# Pervasive aqueous alteration in the early Solar System revealed by potassium isotopic variations in Ryugu samples and carbonaceous chondrites

Yan Hu, Frédéric Moynier, Wei Dai, Marine Paquet, Tetsuya Yokoyama, Yoshinari Abe, Jérôme Aléon, Conel M. O'D. Alexander, Sachiko Amari, Yuri Amelin, et al.

► To cite this version:

Yan Hu, Frédéric Moynier, Wei Dai, Marine Paquet, Tetsuya Yokoyama, et al. Pervasive aqueous alteration in the early Solar System revealed by potassium isotopic variations in Ryugu samples and carbonaceous chondrites. Icarus, 2024, 409, 10.1016/j.icarus.2023.115884. insu-04462199

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Distributed under a Creative Commons Attribution 4.0 International License
1 Pervasive aqueous alteration in the early Solar System revealed by

2 potassium isotopic variations in Ryugu samples and carbonaceous chondrites

3

4 Yan Hu¹,*, Frédéric Moynier¹,*, Wei Dai¹, Marine Paquet¹, Tetsuya Yokoyama², Yoshinari Abe³,
5 Jérôme Aléon⁴, Conel M. O'D. Alexander⁵, Sachiko Amari⁶,⁷, Yuri Amelin⁸, Ken-ichi Bajo⁹,
6 Martin Bizzarro¹⁰,¹, Audrey Bouvier¹¹, Richard W. Carlson⁵, Marc Chaussidon¹, Byeon-Gak
7 Choi¹², Nicolas Dauphas¹³, Andrew M. Davis¹³, Tommaso Di Rocco¹⁴, Wataru Fujiya¹⁵, Ryota
8 Fukai¹⁶, Ikshu Gautam², Makiko K. Haba², Yuki Hibiya¹⁷, Hiroshi Hidaka¹⁸, Hisashi Homma¹⁹,
9 Peter Hoppe²⁰, Gary R. Huss²¹, Kiyohiro Ichida²², Tsuyoshi Iizuka²³, Trevor R. Ireland²⁴, Akira
10 Ishikawa², Shoichi Itoh²⁵, Noriyuki Kawasaki⁹, Noriko T. Kita²⁶, Koki Kitajima²⁶, Thorsten
11 Kleine²⁷, Shintaro Komatani²², Alexander N. Krot²¹, Ming-Chang Liu²⁸, Yuki Masuda², Mayu
12 Morita²², Kazuko Motomura²⁹, Izumi Nakai³⁰, Kazuhide Nagashima²¹, David Nesvorný³¹, Ann
13 Nguyen³², Larry Nittler⁵,³³, Morihiko Onose²², Andreas Pack¹⁴, Changkun Park³⁴, Laurette
14 Piani³⁵, Liping Qin³⁶, Sara S. Russell³⁷, Naoya Sakamoto³⁸, Maria Schönbächler³⁹, Lauren
15 Tafla²⁸, Haolan Tang²⁸,³⁶, Kentaro Terada⁴⁰, Yasuko Terada⁴¹, Tomohiro Usui¹⁶, Sohei Wada⁹,
16 Meenakshi Wadhwa³³, Richard J. Walker⁴², Katsuyuki Yamashita⁴³, Qing-Zhu Yin⁴⁴, Shigekazu
17 Yoneda⁴⁵, Edward D. Young²⁸, Hiroharu Yui⁴⁶, Ai-Cheng Zhang⁴⁷, Tomoki Nakamura⁴⁸, Hiroshi
18 Naraoka⁴⁹, Takaaki Noguchi²⁵, Ryuji Okazaki⁴⁹, Kanako Sakamoto¹⁶, Hikaru Yabuta⁵⁰, Masanao
19 Abe¹⁶, Akiko Miyazaki¹⁶, Aiko Nakato¹⁶, Masahiro Nishimura¹⁶, Tatsuaki Okada¹⁶, Toru Yada¹⁶,
20 Kasumi Yogata¹⁶, Satoru Nakazawa¹⁶, Takanao Saiki¹⁶, Satoshi Tanaka¹⁶, Fuyuto Terui⁵¹, Yuichi
21 Tsuda¹⁶, Sei-ichiro Watanabe¹⁸, Makoto Yoshikawa¹⁶, Shogo Tachibana⁵², Hisayoshi Yurimoto⁹
22

23 Affiliations

24 ¹Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, 75005 Paris, France.

25 ²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan.

26 ³Graduate School of Engineering, Tokyo Denki University, Tokyo 120-8551, Japan.

27 ⁴Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Université, Museum
28 National d'Histoire Naturelle, CNRS UMR 7590, IRD, 75005 Paris, France.

29 ⁵Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, 20015, USA.

30 ⁶McDonnell Center for the Space Sciences and Physics Department, Washington University, St. Louis,
31 MO 63130, USA.

32 ⁷Geochemical Research Center, The University of Tokyo, Tokyo, 113-0033, Japan

33 ⁸Korea Basic Science Institute, Ochang, Cheongwon, Cheongju, Chungbuk 28119, Republic of Korea

34 ⁹Department of Natural History Sciences, Hokkaido University, Sapporo 001-0021, Japan.

35 ¹⁰Centre for Star and Planet Formation, GLOBE Institute, University of Copenhagen, Copenhagen, K
36 1350, Denmark.

37 ¹¹Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth 95447, Germany.

38 ¹²Department of Earth Science Education, Seoul National University, Seoul 08826, Republic of Korea.

39 ¹³Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734
40 South Ellis Avenue, Chicago 60637, USA.
41 <sup>14</sup>Faculty of Geosciences and Geography, University of Göttingen, Göttingen, D-37077, Germany.

42 <sup>15</sup>Faculty of Science, Ibaraki University, Mito 310-8512, Japan.

43 <sup>16</sup>ISAS/JSEC, JAXA, Sagamihara 252-5210, Japan.

44 <sup>17</sup>Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan.

46 <sup>18</sup>Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8601, Japan.

47 <sup>19</sup>Osaka Application Laboratory, SBUWDX, Rigaku Corporation, Osaka 569-1146, Japan.

48 <sup>20</sup>Max Planck Institute for Chemistry, Mainz 55128, Germany.

49 <sup>21</sup>Hawaiʻi Institute of Geophysics and Planetology, University of Hawaiʻi at Mānoa, Honolulu, HI 96822, USA.

51 <sup>22</sup>Analytical Technology, Horiba Techno Service Co., Ltd., Kyoto 601-8125, Japan.

52 <sup>23</sup>Department of Earth and Planetary Science, The University of Tokyo, Tokyo 113-0033, Japan.

53 <sup>24</sup>School of Earth and Environmental Sciences, The University of Queensland, St Lucia QLD 4072, Australia.

55 <sup>25</sup>Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan.

56 <sup>26</sup>Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA.

57 <sup>27</sup>Max Planck Institute for Solar System Research, 37077 Göttingen, Germany.

58 <sup>28</sup>Department of Earth, Planetary, and Space Sciences, UCLA, Los Angeles, CA 90095, USA.

