Chondrule-like objects and CAIs in asteroid Ryugu: Earlier generations of chondrules
D. Nakashima1, T. Nakamura1, M. Zhang2, N. T. Kita2, T. Mikouchi3, H. Yoshida3, Y. Enokido1, T. Morita1, M. Kikuiri1, K.
Amano1, E. Kagawa1, T. Yada4, M. Nishimura4, A. Nakato4, A. Miyazaki4, K. Yogata4, M. Abe4, T. Okada4, T. Usui4, M. Yoshikawa4, T. Saiki4, S. Tanaka4, S. Nakazawa4, F. Terui5, H. Yurimoto6, T. Noguchi7, H. Yabuta8, H. Naraoka9, R. Okazaki9, K. Sakamoto4, S. Watanabe10, S. Tachibana3, and Y. Tsuda4
1Tohoku University, Japan (dnaka@tohoku.ac.jp), 2University of Wisconsin-Madison, USA, 3University of Tokyo, Japan,4
ISAS/JAXA, Japan, 5Kanagawa Institute of Technology, Japan, 6Hokkaido University, Japan, 7Kyoto University, Japan,8Hiroshima University, Japan, 9Kyushu University, Japan, 10Nagoya University, Japan.
Introduction: The Hayabusa2 spacecraft returned samples from C-type asteroid Ryugu [1]. The “stone” team, one of the six
initial analysis teams, received 16 stone samples from the ISAS curation facility and conducted analyses for elucidation of early
evolution of asteroid Ryugu [2]. The Ryugu samples mineralogically and chemically resemble CI chondrites [2-5]. It was suggested that the Ryugu original parent body formed beyond the H2O and CO2 snow lines (> 3 – 4 au) in the solar nebula at 1.8 – 2.9 Myr
after CAI formation [2]. Here we report oxygen isotope analyses (~ 1 µm spots; IMS-1280 at UW-Madison) of chondrule-like objects and CAIs observed in the Ryugu samples [2] and discuss the significance of the presence of chondrule-like objects and CAIsin asteroid Ryugu and their origins.
Results: Small chondrule-like objects and CAIs (< 30 µm; Fig. 1) occur with isolated olivine, pyroxene, and spinel in lessaltered clasts in the polished section C0002-P5 and in polished sections of C0040-02 and C0076-10. The three chondrule-like objects have rounded-to-spherical shapes and consist of olivine with Mg# of ~ 99, FeNi metal, sulfide, and Al-Ti-free diopside but
free from glass or glass-altered phase. Two of the three chondrule-like objects contain low-iron, manganese-enriched (LIME) olivine [6], and one of them shows sub-µm-sized 120° triple junctions, which is evidence of annealing [7]. The two CAIs consist ofspinel and hibonite along with tiny perovskite inclusions. The oxygen isotope ratios show a bimodal distribution at peaks of ~ –43‰ and ~ 0‰ in 18O along the CCAM and the PCM lines [8,9] (Fig. 2). Oxygen isotope ratios of the individual objects are indistinguishable within the uncertainty. Two out of the three chondrule-like objects are 16O-rich with 17O of ~ –23‰ (C0002-P5C2-Chd and C0040-02-Chd), while the other is 16O-poor with 17O of ~ –3‰ (C0002-P5-C1-Chd). The two CAIs are 16O-rich with
17O of ~ –23‰ (C0040-02-CAI and C0076-10-CAI).
Discussion: Mg# of ~ 99 and the 17O value of ~ –3‰ for C0002-P5-C1-Chd are characteristic for type I chondrules in carbonaceous chondrites [10]. The two 16O-rich chondrule-like objects with Mg-rich olivine share characteristics with AOAs [11] andare likely to have been originally AOAs (or fragments) and melted (and annealed) by a heating event in the 16O-rich environment
possibly near the Sun. The three chondrule-like objects, which are dominated by Mg-rich olivine and free from glass, resemble
what has been proposed as earlier generations of chondrules [12]. If the three objects are earlier generations of chondrules, the two
distinct oxygen isotope ratios of 16O-rich and -poor are evidence for the argument that 16O-rich and -poor isotope reservoirs existed
in the early stage of the chondrule formation [10].
