Oxygen isotope analyses of magnetite in Ryugu
Noriko T. Kita1, Kouki Kitajima1, Kazuhide Nagashima2, Noriyuki Kawasaki3, Naoya Sakamoto3, Hisayoshi Yurimoto3,
The Hayabusa2-initial-analysis chemistry team, The Hayabusa2-initial-analysis core.
1
Department of Geoscience, University of Wisconsin-Madison, USA2
University of Hawaii, USA and 3Hokkaido University, Japan
Samples from the CI-like asteroid Ryugu consist mainly of minerals that were produced by aqueous alteration in its
parent asteroid, such as phyllosilicates, dolomite, magnetite, and pyrrhotite [1]. A secondary ion mass spectrometer (SIMS) has
been used to analyze in-situ oxygen 3-isotope ratios (δ17O and δ18O) of these alteration minerals in order to understand theaqueous activity that Ryugu parent body had experienced. Yokoyama et al. [1] reported δ17O, δ18O, and ∆17O (= δ17O –0.52×δ18O) of dolomite and magnetite in the polished section A0058-C1001 of Ryugu along with those in the Ivuna CI
chondrite. Nagashima et al. [2] reported δ17O, δ18O, and ∆17O of dolomite, calcite, and magnetite in the Ryugu polished sectionC0002-C1001. In A0058-C1001, a pair of magnetite and dolomite grains were found within 100 µm of each other and withindistinguishable ∆17O values of ~ 0‰. By applying oxygen isotope thermometry between dolomite and magnetite, [1]estimated the temperature of co-precipitation of the Ryugu dolomite and magnetite, 37±10°C. Oxygen isotope analyses fromanother magnetite and dolomite pair in C0002-C1001 indicate a higher equilibrium temperature of 104±22˚C [2]. However,multiple analyses of magnetite grains in Ryugu and Ivuna show a significant range of δ18O and ∆17O [1-2], suggesting
magnetite might have recorded multiple stages of aqueous activity on the Ryugu and Ivuna parent asteroid(s).
SIMS oxygen isotope analyses of magnetite are known to produce analytical artifacts due to crystal orientation thatcan degrade the analytical reproducibility of δ18O (2-3‰ in 2SD [3]), though the effect is mass-dependent and would notaffect ∆17O. Huberty et al. [3] used lower primary ion acceleration voltage in the Cameca IMS 1280 (changed from 10 kV to 3kV) and improved the analytical reproducibility (0.7‰; 2SD), likely related to lower total impact energy and smaller incident
angle of primary ions to sample surface. Here, we report a new set of SIMS oxygen 3-isotope analyses of magnetite in theRyugu samples using the IMS 1280 at the University of Wisconsin-Madison under 3 kV primary acceleration voltage in orderto obtain higher accuracy in δ18O analyses.
We analyzed magnetite grains (typically ≤10 µm) from two polished sections of Ryugu, A0058-C1002 and C0002C1001. The primary Cs+ ion beam was accelerated by +3 kV at the Cs ion source and focused to sample surface (at –10 kV)
resulting in an impact energy of 13 keV. The primary beam was focused to 3 µm with 14 pA intensity and rastered over 2 µmsquares, which resulted in 3×4 µm SIMS pits after 10 min of analysis (Fig. 1). The 16O– intensity was typically 1.7×107 cps formagnetite. Other conditions were similar to those in [4]. The external reproducibility of δ18O from 30 randomly oriented
magnetite standard 5830 [3] was 1.0‰ (2SD), similar to both internal and external errors of a single grain (0.7-0.9‰). The
external reproducibility of δ17O and ∆17O were ~1.5‰. After the analyses, SIMS pits were examined using a Hitachi S-3400
scanning electron microscope (SEM).
Figure 1. Two types of SIMS pit textures formagnetite grains in Ryugu after SIMS
oxygen isotope analyses. Magnetite grains
showing smooth SIMS pit textures (A) often
show well-defined crystal faces, indicating
slow growth. Magnetite grains showing
porous SIMS pit textures (B) often show
fibrous textures, suggesting rapid growth.

SEM observations show that all SIMS spots were within magnetite grains analyzed and none of the analyses wererejected. There are two types of pit textures, smooth and porous, which often correspond to magnetite with defined crystal
faces and fibrous grains, respectively (Fig. 1). Results of analyses are shown in Fig. 2. and compared to magnetite analyses
reported by [1] using 20 keV impact energy. Both datasets are very similar to each other, though new data show two groups ofmagnetite analyses according to the range of δ18O values, from –4‰ to –1‰ and from +3‰ to +7‰, which correspond tomagnetite grains with smooth and porous pit textures, respectively. The ∆17O values of the lower δ18O group vary from 0‰ to+2‰, while the higher δ18O group shows consistently higher ∆17O values (2-3‰). Some of the smooth magnetite analyses inA0058-C1002 are very similar to those equilibrated with dolomite in A0058-C1001 (δ18O ~ –3‰ and ∆17O ~ 0‰). Assumingthat 16O-poor H2O ice accreted to the Ryugu parent asteroid, melted, and interacted with relatively 16O-rich anhydrous silicates[2], these results suggest that porous (fibrous) magnetite with high ∆17O values formed during an early stage of aqueous
activity, while other magnetite without porosity mostly formed later stages and likely in oxygen isotope equilibrium with otherminerals. However, small variations in ∆17O values among magnetite with lower δ18O may suggest that fluid activity in theRyugu parent asteroid could have been localized and the oxygen isotope ratios of the fluid might have been variable.

Figure 2. SIMS oxygen 3-isotope analyses of minerals in Ryugu. (A, B) Analyses of magnetite and dolomite in A0058-C1001 and Ivuna CI
chondrite [1]. (C, D) Analyses of magnetite using 13 keV impact energy (This work). TF and CCAM are terrestrial mass fractionation andCarbonaceous chondrite anhydrous mineral lines [5], respectively.

References[1] Yokoyama T. et al. Science, 10.1126/science.abn7850 (2022). [2] Nagashima K. et al. this volume. [3] Huberty J. M. et al.
Chemical Geology, 276, 269–283 (2010). [4] Ushikubo T. et al. Geochim. Cosmochim. Acta 90, 242-264 (2012). [5] Clayton
R. N. et al. Earth Planet. Sci. Lett. 34, 209–224 (1977).
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.