59 <sup>29</sup>Thermal Analysis, Rigaku Corporation, Tokyo 196-8666, Japan.

60 <sup>30</sup>Department of Applied Chemistry, Tokyo University of Science, Tokyo 162-8601, Japan.

61 <sup>31</sup>Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA.

62 <sup>32</sup>Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA.

64 <sup>33</sup>School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA.

65 <sup>34</sup>Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Korea.

66 <sup>35</sup>Centre de Recherches Pétrographiques et Géochimiques, CNRS - Université de Lorraine, 54500 Nancy, France.

68 <sup>36</sup>CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, School of Earth and Space Sciences, Anhui 230026, China.

70 <sup>37</sup>Department of Earth Sciences, Natural History Museum, London, SW7 5BD, UK.

71 <sup>38</sup>Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan.

73 <sup>39</sup>Institute for Geochemistry and Petrology, Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland.

75 <sup>40</sup>Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan.

76 <sup>41</sup>Spectroscopy and Imaging, Japan Synchrotron Radiation Research Institute, Hyogo 679-5198 Japan.

77 <sup>42</sup>Department of Geology, University of Maryland, College Park, MD 20742, USA.

78 <sup>43</sup>Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan.

79 <sup>44</sup>Department of Earth and Planetary Sciences, University of California, Davis, CA 95616, USA.

80 <sup>45</sup>Department of Science and Engineering, National Museum of Nature and Science, Tsukuba 305-0005, Japan.

82 <sup>46</sup>Department of Chemistry, Tokyo University of Science, Tokyo 162-8601, Japan.

83 <sup>47</sup>School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China.

84 <sup>48</sup>Department of Earth Science, Tohoku University, Sendai, 980-8578, Japan.
85 <sup>49</sup>Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 819-0395, Japan.

86 <sup>50</sup>Earth and Planetary Systems Science Program, Hiroshima University, Higashi-Hiroshima 739-8526,

87 Japan.

88 <sup>51</sup>Kanagawa Institute of Technology, Atsugi 243-0292, Japan.

89 <sup>52</sup>UTokyo Organization for Planetary and Space Science, The University of Tokyo, Tokyo 113-0033,

90 Japan.

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Abstract: 233 words

Main text: 4512 words

Figures: 3

Table: 1

Submit to *Icarus*

**Revised version**

(November, 2023)

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*Corresponding authors:

110

Email: yanhu@ipgp.fr; moynier@ipgp.fr
111 **Abstract**

112 C-type asteroids are the presumed home to carbonaceous chondrites, some of which contain 113 abundant life-forming volatiles and organics. For the first time, samples from a C-type asteroid 114 (162173 Ryugu) were successfully returned to Earth by JAXA's Hayabusa2 mission. These 115 pristine samples, uncontaminated by the terrestrial environment, allow a direct comparison with 116 carbonaceous chondrites. This study reports the stable K isotopic compositions (expressed as 117 δ<sup>41</sup>K) of Ryugu samples and seven carbonaceous chondrites to constrain the origin of K isotopic 118 variations in the early Solar System. Three aliquots of Ryugu particles collected at two 119 touchdown sites have identical δ<sup>41</sup>K values, averaged at -0.194 ± 0.038‰ (2SD). The K isotopic 120 composition of Ryugu falls within the range of δ<sup>41</sup>K values measured on representative CI 121 chondrites, and together, they define an average δ<sup>41</sup>K value of -0.185 ± 0.078‰ (2SE), which 122 provides the current best estimate of the K isotopic composition of the bulk Solar System. 123 Samples of CI chondrites with δ<sup>41</sup>K values that deviate from this range likely reflect terrestrial 124 contaminations or compositional heterogeneities at sampled sizes. In addition to CI chondrites, 125 substantial K isotopic variability is observed in other carbonaceous chondrites and within 126 individual chondritic groups, with δ<sup>41</sup>K values inversely correlated with K abundances in many 127 cases. These observations indicate widespread fluid activity occurred in chondrite parent bodies, 128 which significantly altered the original K abundances and isotopic compositions of chondrules 129 and matrices established at their accretion. 130

130 **Keywords:** Asteroid Ryugu; Hayabusa2 mission; Carbonaceous chondrites; Stable potassium 131 isotopes; Aqueous alteration 132
133 **1 Introduction**

134 Primitive asteroids accreted varying amounts of ice that melted to water-rich fluids due to 135 heat released from the radioactive decay of short-lived nuclides (e.g., $ ^{26}Al $, McSween et al., 136 2002) and possibly from impacts (Rubin, 2012). Consequently, fluid activity is believed to have 137 been pervasive in the early history of planetesimal and planet development (Brearley, 2006; 138 Brearley, 2014; Clayton, 1993; Zolensky and McSween, 1988). Potassium (K) is a fluid-mobile, 139 large-ion lithophile element that resides primarily in **chondrule** mesostasis and fine-grained 140 matrix in carbonaceous asteroids. These components react readily with aqueous fluids, leading to 141 redistribution of K$^+$ and its isotopes. In addition, K is a moderately volatile metal that is variably 142 depleted in carbonaceous chondrites and terrestrial planetary bodies. Because both volatility- 143 driven depletion and fluid-rock interaction can lead to mass-dependent fractionations between 144 the two stable K isotopes (i.e., $ ^{39}K $ and $ ^{41}K $), it is essential to understand how these processes 145 affect $ \delta^{41}K $ variations in meteorites to facilitate the interpretation of K isotopic signatures of 146 planetary materials.

147 Carbonaceous Ivuna-type (CI) chondrites are the chemically most primitive meteorites and 148 have been widely used to infer the isotopic composition of the bulk Solar System (Palme et al., 149 2014). Despite their primitive bulk composition, CI chondrites underwent near-complete aqueous 150 alteration of anhydrous precursors to secondary mineral assemblages dominated by 151 phyllosilicates (e.g., Tomeoka and Buseck, 1988) and display significant small-scale chemical 152 heterogeneities (e.g., Barrat et al., 2012; Morlok et al., 2006; Palme and Zipfel, 2022). Because 153 CI chondrites are limited in numbers and masses, previous K isotopic analyses focused on the 154 largest witnessed fall (Orgueil, 14 kg) and the CI-type specimen (Ivuna, 0.7 kg). Earlier analyses 155 of Orgueil returned a $ \delta^{41}K $ value (−0.534 ± 0.097‰, 2SE) that is similar to the bulk Silicate 156 Earth (Wang and Jacobsen, 2016); however, this sample shows elemental enrichments (e.g., K, 157 Rb, Ba, Th, and U) that indicate significant contamination on Earth (Ku and Jacobsen, 2020). 158 Reported $ \delta^{41}K $ values of other Orgueil samples vary between −0.290‰ and −0.039‰ (Hu et al., 159 2022, 2023; Koefoed et al., 2022, 2023; Ku and Jacobsen, 2020; Nie et al., 2021), with the six 160 distinct stones reported by Koefoed et al. (2022; 2023) displaying more uniform $ \delta^{41}K $ values 161 ranging from −0.290‰ to −0.170‰. However, while Koefoed et al. (2023) reported a similar 162 $ \delta^{41}K $ value of −0.180 ± 0.060‰ for Ivuna, a markedly lower value of −0.460 ± 0.046‰ was 163 reported for another Ivuna sample with a similar K abundance (Nie et al., 2021). The cause of
this intra-group difference remains unclear. It may reflect an unrepresentative sampling of the bulk composition due to sample heterogeneity caused by aqueous alteration in the parent body(s) of the CI chondrites or remobilization of K in terrestrial environments. A better-defined K isotopic composition of CI chondrites is critical for constraining the bulk composition of the Solar System.

