Aluminum-rich Inclusions Research Articles

Late Pebble Accretion of Comet 81P/Wild 2 Nucleus: Evidence from a Plagioclase-bearing Chondrule Fragment, Pyxie

Comet 81P/Wild 2 is a ∼4.5 km-sized primordial object that almost has not been modified by internal heating by 26Al decay. Its nucleus could have been formed by hierarchical agglomeration or gravitational collapse of pebble swarms concentrated by streaming instability. To shed light on the cometesimal formation mechanism from laboratory sample analysis, we reexamined the 26Al–26Mg isotope systematics of the plagioclase-bearing fragment, Pyxie (from Wild 2 track 81), with significantly improved analytical precision. The revised upper limit of the initial (26Al/27Al)0 of Pyxie is ≤1.5 × 10−6, 2 times smaller than those estimated from other Wild 2 fragments. Assuming homogenous distribution of 26Al in the early solar system, the minimum crystallization age of Pyxie is estimated to be >3.6 Ma after calcium–aluminum-rich inclusions. Additional petrologic examination demonstrated that it is a chondrule fragment formed in disk environments enriched in moderately volatile elements comparable to the Si-rich rim of CR chondrules before accreting by comet Wild 2. The late accretion of the Wild 2 nucleus with most silicates likely from a common source are not favored by the hierarchical agglomeration model that considers early and continuous accretion. Instead, the results are more in line with comet formation by gentle gravitational collapse of pebbles when the 26Al abundance is extremely low (26Al/27Al ≤ 1.5 × 10−6) before gas dispersal.

Isotopic variation of non-carbonaceous meteorites caused by dust leakage across the Jovian gap in the solar nebula

Abstract High-precision isotopic measurements of meteorites revealed that they are classified into non-carbonaceous (NC) and carbonaceous (CC) meteorites. One plausible scenario for achieving this grouping is the early formation of Jupiter, because massive planets can create gaps that suppress the mixing of dust across the gap in protoplanetary disks. However, the efficiency of this suppression by the gaps depends on dust size and the strength of turbulent diffusion, allowing some fraction of the dust particles to leak across the Jovian gap. In this study, we investigate how isotopic ratios of NC and CC meteorites are varied by the dust leaking across the Jovian gap in the solar nebula. To do this, we constructed a model to simulate the evolution of the dust size distribution and the $^{54}$Cr-isotopic anomaly $\varepsilon ^{54}$Cr in isotopically heterogeneous disks with Jupiter. Assuming that the parent bodies of NC and CC meteorites are formed in two dust-concentrated locations inside and outside Jupiter’s orbit, referred to as the NC reservoir and the CC reservoir, we derive the temporal variation of $\varepsilon ^{54}$Cr at the NC and CC reservoirs. Our results indicate that substantial contamination from CC materials occurs at the NC reservoir in the fiducial run. Nevertheless, the values of $\varepsilon ^{54}$Cr at the NC reservoir and the CC reservoir in the run are still consistent with those of NC and CC meteorites formed around $2\:$Myr after the formation of calcium–aluminum-rich inclusions (Sugiura & Fujiya 2014, Meteorit. Planet. Sci., 49, 772). Moreover, this dust leakage causes a positive correlation between the $\varepsilon ^{54}$Cr value of NC meteorites and the accretion ages of their parent bodies.

Role of natural isotopic fractionation in isotope geo- and cosmo-chronology: A theoretical investigation

We introduce a new isotope chronological model in which the natural mass-dependent isotopic fractionation effects of the radioactive (“parent”) and radiogenic (“daughter”) elements are systematically and rigorously considered. Using this model, we show that internally-normalized radiogenic isotopic ratios, commonly determined for daughter elements such as Sr, Nd, Cr, Ni, Hf, W, and Os, are dependent on the extent of natural isotopic fractionation of the daughter and parent elements at the time of system closure. This dependence indicates that (1) in two samples derived from the same isotopically homogeneous source at the same time and with identical radiogenic ingrowth over time, the present-day internally-normalized radiogenic isotope ratios would be different if they were initially fractionated to different degrees, and (2) if different internally-normalized radiogenic isotopic ratios are observed for two co-genetic objects, the difference between them would include contributions from both radiogenic ingrowth and natural isotopic fractionation. Consequently, the isochron dating equations employed in traditional chronological studies will yield inaccurate results when significant natural isotopic fractionation is present among the studied samples. Modified isochron equations that can be used to retrieve correct chronological information from isotopically-fractionated samples are presented. These theoretical considerations are applied to the 87Rb–87Sr, 147Sm–143Nd, and 146Sm–142Nd isotope systems of calcium–aluminium-rich inclusions (CAIs), a set of samples that have undergone significant natural Sr, Nd, and Sm isotope fractionation during their formation. The large natural Sr isotope fractionation (up to ca. 5.3 ‰ for 88Sr/86Sr) in fine-grained CAIs can generate analytically well-resolvable biases (>120 ppm) in the internally-normalized 87Sr/86Sr ratios and lead to significant scatters of their 87Rb–87Sr isochron (in conjunction with scatters induced by open-system disturbances). The 87Rb–87Sr systems of coarse-grained CAIs, on the contrary, are essentially not affected by natural Sr isotopic fractionation due to their much subdued fractionation degrees, resulting in a more robust isochron. Similarly, the large natural Nd (up to ca. 4.0 ‰ for 146Nd/144Nd) and Sm (up to ca. 7.1 ‰ for 152Sm/148Sm) isotopic fractionation in fine-grained CAIs can induce significant scatters of the 147Sm–143Nd isochron if the natural fractionation followed the kinetic or power law, and 146Sm–142Nd isochron if the natural fractionation followed the equilibrium, Rayleigh, or power law. This implies that when studying radioactive isotope systems in objects whose daughter and parent elements can undergo significant isotope fractionation in nature, accompanying stable isotope analyses are necessary for accurate chronological interpretations.

