Distinct ages and temperatures of aqueous activities recorded in Ryugu samples
Kazuhide Nagashima1, Noriyuki Kawasaki2, Naoya Sakamoto2, Wataru Fujiya3, Hisayoshi Yurimoto2, The Hayabusa2-initialanalysis chemistry team, and The Hayabusa2-initial-analysis core.
1
University of Hawai‘i at Mānoa 1, 2Hokkaido University, 3Ibaraki University
The Ryugu asteroid is a rubble pile body likely composed of materials ejected by impact(s) from a larger parent asteroid[e.g., 1] that may have formed far from the Sun [e.g., 2,3]. The Ryugu samples collected by the Hayabusa2 spacecraft thus may
have originated from different parts of the parent asteroid and experienced different thermal histories. We reported thatRyugu's secondary minerals such as dolomite and magnetite formed at 37±10°C, ~5.2 Ma after the formation of the Solar
System based on oxygen and Mn-Cr isotope systematics of the sample A0058 collected from the first touch-down site on
Ryugu [4]. Here we report oxygen and Mn-Cr isotope systematics of secondary minerals found in a Ryugu sample, C0002,
collected from the 2nd touch-down site.
In contrast to A0058-C1001 section, C0002-C1001 section is composed of different lithological units recognizable bydifferences in BSE contrast and elemental distributions in X-ray maps, including less-altered lithologies [e.g., 3,5]. Most
lithologies are dominated by phyllosilicate matrices with sub-micron magnetite and sulfide grains. Larger grains (>10 µm) ofmagnetite, dolomite, and pyrrhotite are scattered throughout the major lithologies.
Figure 1 shows O-isotope compositions of dolomite, magnetite from the C0002 section, together with those from A0058C1001. The O-isotope compositions of the dolomites are consistent with those in A0058, and are similar to the bulk Ryugu
within uncertainty for their Δ17O values [4,6,7]. In contrast to the majority of magnetites in A0058 having Δ17O of ~ 0‰, allmagnetites but one in C0002 have higher Δ17O, ~ 2−3‰ (see also [8]). Some magnetites with high Δ17O are included inovergrown dolomites with Δ17O of ~ 0‰, suggesting earlier formation of these magnetites than the dolomite. Theseobservations suggest that O-isotope composition of aqueous fluid was Δ17O ~ +3‰ before crystallization of dolomite and was
Δ17O > +3‰ at the beginning stage of aqueous alteration because olivine and pyroxene in Ryugu have lower Δ17O (–24 to –5‰) [3,9,10].
Using O-isotope thermometry [e.g., 11], the dolomite and magnetite pair in A0058 was used to estimate a temperature atwhich these minerals precipitated; their O-isotope compositions correspond to 37±10°C [4]. In C0002, a pair of dolomite andmagnetite in the same lithology has identical Δ17O, ~+0.6‰. Assuming these grains were in O-isotope equilibrium with thesame fluid, their difference in δ18O, 25.2±2.0‰, corresponds to 104±22°C.
Figure 2 shows Mn-Cr isotope systematics of dolomites in A0058-C1001, C0002-C1001, and Ivuna CI chondrite. Note wecollected Mn-Cr isotope data from dolomites from which the O-isotope data were obtained. All 53Cr excesses are wellcorrelated with Mn-Cr ratios. The inferred initial 53Mn/55Mn ratios are (2.55±0.35)×10 6, (3.78±0.34)×10 6, and(3.14±0.25)×10 6 for A0058-C1001 [4], C0002-C1001 [this study], and Ivuna [4, this study], respectively. The initial53
Mn/55Mn ratio we obtained from C0002 is consistent with that in [10] and that from bulk Mn-Cr data [12] within uncertainty.
If we use the initial 53Mn/55Mn ratio of the D’Orbigny angrite and the U-corrected Pb-Pb ages of D’Orbigny and CV CAIs
[13−15], the initial 53Mn/55Mn ratios for A0058 and C0002 suggest that dolomite precipitation occurred at 5.2 (+0.8/−0.7) Ma
and 3.1 (+0.5/−0.5) Ma after the CV CAI formation, at ~40°C and ~100°C, respectively. These ages could be systematicallychanged ±million years due to inconsistencies in CAI ages and proposed initial 53Mn/55Mn ratios of the Solar System [4 andreferences therein]. Other systematic changes may be introduced by inaccurate corrections of Mn/Cr sensitivity for dolomite
by SIMS [4, 16−18]. Despite these systematic uncertainties, the relative age between the two isochrons is robust: 2.1±0.9 Ma.
Therefore, during the aqueous alteration, one location in the Ryugu parent asteroid was at ~100°C. Then ~2 Ma later, possibly
another location in the asteroid experienced ~40°C. These are new conditions to restrict thermal models of the Ryugu parentbody. Thermal modeling to satisfy the conditions is in progress using methods in [19].
References: [1] Sugita S. et al. Science. 364. eaaw0422 (2019). [2] Hopp T. et al. Science Adv., accepted. [3] Kawasaki N. etal. Science Adv., in review. [4] Yokoyama T. et al. Science, 10.1126/science.abn7850 (2022) [5] Nakamura T. et al. Science,10.1126/science.abn8671 (2022). [6] Tang H. et al. Science Adv., in review. [7] Fujiya W. et al. this volume. [8] Kita N. T. etal. this volume. [9] Liu M.-C. et al. Nature Astron., 10.1038/s41550-022-01762-4 (2022). [10] Nakamura E. et al. Proc. JapanAcad. B., 98, 227−282. (2022). [11] Zheng Y.-F. Geochem. J., 45, 341−354 (2011). [12] Yokoyama T. et al. 53rd LPSC
abstract #1272 (2022). [13] Glavin D. P. et al. MAPS, 39, 693−700 (2004). [14] Brennecka G. A. and Wadhwa M. PNAS, 109,9299−9303 (2012). [15] Connelly J. N. et al. Science, 338, 651−655 (2012) [16] Steel R. C. et al. Geochim. Cosmochim. Acta,201, 245−259 (2017). [17] McCain K. A. et al. J. Vac. Sci. Technol., 38, 044005 (2020). [18] Sugawara S. et al. this volume.
[19] Fujiya W. et al. ApJL, 924:L16 (2022).
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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.
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. Kitajima, 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.

Fig. 1. Oxygen isotope compositions of dolomite and magnetite from Ryugu samples, A0058-C1001 [4] and C0002-C1001 [7,this study]. Also shown are bulk O-isotope data of A and C samples [4,6].

Fig. 2. Mn-Cr isotope systematics of dolomites from (a) Ryugu samples, A0058-C1001 [4] and C0002-C1001 [this study], and(b) Ivuna [4, this study]. Several dolomite grains were analyzed from each sample and they are shown as different symbols.
Note data from dolomite grain HK2-#7 in Ivuna, shown as green circles were obtained in the two measurement sessions when
we analyzed A0058-C1001 and C0002-C1001 samples and showed no differences in their initial 53Mn/55Mn ratios.