In December 2020, JAXA's Hayabusa2 mission returned the first samples from the carbonaceous asteroid 162173 Ryugu, which had not been exposed to terrestrial environments prior to laboratory analysis. These samples contain abundant organic material plausibly inherited from the protosolar nebula and/or the interstellar medium, which attests to their primitive nature (Nakamura et al., 2022; Pilorget et al., 2022). Furthermore, chemical and isotopic analyses suggest that Ryugu has a similar bulk composition to CI chondrites (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2022; Yokoyama et al., 2022). In particular, the Ryugu samples do not contain chondrules or refractory inclusions (Pilorget et al., 2022; Yokoyama et al., 2022) and display no depletion of volatile metals relative to the solar photosphere (Yokoyama et al., 2022), which are essential features of CI chondrites. Therefore, the Ryugu samples are a direct asteroidal analogue to CI chondrites. Here, we report the K isotopic compositions of three aliquots of Ryugu particles collected from two touchdown sites and seven carbonaceous chondrites measured with the Nu Sapphire collision-cell multi-collector inductively-coupled plasma mass spectrometer (CC-MC-ICP-MS) to further constrain the pristine composition of CI chondrites and the origin of K isotopic variation in the Solar System.

# 2 Samples and methods

## 2.1 Sample description and pre-chemistry preparation

Ryugu formed as a rubble-pile asteroid (approximately 1 km in diameter) reaccumulated from the shattered debris of a larger parent body (Sugita et al., 2019); therefore, Ryugu likely sampled rock fragments from all depths of its parent body. The Hayabusa2 mission retrieved 5.4 g of pebbles and sands at two locations on Ryugu (Tachibana et al., 2022; Yada et al., 2022). Around 3 g of surface material was collected from the touchdown site 1 and stored in Chamber A (Morota et al., 2020). The second landing targeted at collecting impact ejecta from a ~ 15-m diameter crater artificially made by the onboard Small Carry-on Impactor (SCI) (Arakawa et al.,
2020). Around 2 g of samples containing subsurface material was collected from the touchdown site 2 and stored in Chamber C. Samples collected from these two sites are considered representative of the compositionally homogeneous Ryugu surface based on their consistent spectral features and physical properties (color, shape, surface morphology, and structure) with onboard measurements made by the spacecraft (Kitazato et al., 2019; Nakamura et al., 2022; Pilorget et al., 2022; Tachibana et al., 2022).

Three aliquots of Ryugu particles were analyzed for K isotopes. Sample A0106 is from Chamber A, while samples C0107 and C0108 are from Chamber C. Preliminary sample treatment has been performed at the Tokyo Institute of Technology (Tokyo Tech). Samples A0106 and C0107 were first treated to extract soluble organic matter (Naraoka et al., 2023). Then, together with C0108, they were dissolved in pre-cleaned perfluoroalkoxy (PFA) vials in a mixture of concentrated HF and HNO₃, as described in Yokoyama et al. (2022). All three Ryugu samples were first processed to separate the Zn fractions from other constituent elements in the samples (Paquet et al., 2022). This was followed by a three-step sequential chemical separation procedure for the isotopic measurements of Fe (Hopp et al., 2022), Ti, and Cr (Yokoyama et al., 2022), after which we obtained a fraction containing mainly K, Mg, and Ni. Aliquots of the K-Mg-Ni fraction for the Ryugu samples were transferred to the Institut de Physique du Globe de Paris (IPGP) for K isotopic analyses. The amounts of K finally obtained from the three Ryugu samples ranged from 1365 to 2867 ng. To verify whether the multi-stage chemical separation fractionated the K isotopes, the K-Mg-Ni fractions of three carbonaceous chondrites, including Tarda (C2-ungrouped), Tagish Lake (C2-ungrouped), and Allende (CV3), which were subjected to the same processing as the Ryugu samples at Tokyo Tech, were chemically purified for K isotopic analyses at IPGP.

We also analyzed four geostandards (BIR, BCR-2, AGV-2, and seawater) and seven in-house carbonaceous chondrites at IPGP to examine possible sample heterogeneity and to compare with the K isotopic compositions of the Ryugu samples. The additional chondrite samples were: Orgueil (CI1), Tagish Lake (C2-ungrouped), Tarda (C2-ungrouped), Cold Bokkeveld (CM2), Murchison (CM2), Lancé (CO3.5), and Allende (CV3). At IPGP, approximately 8-15 mg of the sample powders were dissolved in Savillex screw-top PFA beakers using sequential addition of concentrated HF-HNO₃ (3:1), HCl-HNO₃ (3:1), and HNO₃. The dissolved sample solutions were refluxed with and redissolved in 0.5 mol/L HNO₃.
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## 2.2 Potassium purification and isotope measurement

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Potassium was isolated from other elements by cation-exchange chromatography described in Xu et al. (2019). Two mL of Bio-Rad AG 50W-X8 cation exchange resin (200-400 mesh) was filled into pre-cleaned Bio-Rad Poly-Prep columns. The resin was cleaned by three passes with 6 mol/L HCl, one pass with 6 mol/L HNO₃, and one pass with 1 mol/L HNO₃. Each acid pass was rinsed with 10 mL of Milli-Q deionized water. The resin was then conditioned with 10 mL of 0.5 mol/L HNO₃. Dissolved sample solutions were loaded onto the resin in 1 mL of 0.5 mol/L HNO₃. A volume of 13 mL of 0.5 mol/L HNO₃ was passed through the column, and the K fraction was collected in the subsequent 22 mL of the same acid. The sample solutions were passed through the column four times. This chromatography protocol provides a K yield greater than 99.3% and effectively separates matrix elements from the K fractions, as have been evaluated in previous studies (Xu et al., 2019; Hu et al., 2022). The K blank introduced by each pass of column chemistry is ~ 0.2 ng, and the whole-procedure K blank varies between 3.7 ng and 6.2 ng (Hu et al., 2022), which is insignificant (< 1%) compared to the amounts of K in processed samples.

