Chemical abundances in Ryugu, nugget effect, and cosmic composition
Nicolas Dauphas1, Tetsuya Yokoyama2, Ryota Fukai3, Tomohiro Usui3, Shogo Tachibana4, Maria Schönbächler5, Andrew M.
Davis1, Hisayoshi Yurimoto6, The Hayabusa2-initial-analysis chemistry team, The Hayabusa2-initial-analysis core.
1
Origins Lab, Dept. of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, USA
2
Dept. of Earth & Planetary Sciences, Tokyo Institute of Technology, Japan
3
Dept. of Solar System Sciences, ISAS, JAXA
4
Dept. of Earth and Planetary Science, University of Tokyo, Japan
5
Inst. für Geochemie und Petrologie, ETH Zürich, Switzerland
6Dept. of Earth and Planetary Sciences, Hokkaido University, Japan
Approximately 99.86% of the mass of the solar system is locked up in the Sun, which is largely inaccessible and whose
composition is only known through spectroscopy and analysis of solar wind. Much of our knowledge of the chemical and isotopic
compositions of the bulk solar system comes from analysis of meteorites, and among those CI (Ivuna-type) carbonaceouschondrites, which contain most elements except the most volatile ones in proportions that match estimates from solar photosphereobservations [1,2]. CI chondrites are rare and most of them have been in meteorite collections for many decades (the largest one,Orgueil, fell in 1864), where they have been exposed to moisture, airborne and handling contaminants. The Hayabusa2 mission
sampled ~5.4 g of pristine material from Ryugu, a Cb-type asteroid [3]. Laboratory analyses of the returned sample show that
Ryugu is related to CI chondrites and probably originated from the outskirts of the solar system [4-7]. The only known
contaminant for Ryugu samples is Ta in the projectile used to liberate dust and fragments from Ryugu's surface [8]. Given theirpristine nature, Ryugu samples are ideally suited to constrain the composition of outer-solar system material, which we take asa proxy for the solar composition. A caveat to estimating the solar composition using Ryugu or CI chondrites is that these may
contain extraneous refractory dust compared to other inner solar system objects. This was most clearly demonstrated with rareearth elements (REEs), as carbonaceous chondrites all show Tm anomalies relative to non-carbonaceous chondrites [9,10],
presumably due to the presence of refractory dust with a group II REE pattern characterized by large positive Tm anomalies
produced by evaporation/condensation processes in the solar nebula [11]. At this stage, it is unclear if the Sun would contain thesame complement of refractory dust as CI chondrites, or if inner solar system objects are more representative.
33

1.5

1%

1CI-dolom

0.5
0
00.5

1(Rb/Sr)N

1.5

ite

2-dolom(Fe/Mn)N

22.5

ite(Mn/Cr)N6%

molo+d2.5

1.5
10.5

ite

2molo+d000.5ite

1(Rb/Sr)N

1.52Fig. 1. Correlations between Mn/Cr, Fe/Mn, and Rb/Sr ratios. The yellow star is the CI composition. The red points
are bulk fragment measurements for Ryugu [4,7]. The curves are calculated mixing curves between bulk CI anddolomite in CI chondrites [12,13].
Yokoyama et al. [4] and Nakamura E. et al. [7] reported high precision chemical composition analyses of Ryugu fragments
by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS), the gold standard for such analyses. Thosemeasurements show that Ryugu has almost unfractionated chemical abundances relative to CI chondrites, strengthening the casefor a link between Cb-asteroid and CI chondrites. The fragments analyzed are relatively small (from ~0.2 to ~30 mg) and show
significant departures relative to CI chondrites that are unrelated to volatility. This is true even in the largest fragments analyzed,
which show significant enrichments in REEs, Ca, Sr, Ba, P, and Mn [4]. Chemical abundance variations have been seen
previously in chondrites that presumably reflect heterogeneous distribution (and unrepresentative sampling) of phases highly
enriched in some elements [9]. This is known as a nugget effect and represents a limit in the precision that can be attained inbulk rock analyses. Carbonate and phosphate phases have been found in Ryugu samples [4,6,7] and could be responsible for thechemical variations seen in bulk fragment analyses.

