物、炭素同位体比
The Ryugu samples returned by the Hayabusa2 spacecraft are CI chondrite-like materials [1]. They underwent extensive aqueous alteration and consist predominantly of secondary minerals (phyllosilicates, carbonates, magnetite, sulfides) likely formed in the presence of liquid water in the putative Ryugu parent body. Carbonates are of the major C-bearing components in Ryugu and in aqueously altered chondrites. However, the sources of their C are not well understood. Here, we report the C-isotope compositions of the Ryugu dolomites (CaMg(CO3)2), and investigate the carbon sources and the volatile materials accreted to the Ryugu parent body.
We analyzed 6 and 7 dolomite grains from the polished sections of A0058-C1001 and C0002-C1001, respectively. We also analyzed 7 dolomite grains from Ivuna (CI) as a reference. Carbon-isotope measurements of dolomites were performed with the CAMECA ims-1280HR SIMS at Hokkaido University. We used a suite of dolomite-ankerite standards [2].
The analyzed Ryugu dolomites have a relatively narrow range of δ13CPDB values for both samples, 67–75‰ (Fig. 1). These δ13C values are consistent with those of bulk measurements of Ivuna and Orgueil (CI) carbonates (71‰ and 69‰, respectively) [3] and Ivuna dolomites measured in situ (67–72‰; this study). They are also close to δ13C of Tagish Lake (C2 ungrouped) carbonates (68‰) [4]. Carbonates from CM chondrites exhibit much larger δ13C variations (e.g., 20–80‰) [5]. The carbonate C abundance of the Ryugu samples is ~1.6 wt.%, which is much higher than that of CIs (0.1–0.3 wt.%) and close to Tagish Lake (~1.3 wt.%) [4]. The high δ13C values and high abundance of Ryugu carbonates could be explained by a large amount of 13C-rich inorganic ice (likely CO2) accreted by the Ryugu parent body, as proposed for Tagish Lake carbonates [6]. Alternatively, given the low-temperature (37±10°C) formation of Ryugu dolomites [1], the 13C-rich compositions could have resulted from a large C-isotope fractionation between carbonates and CO and/or CH4 at low temperatures, as proposed to explain the large δ13C variations of CM carbonates [3]. Although oxidation of organic matter likely produced some of the carbonate C, this source could only account for not more than one-third of the carbonate C [3]. Thus, assuming that the observed carbonate abundance represents that of Ryugu and that the aqueous alteration occurred in a closed system, both of which must be verified by future works, the high abundance of Ryugu dolomites may suggest a significant amount of CO2/CO/CH4-bearing ice accreted by the Ryugu parent body.
References: [1] Yurimoto et al. (2022) 53th LPSC, #1377. [2] Śliwiński et al. (2016) Geostand. Geoanal. Res. 40, 157. [3] Alexander et al. (2015) MAPS 50, 810. [4] Grady et al. (2002) MAPS 37, 713. [5] Vacher et al. (2017) GCA 213, 271. [6] Fujiya et al. (2019) Nat. Astron. 3, 910. [7] Nagashima et al. (2022) this meeting.
The Hayabusa2-initial-analysis chemistry team: T. Yokoyama, K. Nagashima, Y. Abe, J. Aléon, C.M.O'D. Al-exander, S. Amari, Y. Amelin, K. Bajo, M. Bizzarro, A. Bouvier, R. W. Carlson, M. Chaussidon, B.-G. Choi, N. Dau-phas, A.M. Davis, T. Di 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. Kitaji-ma, T. Kleine, S. Komatani, A.N. Krot, M.-C. Liu, Yuki 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. δ13C vs. δ18O values of Ryugu dolomites. δ18O values are from [7].
We analyzed 6 and 7 dolomite grains from the polished sections of A0058-C1001 and C0002-C1001, respectively. We also analyzed 7 dolomite grains from Ivuna (CI) as a reference. Carbon-isotope measurements of dolomites were performed with the CAMECA ims-1280HR SIMS at Hokkaido University. We used a suite of dolomite-ankerite standards [2].
The analyzed Ryugu dolomites have a relatively narrow range of δ13CPDB values for both samples, 67–75‰ (Fig. 1). These δ13C values are consistent with those of bulk measurements of Ivuna and Orgueil (CI) carbonates (71‰ and 69‰, respectively) [3] and Ivuna dolomites measured in situ (67–72‰; this study). They are also close to δ13C of Tagish Lake (C2 ungrouped) carbonates (68‰) [4]. Carbonates from CM chondrites exhibit much larger δ13C variations (e.g., 20–80‰) [5]. The carbonate C abundance of the Ryugu samples is ~1.6 wt.%, which is much higher than that of CIs (0.1–0.3 wt.%) and close to Tagish Lake (~1.3 wt.%) [4]. The high δ13C values and high abundance of Ryugu carbonates could be explained by a large amount of 13C-rich inorganic ice (likely CO2) accreted by the Ryugu parent body, as proposed for Tagish Lake carbonates [6]. Alternatively, given the low-temperature (37±10°C) formation of Ryugu dolomites [1], the 13C-rich compositions could have resulted from a large C-isotope fractionation between carbonates and CO and/or CH4 at low temperatures, as proposed to explain the large δ13C variations of CM carbonates [3]. Although oxidation of organic matter likely produced some of the carbonate C, this source could only account for not more than one-third of the carbonate C [3]. Thus, assuming that the observed carbonate abundance represents that of Ryugu and that the aqueous alteration occurred in a closed system, both of which must be verified by future works, the high abundance of Ryugu dolomites may suggest a significant amount of CO2/CO/CH4-bearing ice accreted by the Ryugu parent body.
References: [1] Yurimoto et al. (2022) 53th LPSC, #1377. [2] Śliwiński et al. (2016) Geostand. Geoanal. Res. 40, 157. [3] Alexander et al. (2015) MAPS 50, 810. [4] Grady et al. (2002) MAPS 37, 713. [5] Vacher et al. (2017) GCA 213, 271. [6] Fujiya et al. (2019) Nat. Astron. 3, 910. [7] Nagashima et al. (2022) this meeting.
The Hayabusa2-initial-analysis chemistry team: T. Yokoyama, K. Nagashima, Y. Abe, J. Aléon, C.M.O'D. Al-exander, S. Amari, Y. Amelin, K. Bajo, M. Bizzarro, A. Bouvier, R. W. Carlson, M. Chaussidon, B.-G. Choi, N. Dau-phas, A.M. Davis, T. Di 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. Kitaji-ma, T. Kleine, S. Komatani, A.N. Krot, M.-C. Liu, Yuki 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. δ13C vs. δ18O values of Ryugu dolomites. δ18O values are from [7].