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Potassium isotopic analyses followed the protocol described by Moynier et al. (2021). Purified K fractions were diluted in 0.5 mol/L HNO₃ to have K concentrations between 60 ppb and 70 ppb. They were introduced into the mass spectrometer using an Apex Omega desolvation nebulizer system fitted with a 100 µL/min nebulizer. The ion beam was first decelerated to enter a hexapole collision cell to remove interfering species (e.g., ⁴⁰ArH⁺ on ⁴¹K⁺), and the ⁴¹K⁺ and ³⁹K⁺ ion beams were accelerated and directed through the magnet to the Faraday collectors. Each analysis consisted of a 90-second transfer time, a 60-second zero measurement in a blank solution, and 50 cycles of sample measurement with 5-second integration. A 150-second wash was performed between analyses. Each sample solution was paired with a standard solution diluted to the same K concentration (to within 2%). Standard and sample solutions were analyzed alternatively during a sequence to correct for instrumental mass bias. Potassium isotopic data are reported as the average of repeated analyses (4 to 8 times) relative to NIST SRM 3141a in delta notation:
$$ \delta^{41}\text{K} (\text{\textperthousand}) = \left\{ \frac{\left( \frac{^{41}\text{K}}{^{39}\text{K}} \right)_{\text{sample}}}{\left( \frac{^{41}\text{K}}{^{39}\text{K}} \right)_{\text{NIST SRM 3141a}}} - 1 \right\} \times 1000 $$

Analytical uncertainties are reported as 2SD (standard deviation) and 95% c.i. (confidence interval) in Table 1 and Table S1. In the following sections, analytical uncertainty used for individual sample is 95% c.i. and that used for group average is 2SD unless stated otherwise.

# 3 Results and discussions

## 3.1 Analytical accuracy

We first evaluate the accuracy of our analyses by comparing the $\delta^{41}\text{K}$ values of representative terrestrial standards and chondrites from analyses in this study to previous results. The $\delta^{41}\text{K}$ values of basalt BIR (–0.418 ± 0.031‰) and BCR-2 (–0.426 ± 0.030‰), andesite AGV-2 (–0.455 ± 0.030‰), and Pacific seawater (0.126 ± 0.028‰) analyzed during the course of this study agree well with published results (Chen et al., 2019; Hille et al., 2019; Hu et al., 2018; Moynier et al., 2021; Xu et al., 2019). In addition, Orgueil (–0.081 ± 0.066‰), Murchison (–0.250 ± 0.043‰), Allende (–0.169 ± 0.030‰), Cold Bokkeveld (–0.267 ± 0.035‰), and Lancé (–0.382 ± 0.040‰) processed and analyzed in this study yielded consistent values with our previous analyses (Hu et al., 2022; Hu et al., 2023). This consistency confirms the accuracy of our analyses.

We then evaluate whether the multi-stage chemical separation procedures performed at Tokyo Tech affected the isotopic compositions of K in the samples. For Tagish Lake, the K extracted from the K-Mg-Ni fraction has a $\delta^{41}\text{K}$ value of –0.180 ± 0.049‰, which is indistinguishable from the values obtained from two new dissolutions of separate pieces of this meteorite from Tokyo Tech (–0.215 ± 0.043‰) and IPGP (–0.215 ± 0.032‰). Likewise, the K purified from the K-Mg-Ni fraction of Tarda (–0.223 ± 0.037‰) is isotopically identical to that purified from a separate piece of this meteorite at IPGP (–0.205 ± 0.024‰). This similarity in $\delta^{41}\text{K}$ values suggests that the preliminary chemical processing performed at Tokyo Tech affected K negligibly. In support of this inference, the anion resin used to separate Zn from other constituent elements in the samples should not retain any K⁺, which would be eluted from the resin. Therefore, it is reasonable to assume that the $\delta^{41}\text{K}$ values of K extracted from the K-Mg-Ni fraction of the three Ryugu samples are representative of these samples. While the K-Mg-Ni
fraction of Allende (–0.292 ± 0.044‰) is isotopically different from our in-house Allende sample (–0.169 ± 0.030‰), the δ<sup>41</sup>K values of both samples fall within the reported range from –0.620 ± 0.050‰ to –0.082 ± 0.030‰ (Bloom et al., 2020; Ku and Jacobsen, 2020; Nie et al., 2021; Jiang et al., 2021; Hu et al., 2023; Koefoed et al., 2023). The discrepancy between the two Allende samples thus likely reflects sample heterogeneity.

## 3.2 Stable K isotopic compositions of CI chondrites and the bulk Solar System

The three Ryugu samples have K abundances that are comparable to those of the CI chondrites (Fig. 1A). In addition, they display consistent δ<sup>41</sup>K values (Fig. 1B), with no significant differences between the samples collected from the touchdown site 1 (Chamber A sample A0106 = –0.172 ± 0.039‰) and the touchdown site 2 (Chamber C samples C0107 = –0.204 ± 0.048‰ and C0108 = –0.207 ± 0.055‰). Furthermore, the two samples from Chamber C have essentially identical δ<sup>41</sup>K values, indicating that the extraction of soluble organic matter from samples C0107 and A0106 prior to K separation did not affect their K isotopic compositions. The three Ryugu samples yield an average δ<sup>41</sup>K value of –0.194 ± 0.038‰ (2SD), which falls within the range of CI chondrites (Fig. 1B). This result is consistent with previous observations of CI-like isotopic compositions of Ryugu for the moderately volatile elements Zn and Cu (Paquet et al., 2022) and the more refractory elements Fe, Cr, Ti, and Ca (Hopp et al., 2022; Moynier et al., 2022; Yokoyama et al., 2022). The stable K isotope ratios thus further strengthen the compositional links between Ryugu and CI chondrites.

The new data from Ryugu samples provide critical constraints on the K isotopic composition of the bulk Solar System, which can otherwise only be inferred from CI chondrites. Previous analyses of CI chondrites show highly variable δ<sup>41</sup>K values from –0.534‰ to –0.039‰, covering almost the entire variation measured in carbonaceous chondrites (Hu et al., 2022, 2023; Koefoed et al., 2022, 2023; Ku and Jacobsen, 2020; Nie et al., 2021; Wang and Jacobsen, 2016). Ku and Jacobsen (2020) found that the lowest δ<sup>41</sup>K values were measured for a piece of Orgueil contaminated with 8% continental crust material, resulting in its substantially higher K abundance and lower δ<sup>41</sup>K value than other Orgueil samples (Fig. 1). In contrast, the 11 other Orgueil samples show a limited K isotopic range from –0.290‰ to –0.039‰, with six stones reported by Koefoed et al. (2023) having similar values between –0.290‰ and –0.170‰ (Fig. 1).
1B). All these analyses together give an average δ⁴¹K value of -0.182 ± 0.148‰ (2SD), suggesting a largely homogeneous K isotopic composition in Orgueil. The small variability between different Orgueil samples could be due to aqueous redistribution, as suggested by a slightly negative correlation between the δ⁴¹K value and K abundance (Fig. 1B).

