A Ca-carbonate grain designated ‘Ca 2’ has Δ17O = + 2.2‰, the highest value of Δ17O we have measured in Ryugu carbonate, which suggests that it recorded an early phase of fluid evolution when relatively 17O- and 18O-enriched fluid21 was less equilibrated with 16O-rich nebular solids22. The petrology of ‘Ca 2’ is distinct from the other Ca-carbonates, further supporting that its formation conditions were distinct from the other Ca-carbonates37. Figure 3 shows that ‘Ca 2’ is also enriched in 13C at δ13C = + 96.9‰, suggesting that carbon in the fluid was initially isotopically heavy and derived from outer solar system CO2 ices, similar to what has been inferred for some carbonaceous chondrites25,26,38. Therefore, we conclude that Ryugu accreted in the outer solar system beyond the CO2 ice line, consistent with previous observations of bulk H and N isotopes that are consistent with an outer solar system origin3.
The population of Ca-carbonate in particle C0009 shows a range in Δ17O of ~ 0 to + 2.2‰, following a mass-independent trend which requires that the O isotopic composition of the fluid evolved over the course of Ca-carbonate precipitation. This is in contrast to calcite grains found in Orgueil20, which follow a mass-dependent trend with constant Δ17O with a restricted range in δ18O. We suggest that this difference reflects variation in alteration processes between Ryugu and Orgueil: The Ca-carbonate in Ryugu recorded the progress of equilibration between fluid and 16O-poor anhydrous silicate22, while calcite in Orgueil precipitated after this equilibration had occurred.
Magnetite in A0037 and the “Ca 2” Ca-carbonate grain (Fig. 1b; see also Fig. 4b in Yamaguchi et al., 2022) in C0009 share the same Δ17O values (within uncertainty) that is higher than the Δ17O of dolomite and other Ca-carbonates, reflecting a less-equilibrated fluid composition. We conclude that magnetite and Ca-carbonate like ‘Ca 2’ were among the earliest minerals to precipitate during the alteration of the Ryugu protolith, predating most carbonate formation. Though magnetite in A0037 and ‘Ca 2’ in C0009 are from different particles, if we assume that alteration was sufficiently widespread on the Ryugu parent body so that ‘Ca 2’ and A0037 magnetite formed in equilibrium37, we estimate the formation temperature at this early stage of alteration using equilibrium thermometry of calcite and magnetite to be 0 − 20°C39. Further discussion of magnetite-H2O and calcite-H2O fractionation can be found in the Supplementary Information.
Dolomite in this study and bulk Ryugu particles share a value of Δ17O within our uncertainties4. The bulk oxygen isotopic composition is dominated by phyllosilicates, with variable contributions from carbonates which can increase the δ18O of the bulk analysis. If we suppose that the weighted average of bulk Ryugu δ18O from Greenwood et al., 2022 of 15.88 ± 4.85‰ (2SD) represents the composition of phyllosilicate, we may calculate an equilibrium formation temperature using this phyllosilicate-dominated bulk analysis and our observed range of dolomite δ18O values (+ 25–34‰). Using experimentally-determined fractionation factors for dolomite40 and brucite41, we constrain the equilibration temperature of dolomite and phyllosilicate to 88–240°C. Further discussion of phyllosilicate-H2O and dolomite-H2O fractionation can be found in the Supplementary Information.
We suggest the following order for the sequence of aqueous alteration on Ryugu: first, magnetite and Ca-carbonates like ‘Ca 2’ precipitated from aqueous fluids with high Δ17O at T < 20°C with C composition dominated by CO2 ice. As the fluid continued to exchange oxygen with 16O-rich anhydrous silicates22, additional Ca-carbonate precipitated as Δ17O fell from ~ + 1.1 to 0‰. Finally, most dolomite formed at about Δ17O = + 0.4‰ after Mg had been added to the fluid by alteration of Mg-rich silicates to form phyllosilicates with similar Δ17O to dolomite. The relatively homogeneous Δ17O composition of dolomite indicates that the pace of evolution of the fluid’s oxygen isotopic composition had slowed by the time of dolomite formation. Petrographic observations of magnetite inclusions enclosed in dolomite but not in calcite support this sequence of events37.
