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Preliminary analyses of asteroid Ryugu samples show kinship to aqueously altered CI (Ivuna-type) chondrites, suggesting similar origins. We report identification of C-rich, particularly primitive clasts in Ryugu samples that contain preserved presolar silicate grains and exceptional abundances of presolar SiC and isotopically anomalous organic matter. The high presolar silicate abundance (104 ppm) indicates that the clast escaped extensive alteration. The 5 to 10 times higher abundances of presolar SiC (~235 ppm), N-rich organic matter, organics with N isotopic anomalies (1.2%), and organics with C isotopic anomalies (0.2%) in the primitive clasts compared to bulk Ryugu suggest that the clasts formed in a unique part of the protoplanetary disk enriched in presolar materials. These clasts likely represent previously unsampled outer solar system material that accreted onto Ryugu after aqueous alteration ceased, consistent with Ryugus rubble pile origin.

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (P CO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and P CO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H 2 O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and P CO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R −1 (1/ T − 1/1200)] was obtained. We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H 2 O gases are larger than those of gaseous isotope exchange between CO and H 2 O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H 2 O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H 2 O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H 2 O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K). [ABSTRACT FROM AUTHOR]

Al–Mg mineral isochron studies using secondary ion mass spectrometry (SIMS) have revealed the initial 26Al/27Al ratios, (26Al/27Al)0, for individual Ca‐Al‐rich inclusions (CAIs) in meteorites. We find that the relative sensitivity factors of 27Al/24Mg ratio for SIMS analysis of hibonite, one of the major constituent minerals of CAIs, exhibit variations based on their chemical compositions. This underscores the critical need for using appropriate hibonite standards to obtain accurate Al−Mg data. We measured the Al−Mg mineral isochron for hibonite in a fine‐grained CAI (FGI) from the Northwest Africa 8613 reduced CV chondrite by SIMS using synthesized hibonite standards with 27Al/24Mg of ~30, ~100, and ~400. The obtained mineral isochron of hibonite in the FGI yields (26Al/27Al)0 of (4.73 ± 0.09) × 10−5, which is identical to that previously obtained from the mineral isochron of spinel and melilite in the same FGI (Kawasaki et al., 2020). The uncertainties of (26Al/27Al)0 indicate that the constituent minerals in the FGI formed within ~0.02 Myr in the earliest solar system. The disequilibrium O‐isotope distributions of the minerals in the FGI suggest that the O‐isotope compositions of the nebular gas from which they condensed underwent a transitional change from 16O‐rich to 16O‐poor within ~0.02 Myr in the earliest solar system. Once formed, the FGI may have been removed from the forming region within ~0.02 Myr and transported to the accretion region of the parent body. [ABSTRACT FROM AUTHOR]

Coarse-grained igneous calcium-aluminum-rich inclusions (CAIs) are suggested to have experienced gas–melt isotope exchange of oxygen during the melting events of their precursors. Therefore, their oxygen isotope variation would preserve information about the high-temperature processes in the earliest Solar System. We experimentally determined oxygen isotope exchange kinetics between CAI analog melt and carbon monoxide (CO) gas at 1420 °C and 1460 °C under CO gas partial pressures of 0.1, 0.5, and 1 Pa to understand the role of CO gas on the oxygen isotope exchange. We observed oxygen isotope zoning profiles inside the reacted samples that formed through the oxygen isotope exchange reaction at the melt surface and oxygen diffusion in the melt. The zoning profiles were fitted using a three-dimensional spherical diffusion model with time-dependent surface concentration. The oxygen isotope exchange efficiency for colliding CO molecules is estimated to be ∼3.3 × 10–4, which is much smaller than that for H 2 O (0.28). The oxygen diffusion coefficient obtained in this study is similar to that obtained in the oxygen isotope exchange experiments between the CAI melt and H 2 O, suggesting that the diffusion species in the melt is O2–, despite the surrounding atmospheres. A comparison of the isotope exchange reaction kinetics between (1) CAI melt and CO gas, (2) CAI melt and H 2 O gas, and (3) CO and H 2 O gases shows that the reaction rate decreases in the order of (3), (2), and (1). The rapid isotope exchange of the reaction (1) indicates that the oxygen isotopic compositions of H 2 O and CO should have been equilibrated during the melting and crystallization processes of igneous CAIs. Both H 2 O and CO change the oxygen isotope compositions of molten CAI in the same direction, although reaction (2) controls the isotope exchange timescale between the CAI melt and surrounding gas. Our dataset demonstrates that type B CAIs having melilite with homogeneous oxygen isotope composition should have been heated for 2–3 days at P H2 > 100 Pa above the melilite liquidus (∼1400 °C) in the solar protoplanetary disk. [ABSTRACT FROM AUTHOR]

