11:00 〜 13:00
[PPS08-P04] Oxygen isotope compositions of aqueously formed minerals in Ryugu.
キーワード：Ryugu, oxygen isotopes, aqueous alteration, temperature, secondary minerals
Introduction: The Ryugu samples are mainly composed of materials related to the CI (Ivuna-like) carbonaceous chondrites and are dominated by minerals produced by extensive aqueous alteration in the parent planetesimal from which Ryugu was derived [1]. Oxygen-isotope study of Ryugu samples could provide important constraints on the origin and evolution of the water that has caused aqueous activity. Here we report O-isotope compositions of aqueously formed minerals from the 2nd touchdown site on Ryugu.
Experimental: Oxygen-isotope compositions in dolomite, magnetite, and calcite in a polished section C0002-C1001 were measured with the ims-1280HR SIMS at Hokkaido University [2,3] using a suite of matrix-matched standards [4].
Results and Discussion: The C0002-C1001 section is mainly composed of a phyllosilicate matrix. Based on differences in BSE contrast and S content, several lithologies can be identified (Fig. 1a). The area shown in Fig. 1a contains large (>10 µm) grains of pyrrhotite, dolomite, and magnetite surrounded by phyllosilicates.
Figure 1b shows O-isotope compositions of dolomite, magnetite, and calcite from the C0002 section. The O-isotope compositions of the dolomites are consistent with those in A0058, and are similar to the bulk Ryugu [5] within uncertainty for their Δ17O values. In contrast to the majority of magnetites in A0058 having Δ17O of ~ 0‰, all magnetites but one in C0002 have higher Δ17O, ~+2−3‰. The magnetites with high Δ17O include two grains enclosed in a dolomite (indicated by a yellow arrow in Fig. 1a), suggesting earlier formation of these magnetites than the dolomite. The rare calcite grains in C0002 have large variations in their O-isotope compositions: 3 out of 4 grains have Δ17O ~ +2−3‰. These observations and O-isotope compositions of olivine and pyroxene in Ryugu having Δ17O ranging from −24‰ to −5‰ [6] suggest the O-isotope composition of water before extensive interaction with anhydrous silicates could have Δ17O > +3‰.
Using O-isotope thermometry [e.g., 7], the dolomite and magnetite pair in A0058 was used to estimate a temperature at which these minerals precipitated; their O-isotope compositions correspond to 37±10 °C [3]. In C0002, dolomite and magnetite shown in a yellow box in Fig. 1a have identical Δ17O, ~+0.6‰. Assuming these grains were in O-isotope equilibrium with the same fluid, the differences in δ18O between them corresponds to 104±22 °C.
References: [1] Yurimoto et al. (2022) 53th LPSC, #1377. [2] Kawasaki et al. (2018) GCA, 221, 318. [3] Nagashima et al. (2022) 53th LPSC, #1689. [4] Śliwiński et al. (2016) Geostand. Geoanal. Res. 40, 157. [5] Young et al. (2022) 53th LPSC, #1290. [6] Kawasaki et al. (2022) JpGU, this meeting. [7] Zheng (2011) Geochem. J. 45, 341-354.
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. (a) Backscattered electron (BSE) and combined X-ray (Ca-red, Fe-green, S-blue) images of Ryugu C0002-C1001 sample. Scale bar is 100 µm. dol: dolomite, mgt: magnetite, po: pyrrhotite. (b) Oxygen-isotope compositions of dolomite, magnetite, and calcite in Ryugu C0002-C1001. Also shown are data from A0058 section [3] and bulk Ryugu samples [5]. In the bottom diagram, grains used for calculating an equilibration temperature are outlined by red.
Experimental: Oxygen-isotope compositions in dolomite, magnetite, and calcite in a polished section C0002-C1001 were measured with the ims-1280HR SIMS at Hokkaido University [2,3] using a suite of matrix-matched standards [4].
Results and Discussion: The C0002-C1001 section is mainly composed of a phyllosilicate matrix. Based on differences in BSE contrast and S content, several lithologies can be identified (Fig. 1a). The area shown in Fig. 1a contains large (>10 µm) grains of pyrrhotite, dolomite, and magnetite surrounded by phyllosilicates.
Figure 1b shows O-isotope compositions of dolomite, magnetite, and calcite from the C0002 section. The O-isotope compositions of the dolomites are consistent with those in A0058, and are similar to the bulk Ryugu [5] within uncertainty for their Δ17O values. In contrast to the majority of magnetites in A0058 having Δ17O of ~ 0‰, all magnetites but one in C0002 have higher Δ17O, ~+2−3‰. The magnetites with high Δ17O include two grains enclosed in a dolomite (indicated by a yellow arrow in Fig. 1a), suggesting earlier formation of these magnetites than the dolomite. The rare calcite grains in C0002 have large variations in their O-isotope compositions: 3 out of 4 grains have Δ17O ~ +2−3‰. These observations and O-isotope compositions of olivine and pyroxene in Ryugu having Δ17O ranging from −24‰ to −5‰ [6] suggest the O-isotope composition of water before extensive interaction with anhydrous silicates could have Δ17O > +3‰.
Using O-isotope thermometry [e.g., 7], the dolomite and magnetite pair in A0058 was used to estimate a temperature at which these minerals precipitated; their O-isotope compositions correspond to 37±10 °C [3]. In C0002, dolomite and magnetite shown in a yellow box in Fig. 1a have identical Δ17O, ~+0.6‰. Assuming these grains were in O-isotope equilibrium with the same fluid, the differences in δ18O between them corresponds to 104±22 °C.
References: [1] Yurimoto et al. (2022) 53th LPSC, #1377. [2] Kawasaki et al. (2018) GCA, 221, 318. [3] Nagashima et al. (2022) 53th LPSC, #1689. [4] Śliwiński et al. (2016) Geostand. Geoanal. Res. 40, 157. [5] Young et al. (2022) 53th LPSC, #1290. [6] Kawasaki et al. (2022) JpGU, this meeting. [7] Zheng (2011) Geochem. J. 45, 341-354.
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. (a) Backscattered electron (BSE) and combined X-ray (Ca-red, Fe-green, S-blue) images of Ryugu C0002-C1001 sample. Scale bar is 100 µm. dol: dolomite, mgt: magnetite, po: pyrrhotite. (b) Oxygen-isotope compositions of dolomite, magnetite, and calcite in Ryugu C0002-C1001. Also shown are data from A0058 section [3] and bulk Ryugu samples [5]. In the bottom diagram, grains used for calculating an equilibration temperature are outlined by red.