The Magnesium Isotope Composition of Samples Returned from Asteroid Ryugu

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 mu 26Mg* and mu 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 mu 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 mu 25Mg variability as reflecting heterogeneous sampling of a carbonate phase hosting isotopically light Mg (mu 25Mg similar to -1400 ppm) formed by low temperature equilibrium processes. After correcting for this effect, Ryugu samples return homogeneous mu 26Mg* values corresponding to a weighted mean of 7.1 +/- 0.8 ppm. Thus, Ryugu defines a mu 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.