Spinel-rich CAIs accompanied by altered phases like the two Ryugu CAIs are observed in CM chondrites [e.g., 13]. However,
the two Ryugu CAIs are smaller than the CM-CAIs and as small as CAI-like Wild2 particles [14]. Spinel in the CM-CAIs contain
Cr2O3 mostly less than 0.6 wt% [13], while that in cometary CAIs contains more Cr2O3 than 1.7 wt% [15]. The relatively highCr2O3 contents are explained by addition of Cr from Cr-bearing gas or dust during the remelting events at few Myr after CAI formation [15,16]. Spinel in the two Ryugu CAIs contain Cr2O3 less than 0.2 wt%. It is possible that the two Ryugu CAIs escapedfrom remelting events that supplied Cr. If this is the case, the two Ryugu CAIs are possibly as old as the CM-CAIs.
The Ryugu original parent body formed at > 3 – 4 au from the Sun [2], while CAIs formed near the Sun [11]. Radial transportof the CAIs and the two 16O-rich chondrule-like objects to the region where the Ryugu original parent body formed is required.
Likewise, it has been suggested from the observations of chondrule-like and CAI-like Wild2 particles that chondrules and CAIs
were transported from the inner regions to the Kuiper belt in the solar nebula [14,17]. Given the smaller sizes of the cometarychondrules and CAIs than those in chondrites, radial transport favoring smaller objects to farther locations may have occurred in
the solar nebula; e.g., a combination of advection and turbulent diffusion [18]. If this is the case, the occurrence of chondrule-likeobjects, CAIs, isolated anhydrous grains that are likely to be fragments of chondrules and CAIs, and AOA-like porous olivine[2,19,20], which are as small as the Wild2 particles, suggests that the Ryugu parent body formed at farther location than any otherchondrite parent bodies and acquired these anhydrous objects transported from the inner regions of the solar nebula.
Chondrules in different chondrite groups have distinct chemical, isotopic, and physical properties, suggesting chondrule formation in local disk regions and subsequent accretion to their respective parent bodies without significant inward/outward migration [10]. The rarity of chondrules (and chondrule-like objects) in the Ryugu samples suggests the Ryugu parent body formation ina region scarce in chondrules. Instead, small chondrules and fragments may have been transported with CAIs from the inner regions of the solar nebula. Since the formation age of the Ryugu parent body is as early as those of major types of carbonaceouschondrite chondrules [2,21], chondrules typically observed in chondrites (~ 1 mm) should have presented in the inner regions ofthe solar nebula when forming the Ryugu parent body. Considering radial outward transport favoring smaller objects, small fragments of relatively large chondrules may have also been provided and observed as isolated anhydrous grains in the Ryugu samples.
CAIs in the Ryugu samples are much less abundant (~ 20 ppm) than those in the Wild2 particles (~ 0.5%; [15]), suggesting destruction of the CAIs and chondrules in the Ryugu parent body during the extensive aqueous alteration. The observed chondrulelike objects and CAIs may have survived along with isolated anhydrous grains in less-altered regions in the Ryugu parent body.
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Fig. 2: Oxygen three-isotope ratios of three chondrule-like objects
and two CAIs in the Ryugu samples. TF, PCM, and CCAM represent
the Terrestrial Fractionation line, the Primitive Chondrule Mineralline, and the Carbonaceous Chondrite Anhydrous Mineral line.
Fig. 1: BSE images of three chondrule-like objects and two CAIs inthe Ryugu samples analyzed for oxygen isotopes. SIMS analysis
spots are shown by the vertex of an open triangle. The rectangle areadrown by the dashed line in panel c corresponds to the region extracted by the FIB sectioning. Abbreviations: Ol, olivine; Mt, Fe-Ni metal;
Sul, Fe-sulfide; Ox, oxide; Diop, diopside; Sp, spinel; Hib, hibonite;
Pv, perovskite; Phyl, phyllosilicates.