Unlike Orgueil, however, there is a large discrepancy in the δ⁴¹K values reported for two separate analyses of the CI-type specimen Ivuna. The lower value of -0.460 ± 0.046‰ does not appear to reflect terrestrial contamination because this Ivuna sample has a normal K abundance (458 ppm, Nie et al., 2021) as the uncontaminated Orgueil samples and the other Ivuna sample, which has a δ⁴¹K value (-0.180 ± 0.060‰, Koefoed et al., 2023) similar to the Orgueil average calculated above (Fig. 1B). Given that this lower Ivuna value is significantly lower than those of the Ryugu samples, it likely reflects an unrepresentative sampling of the bulk Ivuna composition. This inference is supported by the heterogenous distribution of soluble elements in CI chondrites, in particular for Ivuna (Barrat et al., 2012; Brearley, 2006; Morlok et al., 2006; Palme and Zipfel, 2022) (Fig. 1A). Excluding the anomalous Ivuna value, representative averages of Ryugu (-0.194 ± 0.022‰, 2SE) and Orgueil (-0.182 ± 0.045‰, 2SE) and a single Ivuna value (-0.180 ± 0.060‰, 2SE, Koefoed et al., 2023) define an arithmetic mean of -0.185 ± 0.078‰ (2SE). This δ⁴¹K mean agrees with the recently proposed average CI value of -0.21 ± 0.05‰ (Koefoed et al., 2023) calculated by averaging 15 published CI data (including the anomalous Ivuna value). The agreement between the two estimates further supports a relatively homogeneous K isotopic composition of the bulk Solar System.

### 3.3 Origin of K isotopic variations in carbonaceous chondrites

A well-established average δ⁴¹K value of CI chondrites is an important reference for understanding the origin of K isotopic variations among different carbonaceous chondritic groups. This is because CI chondrites are generally used to approximate the isotopic composition of the matrix component in carbonaceous chondrites (e.g., Alexander, 2005; Braukmüller et al., 2018), and a robust δ⁴¹K value for CI chondrites is critical for explaining the K isotopic variations in carbonaceous chondrites (Bloom et al., 2020; Ku and Jacobsen, 2020; Nie et al., 2021; Hu et al., 2022, 2023; Koefoed et al., 2022, 2023). The average δ⁴¹K values established for CI chondrites and Ryugu samples in this study agree with each other within uncertainty (Fig. 2).
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Their $\delta^{41}$K values also overlap within uncertainty with those of the two ungrouped C2 chondrites (Tagish Lake and Tarda), which contain the second-highest matrix volume among carbonaceous chondrites (Fig. 2). These observations indicate that the CI-like matrix in carbonaceous chondrites is largely homogeneous in $\delta^{41}$K.

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Compared with CI chondrites and Ryugu samples, carbonaceous chondrites from the CM (Murchison), CO (Lancé), and CV (Allende) groups are variably depleted in the heavier K isotope, and their $\delta^{41}$K values correlate positively with the $\delta^{66}$Zn and $\delta^{65}$Cu values (Fig. 2). Positive correlations are also evident between $\delta^{41}$K with isotopes of other MVEs, including Rb, Sn, and Te, in a larger dataset of carbonaceous chondrites (Hu et al., 2023; Koefoed et al., 2023; Nie et al., 2021). In addition, a broad negative correlation between $\delta^{41}$K and 1/[K] was reported by Nie et al. (2021) for seven carbonaceous chondrites, from which they extrapolated a $\delta^{41}$K value of -0.33 ± 0.12‰ for the chondrule component and of 0.04 ± 0.08‰ for the matrix component. These systematic correlations reflect the early formation of two nebular reservoirs, with a volatile-depleted refractory component (e.g., chondrules) carrying the light isotopic signatures, and a CI-like reservoir with the heaviest isotopic compositions, as initially proposed by Luck et al. (2005; 2003). When all available data are considered, there is a coinciding increase in $\delta^{41}$K with water content (as a proxy for matrix content) and a decrease in $\delta^{41}$K with chondrule content in chondrites, further substantiating the role of chondrules as the carriers of K depletion and low-$\delta^{41}$K signature in chondrites (Hu et al., 2023). Therefore, the inter-group K isotopic variations in carbonaceous chondrites primarily reflect variable mixing between the two main K hosts, i.e., chondrules and the matrix, accreted by the parent bodies of carbonaceous chondrites.

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## 3.4 Potassium isotopic evidence for widespread aqueous alteration in chondrite parent bodies

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Although chondrule-matrix mixing could explain the inter-group $\delta^{41}$K variation among carbonaceous chondrites, the significant variability within individual chondrites remains perplexing. In addition to the anomalously low $\delta^{41}$K value reported for Ivuna, substantial variability has also been documented for Allende and Murchison. Our in-house Allende sample has a higher $\delta^{41}$K value (-0.169 ± 0.030‰) than the sample provided by the Smithsonian

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Museum (–0.292 ± 0.044‰), while they both fall within the previously reported range of –0.620‰ to –0.080‰ for Allende (Bloom et al., 2020; Jiang et al., 2021a; Koefoed et al., 2023; Ku and Jacobsen, 2020; Nie et al., 2021). Furthermore, the Allende sample provided by the Smithsonian Museum is an aliquot from a large batch of homogenized powder, and its δ<sup>41</sup>K value is in the middle of the reported range. Our Murchison sample also has a δ<sup>41</sup>K value (–0.250 ± 0.043‰) that is in the middle of the previously reported range for Murchison (–0.410‰ to –0.101‰, Jiang et al., 2021b; Koefoed et al., 2023; Ku and Jacobsen, 2020; Nie et al., 2021).

To identify the cause of intra-chondrite isotopic variability, we compare the two sets of δ<sup>41</sup>K vs. δ<sup>87</sup>Rb data obtained from IPGP and those reported in Nie et al. (2021), respectively, for four carbonaceous chondrites. While the two datasets differ by varying extents for individual chondrites, the δ<sup>41</sup>K and δ<sup>87</sup>Rb values correlate in both datasets (Fig. 3A). For example, the Murchison specimen analyzed at IPGP has both lower δ<sup>41</sup>K and δ<sup>87</sup>Rb values than the specimen reported by Nie et al. (2021). The correlation between δ<sup>41</sup>K and δ<sup>87</sup>Rb values suggests that the variability reflects isotopic heterogeneity at the analyzed sample scale. This isotopic heterogeneity could reflect different proportions of chondrules present in the two different specimens of a given chondrite. In this case, the specimen containing more chondrules should have a lower K abundance and a lower δ<sup>41</sup>K value because chondrules are inferred to be depleted in K and its lighter isotope compared with the matrix, as indicated by the mixing curve in Fig. 3B. However, the specimen with the lower K abundance is often found to have a higher δ<sup>41</sup>K value than that with the higher K abundance of the same chondrite (Fig. 3B), which is opposite to the expectation of mixing between varying proportions of chondrules with the matrix. Therefore, the observed sample heterogeneity cannot be due to the chondrule nugget effect alone but, to a greater extent, reflects the redistribution of K and its isotopes during aqueous alteration in chondrite parent bodies, which significantly altered the original K abundances and isotopic compositions in chondrules and matrix.

