Nickel isotopic composition of Ryugu and the link between CI and other carbonaceous chondrites
T. Kleine F. Spitzer1, C. Burkhardt1, the Hayabusa2-initial-analysis chemistry team, and the Hayabusa2-initial-analysis core1
Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
The initial analyses of samples returned from Cb-type asteroid 162173 Ryugu by JAXA’s Hayabusa2 mission provided
isotopic, mineralogical, and chemical evidence for a close link of Ryugu to CI chondrites [1,2]. A subsequent study has shownthat Ryugu and CI chondrites share the same nucleosynthetic Fe isotope signatures, which are distinct from those of othercarbonaceous chondrites [6]. This not only demonstrates that Ryugu and CI chondrites are genetically linked, but also that they
derive from another reservoir than all other carbonaceous chondrites [3]. However, the origin of these distinct Fe isotopic
compositions of CI chondrites/Ruygu compared to other carbonaceous chondrites is not well understood. Nickel isotopes holdconsiderable promise to further investigate the genetic link between Ryugu and CI chondrites. When normalized to 61Ni/58Ni,
CI chondrites do not only display the largest µ62Ni and µ64Ni anomalies among carbonaceous chondrites (where µiNi is theparts-per-million deviation from terrestrial standard values), but they also have a distinct µ60Ni composition compared to allother known carbonaceous chondrite-like materials. We obtained four Ryugu samples (A0106–A0107, A0106, C0108, C0107)
along with six carbonaceous chondrites including the CI chondrites Orgueil and Alais, which have been chemically processedalongside the Ryugu samples [2]. In addition, we also measured several grouped and ungrouped carbonaceous chondrites.
Chemical purification of Ni involved a 3-step ion-exchange chromatographic procedure that achieves sufficiently low 58Fe/58Ni
and 64Zn/64Ni ratios in the final purified Ni cuts to allows for accurate and precise correction of isobaric interferences. All
isotope measurements were performed on the Thermo Scientific NeptunePlus MC-ICP-MS at the Institut für Planetologie,
University of Münster. Instrumental mass bias is corrected by internal normalization to either 61Ni/58Ni or 62Ni/61Ni using theexponential law. All data are reported in the µ-notation as the parts per 106 deviation relative to the standard.
The new Ni isotopic data for CM, CO, CV, and CR chondrites agree
well with those reported in previous studies and reveal that thesechondrites are characterized by negative µ60Ni and positive µ62Ni andµ64Ni values (Fig. 1). The two ungrouped carbonaceous chondrites
Tagish Lake (TL) and Tarda, for which no Ni isotopic data have beenreported previously, have Ni isotope anomalies similar to thosecarbonaceous chondrites. By contrast, CI chondrites display distinct
Ni isotopic compositions and particular distinctly larger µ64Ni andµ60Ni compared to other carbonaceous chondrites (Fig. 1).
Importantly, the Ni isotopic compositions of all four Ryugu samples
overlaps with those of the CI chondrites, indicating distinct Ni
isotopic compositions of CI chondrites/ Ryugu compared to all othercarbonaceous chondrites (Fig. 1). This is consistent with theFig. 1: µ60Ni versus µ64Ni for non-carbonaceous (NC) andnucleosynthetic Fe isotope variations among carbonaceous chondrites, carbonaceous (CC) meteorites. CI chondrites and Ryuguwhich also reveal a uniquely distinct composition for CI chondrites have distinct Ni isotopic compositions compared to NC andCC meteorites.
and Ryugu [3].
Isotopic variations among carbonaceous chondrites are thought to reflect variable proportions of three main components
having distinct isotopic compositions: refractory inclusions (e.g., CAI), chondrules/chondrule precursors, and CI chondrite-like
matrix [4, 5]. The incorporation of CI chondrite-like matrix in other groups of carbonaceous chondrites is consistent withsystematic variations of (i) volatile element contents, (ii) mass-dependent isotope fractionations of moderately volatileelements, and (iii) nucleosynthetic 54Cr anomalies with the fraction of matrix in each chondrite. However, in Ni isotope space,
CI chondrites (and Ryugu) are offset from the composition of other carbonaceous chondrites, indicating that CI chondrites are
not fully representative of the matrix in other groups of carbonaceous chondrites. Instead, this matrix appears to have formed
from different precursor material than CI chondrites, either because it formed in a different area of the disk or because it was
modified by processes in the disk prior to its incorporation in carbonaceous chondrites.
References[1] Yada T. et al. 2022. Nature Astronomy 6:214–220. [2] Yokoyama T. et al. 2022. Science abn7850. [3] Hopp T. et al. 2022.
Science Advances 8:8141. [4] Alexander 2019. Geochimica et Cosmochimica Acta 254:277-309. [5] Hellmann et al. 2023.
Astrophysical Journal Letters 946: L34.