The Magnesium Isotope Composition of Samples Returned from Asteroid Ryugu
Authors: Bizzarro Martin , Schiller Martin , Yokoyama Tetsuya , Abe Yoshinari , Aléon Jérôme , Alexander Conel M. O’D. , Amari Sachiko , Amelin Yuri , Bajo Ken-ichi , Bouvier Audrey , Carlson Richard W. , Chaussidon Marc , Choi Byeon-Gak , Dauphas Nicolas , Davis Andrew M. , Di Rocco Tommaso , Fujiya Wataru , 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 , Kawasaki Noriyuki , Kita Noriko T. , Kitajima Kouki , Kleine Thorsten , Komatani Shintaro , Krot Alexander N. , Liu Ming-Chang , Masuda Yuki , Morita Mayu , Moynier Fréderic , Motomura Kazuko , Nakai Izumi , Nagashima Kazuhide , Nesvorný David , Nguyen Ann , Nittler Larry , Onose Morihiko , Pack Andreas , Park Changkun , Piani Laurette , Qin Liping , Russell Sara S. , Sakamoto Naoya , Schönbächler Maria , Tafla Lauren , Tang Haolan , Terada Kentaro , Terada Yasuko , Usui Tomohiro , Wada Sohei , Wadhwa Meenakshi , Walker Richard J. , Yamashita Katsuyuki , Yin Qing-Zhu , 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: November 2023

Abstract
The nucleosynthetic isotope composition of planetary materials provides a record of the heterogeneous distribution of stardust within the early solar system. In 2020 December, the Japan Aerospace Exploration Agency Hayabusa2 spacecraft returned to Earth the first samples of a primitive asteroid, namely, the Cb-type asteroid Ryugu. This provides a unique opportunity to explore the kinship between primitive asteroids and carbonaceous chondrites. We report high-precision μ
26Mg* and μ
25Mg values of Ryugu samples together with those of CI, CM, CV, and ungrouped carbonaceous chondrites. The stable Mg isotope composition of Ryugu aliquots defines μ
25Mg values ranging from –160 ± 20 ppm to –272 ± 30 ppm, which extends to lighter compositions relative to Ivuna-type (CI) and other carbonaceous chondrite groups. We interpret the μ
25Mg variability as reflecting heterogeneous sampling of a carbonate phase hosting isotopically light Mg (μ
25Mg ∼ –1400 ppm) formed by low temperature equilibrium processes. After correcting for this effect, Ryugu samples return homogeneous μ
26Mg* values corresponding to a weighted mean of 7.1 ± 0.8 ppm. Thus, Ryugu defines a μ
26Mg* excess relative to the CI and CR chondrite reservoirs corresponding to 3.8 ± 1.1 and 11.9 ± 0.8 ppm, respectively. These variations cannot be accounted for by in situ decay of 26Al given their respective 27Al/24Mg ratios. Instead, it requires that Ryugu and the CI and CR parent bodies formed from material with a different initial 26Al/27Al ratio or that they are sourced from material with distinct Mg isotope compositions. Thus, our new Mg isotope data challenge the notion that Ryugu and CI chondrites share a common nucleosynthetic heritage.