Calcium-aluminum-rich inclusions (CAIs) are known as the oldest high-temperature mineral assemblages of the Solar System. The CAIs record thermal events that occurred during the earliest epochs of the Solar System formation in the form of heterogeneous oxygen isotopic distributions between and within their constituent minerals. Here, we explored the kinetics of oxygen isotope exchange during partial melting events of CAIs by conducting oxygen isotope exchange experiments between type B CAI-like silicate melt and 18O-enriched water vapor (P H2O = 5 × 10−2 Pa) at 1420 °C. We found that the oxygen isotope exchange between CAI melt and water vapor proceeds at competing rates with surface isotope exchange and self-diffusion of oxygen in the melt under the experimental conditions. The 18O concentration profiles were well fitted with the three-dimensional spherical diffusion model with a time-dependent surface concentration. We determined the self-diffusion coefficient of oxygen to be ∼1.62 × 10−11 m2 s−1, and the oxygen isotope exchange efficiency on the melt surface was found to be ∼0.28 in colliding water molecules. These kinetic parameters suggest that oxygen isotope exchange rate between cm-sized CAI melt droplets and water vapor is dominantly controlled by the supply of water molecules to the melt surface at P H2O < ∼10−2 Pa and by self-diffusion of oxygen in the melt at P H2O > ∼1 Pa at temperatures above the melilite liquidus (1420–1540 °C). To form type B CAIs containing 16O-poor melilite by oxygen isotope exchange between CAI melt and disk water vapor, the CAIs should have been heated for at least a few days at P H2O > 10−2 Pa above temperatures of the melilite liquidus in the protosolar disk. The larger timescale of oxygen isotopic equilibrium between CAI melt and H 2 O compared to that between H 2 O and CO in the gas phase suggests that the bulk oxygen isotopic compositions of ambient gas at ∼1400 °C in the type B CAI-forming region is preserved in the oxygen isotopic compositions of type B CAI melilite. Based on the observed oxygen isotopic composition, we suggest that a typical type B1 CAI (TS34) from Allende was cooled at a rate of ∼0.1–0.5 K h−1 during fassaite crystallization. [ABSTRACT FROM AUTHOR]

Fine-grained Ca-Al-rich inclusions (FGIs) in CV chondrites are suggested to be condensates formed directly from the solar nebular gas. Al–Mg mineral isochrons of seven FGIs from reduced CV chondrites Efremovka, Vigarano, Thiel Mountains 07007, and Northwest Africa 8613 were obtained via in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slopes of the mineral isochrons for seven FGIs exhibit statistically significant variations in initial 26Al/27Al ratios, (26Al/27Al) 0 , ranging from (5.19 ± 0.17) to (3.35 ± 0.21) × 10−5, which correspond to a relative age spread of 0.44 ± 0.07 Myr. Inferred upper limit of (26Al/27Al) 0 for the FGIs is identical to the Solar System (26Al/27Al) 0 of ∼5.2 × 10−5 as determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. The intercepts of the mineral isochrons, the initial 26Mg/24Mg ratios the FGIs formed with, are consistent with Mg-isotope evolution path of a solar-composition nebular gas. The observed variations in (26Al/27Al) 0 for FGIs are essentially similar to those (∼5.2 to ∼4.2 × 10−5) for coarse-grained, igneous CAIs of CV chondrites that are formed by melting and solidification. If 26Al was distributed homogeneously in the forming region, then our data indicate that thermal processes of condensation and melting for CAI formation occurred contemporaneously and continued for at least ∼0.4 Myr at the very beginning of the Solar System. Alternatively, the observed variations in (26Al/27Al) 0 also indicate the possibility of heterogeneous distributions of 26Al in the forming region, corresponding to a range of over at least 3.4 × 10–5 < (26Al/27Al) 0 < 5.2 × 10–5. [ABSTRACT FROM AUTHOR]

