A total of 5.4 grams of surface and subsurface materials of the C-type asteroid 162173 Ryugu were collected by the Hayabusa2 spacecraft during its two touchdowns and then brought back to Earth on December 5th, 2020. Initial characterization of these fragments at the JAXA curation facility revealed that Ryugu materials mineralogically resemble those of CI (Ivuna-type) chondrites1,2, an extremely rare meteorite group in our collections11. From a chemical perspective, CI chondrites are among the most primitive meteorites, having a bulk composition similar to that of the solar photosphere (e.g., ref12,13). However, paradoxically, they also contain very high abundances of mineral phases (e.g., carbonates, magnetite, phyllosilicates, and sulfide) produced by extensive interactions between water and the original mineralogy on their parent asteroids (e.g., refs11,14). Despite having a solar-like bulk elemental composition, the CI chondrites are the furthest of all carbonaceous chondrite meteorites from the solar value in terms of oxygen isotopes (e.g., ref15). Among the samples returned by the Hayabusa2 mission, particle C0009, which was collected during the second touchdown and thus could possibly represent subsurface materials, contains a small amount of anhydrous silicates and thus provides an opportunity to investigate the origins of materials that constituted the protolith of the Ryugu parent body. Here we report the results of the first oxygen isotope measurements of Ryugu anhydrous silicates and discuss their implications for the protolith of Ryugu, and by extension, of the parent asteroids of CI-chondrite meteorites.
There are twenty-five anhydrous silicate targets identified in C0009, of which nineteen are olivine, four are pyroxene, and two are aggregates of the two minerals. They amount to ~0.5 vol% of the fragment (Ito et al. submitted), much lower than the estimated fraction of 2−4 vol% for CI chondrites14. Given the limited number of Ryugu samples characterized to date, it is not yet possible to estimate the fraction of C0009-like (anhydrous silicate-bearing) samples in the total returned mass. Based on their morphology, these silicates can be broadly categorized into (1) isolated angular crystals in the matrix (Figure 1a-b), and (2) irregularly shaped olivine aggregates with abundant pores (Figure 1c). Most olivine and all pyroxene grains in C0009 belong to the first group and are morphologically similar to those mechanically separated from CI chondrites16,17. Three olivine grains belong to the second group and represent the first reported occurrence of the porous olivine morphology in CI-chondrite-like materials. Two individual olivine fragments enclosed in one oval-shaped, phyllosilicate-rich object texturally resemble the grains from the first group except for their subrounded shapes (Figure 1d). Most anhydrous silicates are highly Mg-rich; twelve grains (ten olivines and two low-Ca pyroxenes, see Extended Figure 1 for images and Extended Table 1 for their chemical compositions) with Mg number [Mg#, defined as Mg/(Mg+Fe)×100] in the range of 97−99 were selected for in-situ oxygen isotope measurements with secondary ion mass spectrometry (SIMS).
The oxygen isotopic compositions of the chosen anhydrous silicates fall into two distinct ∆17O groups (Figure 2), which correlate with the olivine/pyroxene morphologies. Six isolated olivines and two isolated pyroxenes (collectively referred to as isolated Mg-rich silicates hereafter) are characterized by ∆17O between −8‰ and −3‰, similar to the range of −6.5‰ ≤ ∆17O ≤ −2‰ found in a handful of mechanically separated Mg-rich olivine grains (Mg# > 95) from the CI chondrite Orgueil (refs16,17; also see Figure 2 for comparison). The oxygen isotopic compositions of these grains appear to form an array extending from the Young and Russell (Y&R) line18 to the right of the carbonaceous chondrite anhydrous mineral (CCAM) line19 with a slope of ~0.44 on a three-isotope plot (Figure 2a). In contrast, the olivine targets with porous textures (referred to as porous olivine aggregates hereafter) are more 16O-rich, plotting on or near the CCAM (or Y&R) line with the ∆17O values ranging from −25‰ to −15‰. The two olivine fragments enclosed in the oval-shaped object plot on the Y&R line but show different oxygen isotopic compositions, with two spot analyses on the larger grain averaging ∆17O = −4.1‰ and one spot on the smaller grain showing ∆17O = −24.5‰, equivalent to the 16O-rich olivine aggregates. This work represents the first discovery of olivine with ∆17O near the solar value (∆17O = −28.4±1.8‰)10 and those of refractory inclusions (e.g., Ca-Al-rich inclusions and hibonite; e.g., ref8,20,21) in an asteroid of CI chondrite characteristics.