A nugget effect for REEs associated with the presence of phosphate was identified in carbonaceous and ordinary chondrites[9]. There are several telltale signatures of such a nugget effect. The first one is that in elemental ratio plots, "bulk" analyses
should define mixing curves between the true bulk and the mineral composition. In ordinary and carbonaceous chondrites, thepresence of phosphate nuggets was identified as a negative correlation between La/Lu ratios and Eu/Eu* (=observedEu/interpolated Eu) anomalies (phosphates in ordinary chondrites have high La/Lu and negative Eu/Eu* anomalies) [9]. The
phosphates in Ryugu lack these characteristic features and are more difficult to detect [7]. Ryugu fragments are rich in dolomite,
which has high Mn/Cr, low Rb/Sr, and low Fe/Mn. In Fig. 1, we plot CI-normalized Mn/Cr and Fe/Mn against Rb/Sr. As shown,the ratios are correlated, and the variations observed can be well explained by admixture or removal of dolomite at the percent
level. Another telltale signature of a nugget effect is that the dispersion in elemental ratios should decrease as the inverse of thesquare root of the mass of sample homogenized [9]. As shown in Fig. 2, the dispersions in elemental ratios are consistent with asampling problem associated with a nugget effect. The dispersion in elemental ratios can be calculated if the nugget size,abundance, and composition are known [9]. Because these are poorly constrained for carbonates, we calculated instead thedispersion for the smaller fragments digested and calculated the predicted dispersion for larger masses. Having established that
unrepresentative sampling of carbonate is likely responsible for the dispersion in specific elemental ratios, we can predict theexpected dispersion if large sample masses are digested. We thus estimate that for 1 g of Ryugu sample homogenized/digested,
the dispersion (2s) of the bulk Mn/Cr and Rb/Sr ratios will be better than ~±5%. Elements that are not concentrated in specificminerals will be much less affected by this uncertainty. Our analysis shows that much of the dispersion in chemical composition
between Ryugu grains is a due to non-representative sampling of mineral phases highly enriched in some elements. This artifact
can be remediated by homogenizing a larger sample mass (~1 g), which is important to constrain the "cosmic" composition, andtest if previous estimates based on CI chondrites are reliable.
0.50.3Log10 (Rb/Sr)N

Log10 (Mn/Cr)N

0.4
0.2
0.1
0-0.1
-0.2
-0.3
0.001

0.01
0.1
Sample mass (g)

1

0.3
0.2
0.1
0-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
0.0001

0.001
0.01
0.1
Sample mass (g)1Fig. 2. Elemental ratios sensitive to carbonate nugget effects against mass homogenized/digested. If a nugget effect ispresent, we would expect the dispersion to decrease as the inverse of the square root of the mass of sample
homogenized/digested (black envelope) [9]. As shown, this seems to be the case.
References. [1] Lodders, K., 2021. Space Science Reviews, 217, 1. [2] Asplund M. et al. 2021. Astronomy & Astrophysics
653, A141. [3] Yada T. et al. 2022. Nature Astronomy 6:214. [4] Yokoyama T. et al. 2022 Science eabn7850. [5] Ito, M et al.
2022. Nature Astronomy, 1, https://doi.org/10.1038/s41550-022-01745-5. [6] Nakamura T. et al., 2022. Science, eabn8671. [7]Nakamura E. et al. (2022) Proceedings of the Japan Academy, Series B 98, 227. [8] Sawada H. et al. 2017. Space Science
Reviews, 208, 81. [9] Dauphas, N. and Pourmand, A., 2015. Geochimica et Cosmochimica Acta, 163, 234. [10] Barrat, J.A.,
Dauphas, N., et al., 2016. Geochimica et Cosmochimica Acta, 176, 1. [11] Hu, J. Y., et al. (2021). Science Advances, 7(2),eabc2962. [12] Endreß, M. and Bischoff, A., 1996. Geochimica et Cosmochimica Acta 60, 489. [13] Macdougall, J.D., et al.
1984. Nature, 307, 249.
The Hayabusa2-initial-analysis chemistry team: T. Yokoyama, K. Nagashima, I. Nakai, E.D. Young, Y. Abe, J. Aléon,C.M.O'D. Alexander, S. Amari, Y. Amelin, K. Bajo, M. Bizzarro, A. Bouvier, R. W. Carlson, M. Chaussidon, B.-G. Choi, N.
Dauphas, A. M. Davis, T. Di Rocco, W. Fujiya, R. Fukai, I. Gautam, M. K. Haba, Y. Hibiya, H. Hidaka, H. Homma, P. Hoppe,G.R. Huss, K. Ichida, T. Iizuka, T.R. Ireland, A. Ishikawa, M. Ito, S. Itoh, N. Kawasaki, N. T. Kita, K. Kitajima, T. Kleine, S.
Komatani, A. N. Krot, M.-C. Liu, Yuki Masuda, K.D. McKeegan, M. Morita, K. Motomura, F. Moynier, A. Nguyen, L. Nittler,M. Onose, A. Pack, C. Park, L. Piani, L. Qin, S.S. Russell, N. Sakamoto, M. Schönbächler, L. Tafla, H. Tang, K. Terada, Y.
Terada, T. Usui, S. Wada, M. Wadhwa, R.J. Walker, K. Yamashita, Q.-Z. Yin, S. Yoneda, H. Yui, A.-C. Zhang, H. Yurimoto.
The Hayabusa2-initial-analysis core: S. Tachibana, T. Nakamura, H. Naraoka, T. Noguchi, R. Okazaki, K. Sakamoto, H.
Yabuta, H. Yurimoto, Y. Tsuda, S. Watanabe.