Most meteorites are believed to be chips from planetesimals and can provide clues to constrain the chemical evolution and dynamic history of the early Solar System. In this study, we report two unique fragments (ALF-1 and ALF-2) enclosed in the CH3 carbonaceous chondrite Sayh al Uhaymir 290. These two fragments are dominated by Ca,Fe-rich olivine with various amounts of Al,Ti-rich augite, anorthite, oxide minerals, Ca-phosphate mineral, FeNi metal, enstatite, and less Al,Ti-rich augite. The Ca-Mg-Fe systematics and Fe/Mn ratios of olivine grains in the two fragments are similar to that of the volcanic angrites. These similar features imply that the parent body of the two fragments might have bulk chemistry, oxygen fugacity, and differentiation resembling the angrite parent body. However, high-precision SIMS measurements reveal oxygen isotope compositions Δ17O = 0.91 ± 0.18‰ (2σ) of olivine in the two fragments are distinctly different from that of known angrite meteorites, possibly representing a new type of basaltic planetesimal. The two fragments also have a few mineralogical features distinct from angrite meteorites. They include: (1) the lack of a typical igneous texture; (2) the coexistence of two spatially associated Al,Ti-rich augites with different contents of CaO, Al 2 O 3 , MgO, and P 2 O 5 ; (3) the presence of an enstatite-dominant rim in ALF-1; (4) the presence of a Cr,Mn-rich margin in ALF-1; and (5) complex microscale heterogeneity in oxide minerals. We argue that these features could be due to complex thermal histories in their parent body and/or after ejection from their parent body. Among these features, the Mn,Cr-rich margin and the enstatite-dominant rim in ALF-1 can be best explained with thermal events in nebular settings. This implies that the differentiation of the parent body of the two fragments might have taken place prior to the dissipation of the nebular gas. [ABSTRACT FROM AUTHOR]

Aqueous alteration in planetesimals is one of the earliest geological processes in the solar system. The timing of aqueous alteration sheds light on the timescale of material evolution through water–rock interaction in small bodies. The 53Mn-53Cr decay system, where a short-lived radionuclide 53Mn decays to 53Cr with a half-life of 3.7 Myr, is a powerful tool for dating carbonates in primitive meteorites that formed during aqueous alteration. In CI chondrites and samples returned from asteroid Ryugu, a major carbonate mineral is dolomite (CaMg(CO 3) 2) and could be dated precisely because of their relatively high Mn abundances. However, the lack of a proper dolomite standard for secondary ion mass spectrometry (SIMS) hinders us from obtaining accurate Mn/Cr ratios of carbonates, resulting in erroneous formation ages. In this work, we synthesized Mn-, Cr-, and Fe-bearing crystalline dolomite as standard materials and evaluated the relative sensitivity factor (RSF) of Mn/Cr for SIMS analysis, namely, the ratio of Mn/Cr obtained using SIMS to true Mn/Cr. We found that the RSF values of the dolomite standards range from 0.8 to 0.9, slightly higher than that of calcite (CaCO 3) (∼0.7), and increase with their Fe contents. We used the newly evaluated RSF values to date dolomite in the Ivuna CI chondrite and obtained an initial 53Mn/55Mn ratio of (3.95 ± 0.49) × 10−6 (95 % confidence interval) and the corresponding absolute age of 4564.0 + 0.6/−0.7 Ma. Our new initial 53Mn/55Mn ratio is 26 ± 19 % higher than that obtained by a previous study for the same dolomite grain using a calcite standard. This difference is consistent with the difference between the RSF values of dolomite and calcite. Based on these results, we updated the initial 53Mn/55Mn ratio previously reported for dolomite in the Ryugu sample A0058 to be (3.21 ± 0.66) × 10−6, which corresponds to an absolute age of 4562.8 + 1.0/−1.2 Ma. This age seems to be the best estimate for the formation age of dolomite in Ryugu currently available. [ABSTRACT FROM AUTHOR]

- Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca-Al-rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core-mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous 16O-poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ18O = 5-10‰, which suggests that the domain was formed in a 16O-poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from 16O-rich (δ18O = −40‰) to 16O-poor (δ18O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more 16O-rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI. [ABSTRACT FROM AUTHOR]