Carbon and oxygen isotopic analyses performed on the same grains were used to explore correlations between the two isotopic systems. Figure 5 illustrates that δ13C is correlated with δ18O (upper panel) and Δ17O (lower panel), similar to trends observed for some CM chondrites28. This observation suggests that methane formation via serpentinization of the protolith followed by loss to space did not strongly affect the δ13C of Ryugu carbonate, as methane release would enrich 13C in the fluid over time16,27. In contrast, we observe that carbonate formed from less-equilibrated water (e.g., with higher δ18O and Δ17O) is also the most enriched in δ13C. One possible scenario could be that the initial unequilibrated fluid composition, presumably similar to the fluid recorded by ‘Ca 2’, evolved towards lower δ13C as the fluid interacted with and oxidized Ryugu’s relatively 13C-depleted organic matter3.
The old ages measured in Ryugu carbonate stand in contrast to ages obtained from carbonate in carbonaceous chondrites, most of which were thought to have formed 4–6 Myr after CAIs7,8,29. This difference arises from our use of matrix-matched standards, as opposed to calcite standards used exclusively in previous studies, to determine the Mn/Cr of the carbonates. Had we corrected measured Mn+/Cr+ using a relative sensitivity factor derived only from analyses of calcite, we would have obtained ages of 3.0 Myr and 3.5 Myr after CAI formation for A0037 and C0009 carbonate respectively, approaching the range of ages previously determined for carbonates in carbonaceous chondrites7,8,29.
These old carbonate formation ages suggest a significantly different formation scenario for Ryugu than those previously proposed for the asteroid parent bodies of carbonaceous chondrites. Our data clearly show that aqueous fluids responsible for carbonate formation were active on Ryugu (or its progenitor asteroid) early in Solar System history, within the first ~ 1.4 Myr after CAI formation. At that time, 26Al in chondritic material was still at the level of 26Al/27Al ~ 10− 5, abundant enough to melt accreted ices and drive aqueous alteration. However, for 26Al heating to not be so intensive as to cause water loss or even silicate melting and chemical differentiation, Ryugu must have accreted as a small asteroid which could effectively conduct heat away from its interior to cool itself by radiation. The inferred presence of co-accreted CO2 ice constrains the initial temperature of the parent body to below the sublimation temperature of CO2. By modeling parent bodies accreting as mixtures of 50% chondritic material and 50% water ice6,42 at an initial temperature of 78 K, we find that parent bodies accreting before 1.4 Myr must be smaller than 17 km in diameter for the internal temperature to remain below 400 K43,44. In these bodies, the interior 4 km reaches the melting point of water within 0.2 Myr after accretion, and remains warm enough to support liquid water for an additional 1.5 Myr.
Alternatively, it could be possible to form Ryugu components in a progenitor body larger than 17 km in diameter which was later disrupted by impact before reaching peak temperatures. Ryugu is a ~ 1 km diameter asteroid inferred, like many asteroids, to be a ‘rubble pile’ characterized by large internal void spaces and a low bulk density (1,190 ± 20 kg m− 3)45. A multi-stage scenario of brecciation and reassembly is also supported by petrographic and shock characteristics observed in Ryugu particles3,37,46. This view is very different from prior estimates of parent body size and accretion times based upon younger carbonate ages, which suggested that CM and CI parent bodies were > 50 km in diameter and accreted ~ 3–3.5 Myr after CAI formation7,8,29.
An early formation scenario for C-type asteroids has implications for models seeking to understand the origins of the so-called ‘isotopic dichotomy’ within the solar nebula. In this framework, the early solar system was divided into two reservoirs, one characterized by isotopic compositions similar to those of the volatile-rich carbonaceous chondrites (CC), and the other being isotopically similar to the compositions of volatile-depleted ordinary-chondrite, enstatite-chondrite, and terrestrial (collectively known as the non-carbonaceous (NC) isotopic reservoir) materials47. Whereas the NC group accreted from materials formed in the inner solar system, the CC group is thought to have accreted in the outer solar system, beyond the snow line. Based on 182Hf-182W ages of iron meteorites with CC affinities, it has been suggested that some planetesimals in the outer solar system accreted within ~ 1 Myr of CAI formation48. This timescale is consistent with such objects having melted and chemically differentiated into core-mantle structures due to 26Al heating, and is also consistent with the accretion time of NWA 011, a basaltic achondrite with CC affinities that accreted within 1.6 Myr of CAI formation49. Based on previous Mn-Cr dating of carbonates it was thought that CM and CI chondrites escaped such heating by virtue of having accreted at later times, after most 26Al had decayed. However, early formation for undifferentiated CC material, such as that from Ryugu, requires an explanation (e.g., formation in a small body or early disruption by impact) for the simultaneous existence of differentiated and unmelted CC materials. Similarly, models of accretion and transport in the disk which invoke a late formation time for carbonaceous chondrite parent bodies50 should consider the implications of early formation of these objects.