The effects of asteroidal aqueous alteration on the redistribution of alkalis in carbonaceous chondrites have been well documented, including the formation of diverse K-bearing phyllosilicate phases at < 100°C and/or anhydrous phases (e.g., nepheline and sodalite) due to Fe-alkali-halogen metasomatism at ~200°-300°C (e.g., Brearley, 2014; Ikeda and Kimura, 1996; Kimura and Ikeda, 1998; Krot et al., 1998; Krot et al., 1995). Ordinary chondrites also show signs, albeit more subtle, of interaction with aqueous fluids at varying temperatures, ranging
from low-temperature alteration (< 200°C, in the least-equilibrated petrologic type 3) to fluid-assisted metamorphism (200-400°C, in petrologic types of 4-6). Before or during the earliest stage of thermal metamorphism, aqueous fluids were suggested to have flowed through the fine-grained matrix, dissolving the mesostasis near the edge of chondrules and migrating alkalis into chondrule cores (e.g., Grossman et al., 2000; Grossman and Brearley, 2005). The presence of fluids is evidenced by the brine-bearing salt crystals found in the Monahans and Zag ordinary chondrites, which are argued to represent fluid samples from the early Solar System (Rubin et al., 2002; Zolensky et al., 1999). More recently, fluid activity on S-type asteroids is further supported by nanometer-sized halite crystals detected in particles returned from the asteroid Itokawa by JAXA's Hayabusa mission (Che and Zega, 2023). These observations collectively suggest the widespread action of fluids on primitive asteroids in the early Solar System.

Fluid-rock interactions have also been invoked to explain K isotopic variations in chondritic components of the Allende and Hamlet (LL4) chondrites (Jiang et al., 2021a; Koefoed et al., 2020). While inter-group variations in δ⁴¹K are attributable to mixing between a CI-like matrix and K-poor, isotopically light chondrules, direct measurements on Allende and Hamlet show highly variable δ⁴¹K in chondrules with values both higher and lower than those of the matrices (Fig. 3C and D). Furthermore, Ca-Al-inclusions (CAIs) formed well above the condensation temperature of K contain anomalously high K concentrations and δ⁴¹K values. These observations are inconsistent with the low δ⁶⁶Zn and δ⁷¹Ga values measured in CAIs interpreted as reflecting incomplete condensation (Kato et al., 2017; Luck et al., 2005). Instead of pristine nebular signatures, Jiang et al. (2021a) considered the high δ⁴¹K values of CAIs as a secondary feature inherited from aqueous fluids assuming that most K in the CAIs was secondary in origin (Fig. 3C). Furthermore, they proposed that as the fluids interact with the porous matrix, the heavier K isotope was preferentially leached from the matrix and entered chondrules or CAIs, overprinting the original K abundances and isotopic signatures of the chondritic components.

The preference for the heavier K isotope by aqueous fluids is supported by the direction of K isotope fractionation during chemical weathering on Earth. Global rivers (–0.59‰ to 0.12‰) and oceans (0.12 ± 0.07‰, 2SD) have heavier K isotopic compositions (Hille et al., 2019; Li et al., 2019; Wang et al., 2020; Wang et al., 2021) than fresh basaltic rocks averaging at –0.42 ± 0.08‰ (2SD) (Hu et al., 2021; Tuller-Ross et al., 2019a; Tuller-Ross et al., 2019b). In addition, studies of weathering profiles and riverine sediments indicate that aqueous fluids preferentially
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leach the heavy K isotope from the bedrock with apparent isotope fractionation factors of 0.08‰ to 0.55‰, leaving behind isotopically light weathered residues (down to -0.94‰, Chen et al., 2020; Li et al., 2019; Teng et al., 2020). Consistent with field analyses, theoretical calculations based on K-O bond strengths also suggest an enrichment of the heavier K isotope in aqueous solutions relative to typical silicate minerals and a fractionation factor of 0.24‰ between aqueous fluids and illite (Zeng et al., 2019).

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At the onset of aqueous alteration, the K abundance and δ<sup>41</sup>K value of matrix would have decreased, while the chondrules or CAIs would have increased K abundances and δ<sup>41</sup>K values. With increasing degrees of alteration, chondrules, CAIs, and matrix would tend to approach equilibrium. The significant δ<sup>41</sup>K variability in Allende and Murchison (Fig. 2) and the fact that the specimen with a lower K abundance often has a higher δ<sup>41</sup>K value (Fig. 3B) suggest that these chondrites record early stages of aqueous alteration. A broad negative correlation between δ<sup>41</sup>K values and K abundances is also observed in major groups of carbonaceous chondrites (Fig. S1), suggesting that aqueous alteration and associated redistribution of K isotopes occurred pervasively in the parent bodies of carbonaceous chondrites. Therefore, K isotopic variations in carbonaceous chondrites record volatility-driven isotope fractionations during chondrule formation, variable chondrule-matrix mixing at the accretion of chondrite parent bodies, and post-accretionary redistribution of K in chondrite parent bodies assisted by aqueous fluids.

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# 4 Conclusions

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Previous estimates of the K isotopic composition of the bulk Solar System were based **heavily** on CI chondrite Orgueil, whereas δ<sup>41</sup>K values reported for two fragments of the CI-type specimen Ivuna show a large discrepancy. This study reports the K isotopic composition of the first samples returned from a carbonaceous asteroid (Ryugu) by JAXA's Hayabusa2 mission, thereby allowing for a direct comparison of CI chondrites with pristine asteroidal samples not contaminated by terrestrial environments. Analyses of three aliquots of Ryugu particles yield similar δ<sup>41</sup>K values between -0.207‰ and -0.172‰. This range is consistent with the K isotopic compositions of most CI chondrites except for lower δ<sup>41</sup>K values found in contaminated Orgueil samples and an anomalously low value previously reported for Ivuna. **Based on the Ryugu samples and representative analyses of CI chondrites, we provide an updated estimate of the δ<sup>41</sup>K**
value for the bulk Solar System (−0.185 ± 0.078‰, 2SE). We suggest that samples of CI chondrites with δ<sup>41</sup>K values significantly deviating from this range may reflect compositional heterogeneities due to aqueous alteration in CI parent bodies. Considerable K isotopic heterogeneity is also observed in other carbonaceous chondrites and within individual chondritic groups, whereby δ<sup>41</sup>K values are broadly negatively correlated with K abundances. These observations suggest the widespread redistribution of K and its isotopes during aqueous alteration in chondrite parent bodies.

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473 Acknowledgements

We appreciate the constructive comments from two anonymous reviewers that significantly improved the manuscript and Editor Alessandro Morbidelli for efficient handling the paper. We thank Julien Moureau and Tu-Han Luu for their assistance with mass spectrometer maintenance. This work was partly supported by the IPGP analytical platform PARI, Region Ile-de-France SESAME Grants no. 12015908, and DIM ACAV+, the ERC grant agreement No. 101001282 (METAL) (F.M.), the UnivEarthS Labex program (numbers: ANR-10-LABX-0023 and ANR-11-IDEX-0005-02) (F.M.), JSPS Kakenhi grants (S.T., H.Y., T.Y.), and the CNES.

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482 All data referred to in this article can be found in the tables.

