The magnesium isotope composition of samples returned from asteroid Ryugu

1

Martin Bizzarro1,2*, Martin Schiller1, Tetsuya Yokoyama3 and the Hayabusa2 Initial Analysis TeamCentre for Star and Planet Formation, Globe Institute, University of Copenhagen, Copenhagen, K 1350, Denmark.*email:bizzarro@sund.ku.dk
2
Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 75005 Paris, France
3
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan.

The nucleosynthetic isotope composition of planetary materials provides a record of the heterogeneous distribution of stardust
within the early Solar System. Thus, nucleosynthetic signatures can be used to infer genetic relationships between early formed
bodies and, ultimately, provide constraints on their accretion regions. In December 2020, the Japan Aerospace Exploration
Agency Hayabusa2 spacecraft returned to Earth the first samples of a primitive asteroids, namely the Cb-type asteroid Ryugu.
This provides a unique opportunity to explore the kindship between primitive asteroids and carbonaceous chondrites. Based onchemistry, mineralogy, petrology and isotope systematics of various elements, it has been proposed that Ryugu samples areclosely related to Ivuna-type (CI) carbonaceous chondrites. Indeed, the bulk nucleosynthetic isotope compositing for tracers
like 54Cr and 50Ti as well as the chemical abundances of most elements are within the range of CI chondrites. Moreover, likeCIs, Ryugu samples experienced extensive aqueous alteration in the presence of water and mainly consist of hydrous silicates(serpentine and saponite) and other secondary minerals (i.e., carbonate, magnetite, and sulfide) interpreted to have formed
during asteroidal fluid circulation.
High-precision Mg isotope measurements can provide a novel perspective on the kinship between Ryugu with CI chondrites
and, by extension, the accretion region of Cb-type asteroids. Magnesium isotope variability in Solar System materials canoriginate from the decay of the short-lived 26Al nuclide as well as primary nucleosynthetic processes. Moreover, Mg isotopes
can also be fractionated according to their masses by high-temperature events in the protoplanetary disk or, alternatively, bylow-temperature parent body secondary. Thus, the combination of high-precision mass-independent and mass-dependent Mg
isotope compositions is useful to understand genetic relationships and accretion history of planetary materials.
Here, we report high-precision µ26Mg* and µ25Mg values of Ryugu samples together with those of CI, CM, CV and ungroupedcarbonaceous chondrites. The stable Mg isotope composition of Ryugu aliquots define µ25Mg values ranging from –160±20ppm to –272±30 ppm, which extends to lighter compositions relative to Ivuna-type (CI) and other carbonaceous chondrite
groups. We interpret the µ25Mg variability as reflecting heterogenous sampling of a carbonate phase hosting isotopically lightMg (µ25Mg ~–1400 ppm) formed by low temperature equilibrium processes. After correction for this effect, Ryugu samples
return homogenous µ26Mg* values corresponding to a weighted mean of 7.1±0.8 ppm. Thus, Ryugu defines a µ26Mg* 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 theCI 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.