All the olivine and pyroxene are either surrounded by or embayed with fine-grained phyllosilicates due to aqueous alteration. Such textures suggest that these grains are indigenous to Ryugu, rather than being xenolithic components added after aqueous activity on Ryugu had ceased. Therefore, the oxygen isotope data together with grain morphology allow us to infer the original materials incorporated into the protolith of Ryugu as they reveal a potential relationship between anhydrous silicates in C0009 and other known high temperature components found in non-CI carbonaceous chondrites (CCs). Most isolated Mg-rich silicates in C0009 plot on either the Y&R or CCAM line, indicating that their oxygen isotopic compositions were derived from mixing between 16O-rich (e.g., solids or gases with solar oxygen isotopic composition10) and 16O-poor (e.g., heavy water, such as that produced during CO self-shielding22–24) components in the solar nebula, similar to that proposed for chondrules (e.g., ref9 and references therein), and were not influenced later by isotope exchange during fluid activities on Ryugu. For the two isolated olivine crystals plotting to the right of the CCAM line, although their (δ17O, δ18O) values appear to be consistent with having experienced mass-dependent fractionation during aqueous processing on the parent body (hence forming a slope ~0.44), they are more likely to represent the original signatures of the grains for the following reasons: (1) the high Mg content of the two grains suggest very limited water-mineral interactions (e.g., refs4,25, also see below), and (2) similar (δ17O, δ18O) have been reported for some Mg-rich olivine of type-I (FeO-poor) chondrules from other more pristine non-CI CCs (e.g., refs8,9). Given all the chemical, isotopic, and morphological resemblances, we infer that the isolated Mg-rich silicates originated from olivine or pyroxene phenocrysts of type-I chondrules that existed in the Ryugu protolith before extensive aqueous alteration destroyed or disintegrated most of them. This inference is consistent with a chondrule origin hypothesized for the isolated olivine and pyroxene in CI chondrites16,17.
In comparison to these possible chondrule-derived isolated Mg-rich silicates, the porous olivine aggregates are more 16O-rich and isotopically and textually akin to porous amoeboid olivine aggregates (AOAs) previously found in other least-altered CCs (e.g., refs5,26). AOAs are considered primary fine-grained condensates from an 16O-rich nebular gas at high temperature and are among the most 16O-rich objects in the Solar System, plotting on or near the CCAM line with corresponding Δ17O of −25‰ to −20‰ (e.g., refs3–7, also see Figure 2). The fact that the oxygen isotopic compositions of the three porous olivine samples fall well within the range defined by porous AOAs (Figure 2a) demonstrates a clear relationship between the two and thus provides strong evidence that AOAs were present in the protolith of Ryugu.
Each porous olivine aggregate was measured twice. In porous-oliv2 and porous-oliv4 (Table1), one spot appears to have an elevated Δ17O value relative to the other (Δ17O = ~−15‰ vs. ~−20‰, also indicated by light and dark blue symbols in Figure 2), but this observation should not be taken as representing oxygen isotope heterogeneity within individual porous aggregates. Rather, the higher Δ17O value in porous-oliv2 and both Δ17O values in porous-oliv4 were affected by overlapping of the ion beam with adjacent mineral phases formed during aqueous alteration (e.g., phyllosilicates, micro-magnetite, micro-carbonates, etc.), consistent with the high 16OH− signals measured in these spots (see Methods and Table 1). Since secondary phases normally have Δ17O around 0‰ to +3‰ (e.g., refs15,17,27,28), we interpret the results to indicate that the spot with Δ17O ~−15‰ in porous-oliv2 should have an oxygen isotopic composition similar to its counterpart point with Δ17O ~− 20‰, and that porous-oliv4 should be more 16O-rich (i.e., Δ17O < − 20‰) than the reported values.
