Disequilibrium oxygen isotope distribution among aqueously altered minerals in Ryugu asteroid returned samples
Authors: Kita Noriko T. , Kitajima Kouki , Nagashima Kazuhide , Kawasaki Noriyuki , Sakamoto Naoya , Fujiya Wataru , Abe Yoshinari , Aléon Jérôme , Alexander Conel M. O'D. , Amari Sachiko , Amelin Yuri , Bajo Ken‐ichi , Bizzarro Martin , Bouvier Audrey , Carlson Richard W. , Chaussidon Marc , Choi Byeon‐Gak , Dauphas Nicolas , Davis Andrew M. , Di Rocco Tommaso , Fukai Ryota , Gautam Ikshu , Haba Makiko K. , Hibiya Yuki , Hidaka Hiroshi , Homma Hisashi , Hoppe Peter , Huss Gary R. , Ichida Kiyohiro , Iizuka Tsuyoshi , Ireland Trevor R. , Ishikawa Akira , Itoh Shoichi , Kleine Thorsten , Komatani Shintaro , Krot Alexander N. , Liu Ming‐Chang , Masuda Yuki , McKeegan Kevin D. , Morita Mayu , Motomura Kazuko , Moynier Frédéric , Nakai Izumi , Nguyen Ann , Nittler Larry , Onose Morihiko , Pack Andreas , Park Changkun , Piani Laurette , Qin Liping , Russell Sara S. , Schönbächler Maria , Tafla Lauren , Tang Haolan , Terada Kentaro , Terada Yasuko , Usui Tomohiro , Wada Sohei , Wadhwa Meenakshi , Walker Richard J. , Yamashita Katsuyuki , Yin Qing‐Zhu , Yokoyama Tetsuya , Yoneda Shigekazu , Young Edward D. , Yui Hiroharu , Zhang Ai‐Cheng , Nakamura Tomoki , Naraoka Hiroshi , Noguchi Takaaki , Okazaki Ryuji , Sakamoto Kanako , Yabuta Hikaru , Abe Masanao , Miyazaki Akiko , Nakato Aiko , Nishimura Masahiro , Okada Tatsuaki , Yada Toru , Yogata Kasumi , Nakazawa Satoru , Saiki Takanao , Tanaka Satoshi , Terui Fuyuto , Tsuda Yuichi , Watanabe Sei‐ichiro , Yoshikawa Makoto , Tachibana Shogo , Yurimoto Hisayoshi 
Published: April 2024

Oxygen 3‐isotope ratios of magnetite and carbonates in aqueously altered carbonaceous chondrites provide important clues to understanding the evolution of the fluid in the asteroidal parent bodies. We conducted oxygen 3‐isotope analyses of magnetite, dolomite, and breunnerite in two sections of asteroid Ryugu returned samples, A0058 and C0002, using a secondary ion mass spectrometer (SIMS). Magnetite was analyzed by using a lower primary ion energy that reduced instrumental biases due to the crystal orientation effect. We found two groups of magnetite data identified from the SIMS pit morphologies: (1) higher δ18O (from 3‰ to 7‰) and ∆17O (~2‰) with porous SIMS pits mostly from spherulitic magnetite, and (2) lower δ18O (~ −3‰) and variable ∆17O (0‰–2‰) mostly from euhedral magnetite. Dolomite and breunnerite analyses were conducted using multi‐collection Faraday cup detectors with precisions ≤0.3‰. The instrumental bias correction was applied based on carbonate compositions in two ways, using Fe and (Fe + Mn) contents, respectively, because Ryugu dolomite contains higher amounts of Mn than the terrestrial standard. Results of dolomite and breunnerite analyses show a narrow range of ∆17O; 0.0‰–0.3‰ for dolomite in A0058 and 0.2‰–0.8‰ for dolomite and breunnerite in C0002. The majority of breunnerite, including large ≥100 μm grains, show systematically lower δ18O (~21‰) than dolomite (25‰–30‰ and 23‰–27‰ depending on the instrumental bias corrections). The equilibrium temperatures between magnetite and dolomite from the coarse‐grained lithology in A0058 are calculated to be 51 ± 11°C and 78 ± 14°C, depending on the instrumental bias correction scheme for dolomite; a reliable temperature estimate would require a Mn‐bearing dolomite standard to evaluate the instrumental bias corrections, which is not currently available. These results indicate that the oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially, or temporary. Initial water ice accreted to the Ryugu parent body might have ∆17O > 2‰ that was melted and interacted with anhydrous solids with the initial ∆17O < 0‰. In the early stage of aqueous alteration, spherulitic magnetite and calcite formed from aqueous fluid with ∆17O ~ 2‰ that was produced by isotope exchange between water (∆17O > 2‰) and anhydrous solids (∆17O < 0‰). Dolomite and breunnerite, along with some magnetite, formed at the later stage of aqueous alteration under higher water‐to‐rock ratios where the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, δ18O of carbonates decreased in the order calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.