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<table><caption>Table 1. Potassium abundance (ppm) and isotopic composition (‰) of Ryugu samples and carbonaceous chondrites analyzed in this study.</caption><thead><tr><th>Sample</th><th>Group</th><th>δ<sup>41</sup>K (‰)</th><th>2SD (‰)</th><th>95% c.i. (‰)</th><th>N</th><th>[K] (ppm)</th><th>Source</th><th>Treatment</th></tr></thead><tbody><tr><td colspan="9"><strong>Ryugu particles</strong></td></tr><tr><td>A0106</td><td></td><td>-0.150</td><td>0.065</td><td>0.060</td><td>4</td><td>436</td><td>Tokyo Tech</td><td>SOM, Zn,Fe,Ti,Cr chemistry</td></tr><tr><td>duplicate</td><td></td><td>-0.172</td><td>0.085</td><td>0.056</td><td>5</td><td></td><td></td><td></td></tr><tr><td>repeat</td><td></td><td>-0.191</td><td>0.059</td><td>0.048</td><td>5</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.172</td><td>0.040</td><td>0.039</td><td></td><td></td><td></td><td></td></tr><tr><td>C0107</td><td></td><td>-0.192</td><td>0.070</td><td>0.040</td><td>6</td><td>457</td><td>Tokyo Tech</td><td>SOM, Zn,Fe,Ti,Cr chemistry</td></tr><tr><td>duplicate</td><td></td><td>-0.214</td><td>0.066</td><td>0.056</td><td>5</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.204</td><td>0.030</td><td>0.048</td><td></td><td></td><td></td><td></td></tr><tr><td>C0108</td><td></td><td>-0.188</td><td>0.086</td><td>0.059</td><td>5</td><td>604</td><td>Tokyo Tech</td><td>Zn,Fe,Ti,Cr chemistry</td></tr><tr><td>repeat</td><td></td><td>-0.220</td><td>0.072</td><td>0.069</td><td>4</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.207</td><td>0.045</td><td>0.055</td><td></td><td></td><td></td><td></td></tr><tr><td>Ryugu average</td><td></td><td>-0.194</td><td>0.038</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td colspan="9"><strong>Carbonaceous</strong></td></tr><tr><td>Orgueil</td><td>CI1</td><td>-0.081</td><td>0.070</td><td>0.066</td><td>5</td><td>568</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>Tagish lake-1</td><td>C2-ung</td><td>-0.178</td><td>0.097</td><td>0.050</td><td>5</td><td>358</td><td>Tokyo Tech</td><td>Zn,Fe,Ti,Cr chemistry</td></tr><tr><td>duplicate</td><td></td><td>-0.181</td><td>0.057</td><td>0.048</td><td>5</td><td></td><td></td><td></td></tr><tr><td>replicate</td><td></td><td>-0.215</td><td>0.057</td><td>0.043</td><td>6</td><td></td><td></td><td>Powder dissolution</td></tr><tr><td>Wtd average</td><td></td><td>-0.195</td><td>0.042</td><td>0.037</td><td></td><td></td><td></td><td></td></tr><tr><td>Tagish lake-2</td><td></td><td>-0.201</td><td>0.042</td><td>0.023</td><td>8</td><td>360</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>repeat</td><td></td><td>-0.236</td><td>0.051</td><td>0.030</td><td>7</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.215</td><td>0.049</td><td>0.032</td><td></td><td></td><td></td><td></td></tr><tr><td>Tagish lake average</td><td></td><td>-0.205</td><td>0.029</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Tarda-1</td><td>C2-ung</td><td>-0.217</td><td>0.067</td><td>0.054</td><td>6</td><td>416</td><td>Tokyo Tech</td><td>Zn,Fe,Ti,Cr chemistry</td></tr><tr><td>duplicate</td><td></td><td>-0.230</td><td>0.076</td><td>0.037</td><td>4</td><td></td><td></td><td></td></tr><tr><td>repeat</td><td></td><td>-0.224</td><td>0.056</td><td>0.045</td><td>5</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.223</td><td>0.013</td><td>0.037</td><td></td><td></td><td></td><td></td></tr><tr><td>Tarda-2</td><td></td><td>-0.166</td><td>0.063</td><td>0.050</td><td>5</td><td>416</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>repeat</td><td></td><td>-0.222</td><td>0.054</td><td>0.047</td><td>5</td><td></td><td></td><td></td></tr><tr><td>repeat</td><td></td><td>-0.218</td><td>0.058</td><td>0.039</td><td>4</td><td></td><td></td><td></td></tr><tr><td>replicate</td><td></td><td>-0.204</td><td>0.056</td><td>0.038</td><td>7</td><td></td><td></td><td>Powder dissolution</td></tr><tr><td>duplicate</td><td></td><td>-0.189</td><td>0.060</td><td>0.047</td><td>6</td><td></td><td></td><td></td></tr><tr><td>repeat</td><td></td><td>-0.222</td><td>0.060</td><td>0.050</td><td>5</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.205</td><td>0.044</td><td>0.024</td><td></td><td></td><td></td><td></td></tr><tr><td>Tarda average</td><td></td><td>-0.214</td><td>0.026</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Murchison</td><td>CM2</td><td>-0.250</td><td>0.049</td><td>0.043</td><td>7</td><td>411</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>Cold Bokkeveld</td><td>CM2</td><td>-0.267</td><td>0.064</td><td>0.035</td><td>7</td><td>476</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>Lancé</td><td>CO3.5</td><td>-0.382</td><td>0.087</td><td>0.040</td><td>7</td><td>430</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>Allende (A)-1</td><td>CV3</td><td>-0.293</td><td>0.056</td><td>0.052</td><td>5</td><td>296</td><td>Tokyo Tech</td><td>Zn,Fe,Ti,Cr chemistry</td></tr><tr><td>repeat</td><td></td><td>-0.292</td><td>0.072</td><td>0.048</td><td>5</td><td></td><td></td><td></td></tr><tr><td>Wtd average</td><td></td><td>-0.292</td><td>0.001</td><td>0.044</td><td></td><td></td><td></td><td></td></tr><tr><td>Allende-2</td><td></td><td>-0.151</td><td>0.062</td><td>0.059</td><td>5</td><td>267</td><td>IPGP</td><td>Powder dissolution</td></tr><tr><td>repeat</td><td></td><td>-0.181</td><td>0.062</td><td>0.055</td><td>4</td><td></td><td></td><td></td></tr><tr><td>repeat</td><td></td><td>-0.168</td><td>0.052</td><td>0.045</td><td>6</td><td></td><td></td><td></td></tr><tr><td>replicate</td><td></td><td>-0.178</td><td>0.065</td><td>0.031</td><td>4</td><td></td><td></td><td>Powder dissolution</td></tr><tr><td>Wtd average</td><td></td><td>-0.169</td><td>0.027</td><td>0.030</td><td></td><td></td><td></td><td></td></tr></tbody></table>