Given the significantly different oxygen isotopic compositions between the two olivine fragments in the oval-shaped object (green triangles in Figure 2), it is more challenging to definitively relate them to any known refractory components in chondritic meteorites. However, if one assumes that both grains were part of the precursor to the oval-shaped object and are survivors of aqueous processing, the precursor material most likely could have been a chondrule-like inclusion because ∆17O = −4.1‰ (larger grain) and −24.5‰ (smaller grain) are consistent with what have been reported for some olivine phenocrysts and relicts, respectively, inside some CC-chondrules (e.g., refs9,29,30). If the two fragments are genetically unrelated, meaning that their co-existence in the oval-shaped object is purely coincidental, one can still infer that the relatively 16O-poor grain may be of chondrule origin, and the lower grain could be a remnant of an AOA or a relict olivine of a chondrule.
From our oxygen isotope and textural observations, we conclude that chondrules and refractory AOAs were incorporated into the protolith of Ryugu, and that portions of these materials survived various degrees of fluid activities. Chondrule mesostasis, composed primarily of glass, would be quickly transformed into phyllosilicates upon interacting with water. Observations of phyllosilicates replacing chondrule mesostasis in much less aqueously altered chondrites (e.g., CM2.8) support this inference (e.g., ref31). AOAs are composed of more refractory minerals than chondrule mesostasis but are also susceptible to destruction during aqueous alteration because of their fine-grained, porous morphologies with high surface area to volume ratios. Chondrule phenocrysts of olivine and pyroxene would be more resistant to alteration than AOAs but can still be completely transformed into phyllosilicates under extensive aqueous processing. This explains why AOA remnants are much less abundant than isolated olivine and pyroxene in C0009, and why traces of AOAs and chondrules have not been identified in other Ryugu samples that contain higher abundances of secondary phases as a result of higher degrees of aqueous alteration (Ito et al., submitted).
The presence of AOA remnants in particle C0009 also has important implications for the average oxygen isotopic compositions of constituents in the parent body (or bodies) of Ryugu and CI chondrites. Previous attempts at constraining the oxygen isotopic evolution of CI chondrite materials by invoking water-rock interactions in a closed system (e.g., refs16,32,33) have assumed the average oxygen isotopic composition of initial anhydrous silicates (i.e., prior to aqueous alteration) to be the same as the average value of olivine/pyroxene separated from either CM2 Murchison [(δ17O, δ18O) = (−7.4‰, −4.2‰); ref15], or CI Orgueil [(δ17O, δ18O) = (+1.8‰, +4.8‰); ref16]. Our discovery of AOA remnants suggests that the precursor silicate components in at least part of the Ryugu protolith could have been more 16O-rich than previously considered for the CI chondrite parent body. Since the bulk Ryugu particles and CI chondrites share very similar ∆17O (Ito et al. submitted), the initial fluid in the Ryugu protolith must have been more 17,18O-rich than that inferred for CI chondrites15–17.
The chemical, mineralogical, and isotopic similarities between most of the Ryugu particles (C0009 clearly is slightly different) and CI chondrite samples suggest that both may have been derived from parent bodies of similar composition and evolutionary history. Whether AOAs were also incorporated into the protoliths of CI chondrites will require additional studies of these meteorites. However, one can still gain insight into the original oxygen isotope compositions of isolated anhydrous silicates in CI chondrites before aqueous alteration took place. Previous oxygen isotope measurements of anhydrous silicates separated from CI chondrites have shown that most olivine and pyroxene have Mg# < 95 (20 out of 24) and are characterized by Δ17O ~0‰. Negative Δ17O values have only been found in CI olivine of high Mg# (≥ 95) (4 out of 24; see Figure 2; refs16,17). Although Mg-poor grains were not measured in this study, our data on highly Mg-rich Ryugu olivine and pyroxene (Mg# = 97−99) still partly corroborate the relationship. This implies that the Δ17O ~0‰ associated with anhydrous silicates of relatively low Mg numbers (Mg# < 95) observed in CI-chondrite samples primarily resulted from isotope exchange during more substantial water-rock interaction on the parent body, and thus should not be included to infer the initial average oxygen isotopic composition of anhydrous silicates, contrary to what has been previously assumed (refs16,17). Instead, the initial anhydrous minerals in the pre-alteration CI chondrite parent bodies should have been at least as 16O-rich as olivine/pyroxene of type-I chondrules in the least altered chondrites, or more so if AOAs co-accreted. This raises a possibility that the protoliths of CI and other carbonaceous chondrites incorporated similar types of anhydrous silicate inclusions (Mg-rich chondrules ± AOAs), and consequently, more olivine/pyroxene grains of very high Mg# and remnants of AOAs should be found in CI chondrite samples with more searches.