Note: 'repeat' indicates repeat instrumental analysis. 'duplicate' indicates repeat column chemistry and instrumental analysis. 'replicate' indicates repeat sample dissolution, column chemistry, and instrumental analysis. The number '-1' and '-2' attached to the same names indicate different pieces of a given chondrite. 'SOM' is short for 'Soluble Organic Matter extraction.'
<table><caption>491 **Table S1.** Potassium isotopic compositions (‰) of terrestrial samples analyzed in this study.</caption><thead><tr><th>Geostandard</th><th>Description</th><th>δ<sup>41</sup>K (‰)</th><th>2SD (‰)</th><th>95% c.i. (‰)</th><th>N</th></tr></thead><tbody><tr><td>BIR</td><td rowspan="3">Basalt</td><td>-0.448</td><td>0.059</td><td>0.036</td><td rowspan="3">7</td></tr><tr><td>replicate</td><td>-0.400</td><td>0.052</td><td>0.076</td></tr><tr><td>repeat</td><td>-0.414</td><td>0.050</td><td>0.062</td></tr><tr><td>Wtd average</td><td></td><td>-0.418</td><td>0.050</td><td>0.031</td><td></td></tr><tr><td>BCR-2</td><td rowspan="3">Basalt</td><td>-0.404</td><td>0.051</td><td>0.019</td><td rowspan="3">8</td></tr><tr><td>repeat</td><td>-0.443</td><td>0.044</td><td>0.024</td></tr><tr><td>replicate</td><td>-0.425</td><td>0.066</td><td>0.034</td></tr><tr><td>Wtd average</td><td></td><td>-0.426</td><td>0.039</td><td>0.030</td><td></td></tr><tr><td>AGV-2</td><td>Andesite</td><td>-0.455</td><td>0.042</td><td>0.030</td><td>7</td></tr><tr><td>Pacific seawater</td><td></td><td>0.152</td><td>0.060</td><td>0.054</td><td>4</td></tr><tr><td>repeat</td><td></td><td>0.128</td><td>0.078</td><td>0.050</td><td>6</td></tr><tr><td>repeat</td><td></td><td>0.113</td><td>0.039</td><td>0.037</td><td>7</td></tr><tr><td>repeat</td><td></td><td>0.133</td><td>0.084</td><td>0.056</td><td>5</td></tr><tr><td>Wtd average</td><td></td><td>0.126</td><td>0.032</td><td>0.028</td><td></td></tr></tbody></table>

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**Figure 1.** Potassium abundances ([K], ppm) and isotope compositions (δ<sup>41</sup>K, ‰) of CI chondrites and Ryugu samples. (A) A histogram of K abundances in CI chondrites reported in the literature (Ahrens et al., 1969; Anders and Grevesse, 1989; Barrat et al., 2012; Beer et al., 1984; Braukmüller et al., 2018; Edwards and Urey, 1955; Jarosewich, 1990; Kallemeyn and Wasson, 1981; Kaushal and Wetherill, 1970; Makishima and Nakamura, 2006; Mittlefehldt, 2002; Nakamura, 1974; Nichiporuk and Moore, 1974; Palme and Zipfel, 2022; Von Michaelis et al., 1969; Wiik, 1956). (B) The three aliquots of Ryugu particles have indistinguishable δ<sup>41</sup>K values and fall within the range of CI chondrites except for an anomalously lower δ<sup>41</sup>K value previously reported for Ivuna. The horizontal black line and pink field indicate the average δ<sup>41</sup>K value of Ryugu samples and CI chondrites (−0.185 ± 0.078‰), representing the current best estimate of the bulk K isotopic composition of the Solar System. The dashed grey line is the average CI chondrites reported by Koefoed et al. (2023) and is plotted for comparison. Crustal contamination results in elevated K abundances and crust-like, lower δ<sup>41</sup>K compared with pristine CI chondrite values (Ku and Jacobsen, 2020; Wang and Jacobsen, 2016). The K abundance of Ryugu C0108 is from Yokoyama et al. (2022), while those of A0106 and C0107 are new data reported in this study (Table 1).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Positive correlations of δ<sup>41</sup>K values with (A) δ<sup>65</sup>Cu values and (B) δ<sup>66</sup>Zn values in Ryugu samples and different groups of carbonaceous chondrites. These correlations suggest inter-group variations in δ<sup>41</sup>K primarily reflect mixing of a CI-, or Ryugu-like matrix component with the heaviest isotopic compositions and a volatile-depleted refractory component such as chondrules carrying the light isotopic signatures. The colored symbols are data from this study and the grey symbols are literature values. In (A), δ<sup>41</sup>K and δ<sup>65</sup>Cu values (Paquet et al., 2022) for Ryugu, Tarda, Allende were measured from the same specimens. δ<sup>65</sup>Cu values for Orgueil and Murchison are from Paquet et al. (2022), for Tagish Lake, Cold Bokkeveld, and Lancé are from Luck et al. (2003). In (B), δ<sup>41</sup>K and δ<sup>66</sup>Zn values (Paquet et al., 2022; Pringle et al., 2017) for Ryugu, Tagish Lake, Tarda, Murchison, Cold Bokkeveld, Lancé, and Allende were measured from the same specimens. δ<sup>66</sup>Zn value for Orgueil is from Paquet et al. (2022). Literature δ<sup>41</sup>K values are from Bloom et al. (2020); Jiang et al. (2021a); Jiang et al. (2021b); Koefoed et al. (2023); Koefoed et al. (2022); Ku and Jacobsen (2020); Nie et al. (2021). Although literature δ<sup>41</sup>K values were plotted with δ<sup>65</sup>Cu and δ<sup>66</sup>Zn values measured from different specimens of a given chondrite, δ<sup>65</sup>Cu and δ<sup>66</sup>Zn values reported for different specimens of Orgueil, Murchison, and Allende generally agree well (Barrat et al., 2012; Luck et al., 2005; Luck et al., 2003; Paquet et al., 2022; Pringle et al., 2017).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** K isotopic evidence of aqueous alteration of chondrites in their parent bodies. (A) Positively correlated δ<sup>41</sup>K and δ<sup>87</sup>Rb values indicate sample heterogeneity for a given chondrite. The larger symbols are data from this study and the smaller symbols are from Nie et al. (2021). (B) The broad positive correlation between δ<sup>41</sup>K values and K abundances across different carbonaceous chondrite groups indicates mixing of the CI-like, isotopically heavy matrix with K-depleted, isotopically light chondrules, as indicated by the mixing curve using endmember compositions extrapolated in Nie et al. (2021). However, for a given chondrite (coded with the same color), the specimen with the higher K abundance often has a lower δ<sup>41</sup>K value than that with the lower K abundance. The large solid triangle down is an Allende specimen from IPGP, and the open triangle down is an aliquot of pulverized Allende provided by the Smithsonian Museum. The blue diamond symbol indicates the new estimate of the composition of the bulk Solar System from this study. (C) and (D) δ<sup>41</sup>K values measured in chondrules, calcium-aluminum-rich inclusions (CAIs), and matrices in Allende and Hamlet chondrites show significant alteration of the original K abundances and δ<sup>41</sup>K values of all chondritic components. Data source: Data presented in (A) and (B) are from this study and the literature (Nie et al., 2021; Pringle and Moynier, 2017), whereas those presented in (C) and (D) are from Jiang et al. (2021a); Ku et al. (2022), and Koefoed et al. (2020).</figcaption></figure>
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**Declaration of interests**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.