Oxygen Isotopic Variations in the Calcium, Aluminum-rich Inclusion–forming Region Recorded by a Single Refractory Inclusion from the CO3.1 Carbonaceous Chondrite Dar al Gani 083

Calcium, aluminum-rich inclusions (CAIs) are the oldest solids dated that formed in the solar system. Most CAIs in unmetamorphosed chondritic meteorites (chondrites; petrologic type ≤3.0) have uniform solar-like 16O-rich compositions (Δ17O ∼ −24‰) and a high initial 26Al/27Al ratio [(26Al/27Al)0] of ∼(4–5) × 10−5, consistent with their origin in a gas of approximately solar composition during a brief (<0.3 Ma) epoch at the earliest stage of our solar system. The nature of O-isotope heterogeneity in CAIs (Δ17O range from ∼−24 up to ∼+5‰) from weakly metamorphosed chondrites (petrologic type >3.0) remains an open issue. This heterogeneity could have recorded fluctuations of O-isotope composition of nebular gas in the CAI-forming region and/or postcrystallization O-isotope exchange of CAI minerals with aqueous fluids on the chondrite parent asteroids. To obtain insights into possible processes resulting in this heterogeneity, we investigated the mineralogy, rare-earth element abundances, and O- and Mg-isotope compositions of a CAI from the CO3.1 chondrite Dar al Gani 083. This concentrically zoned inclusion has a Zn-hercynite core surrounded by layers of (from core to edge) grossite, spinel, melilite, and Al-diopside. The various phases have heterogeneous Δ17O (from core to edge): −2.2 ± 0.6‰, −0.9 ± 2.1‰, −13.7 ± 2.1‰, −2.6 ± 2.3‰, and −22.6 ± 2.1‰, respectively. Magnesium-isotope compositions of grossite, spinel, melilite, and Al-diopside define an undisturbed internal Al–Mg isochron with (26Al/27Al)0 of (2.60 ± 0.29) × 10−6. We conclude that the variations in Δ17O of spinel and diopside recorded fluctuations in O-isotope composition of nebular gas in the CAI-forming region prior to injection and/or homogenization of 26Al at the canonical level. The 16O depletion of grossite and melilite resulted from O-isotope exchange with asteroidal fluid, which did not disturb Al–Mg isotope systematics of the CAI primary minerals.

The Impact of Serpentinization on the Initial Conditions of Satellite Forming Collisions of Large Kuiper Belt Objects

Kuiper Belt objects are thought to be formed at least a few million years after the formation of calcium–aluminum-rich inclusions (CAIs), at a time when the 26Al isotope—the major source of radiogenic heat in the early solar system—had significantly depleted. The internal structure of these objects is highly dependent on any additional source that can produce extra heat in addition to that produced by the remaining, long-lasting radioactive isotopes. In this paper, we explore how serpentinization, the hydration of silicate minerals, can contribute to the heat budget and to what extent it can modify the internal structure of large Kuiper Belt objects. We find that the extent of restructuring depends very strongly on the start time of the formation process, the size of the object, and the starting ice-to-rock ratio. Serpentinization is able to restructure most of the interior of all objects in the whole size range (400–1200 km) and ice-to-rock ratio range investigated if the process starts early, ∼3 Myr after CAI formation, potentially leading to a predominantly serpentine core much earlier than previously thought (≤5 Myr versus several tens of million years). While the ratio of serpentinized material gradually decreases with the increasing formation time, the increasing ice-to-rock ratio, and the increasing start time of planetesimal formation in the outer solar system, in the case of the largest objects a significant part of the interior will be serpentinized even if the formation starts relatively late, ∼5 Myr after CAI formation. Therefore it is feasible that the interior of planetesimals may have contained a significant amount of serpentine, and in some cases, it could have been a dominant constituent, at the time of satellite-forming impacts.

An inflationary disk phase to explain extended protoplanetary dust disks

Context. Understanding planetesimal formation is an essential first step towards understanding planet formation. The distribution of these first solid bodies drives the locations where planetary embryos, which eventually form fully-fledged planets, grow. Aims. We seek to understand the parameter space of possible protoplanetary disk formation and evolution models of our Solar System. A good protoplanetary disk scenario for the Solar System must meet at least the following three criteria: (1) It must produce an extended gas and dust disk (e.g. 45 au for the dust); (2) within the disk, the local dust-to-gas ratio in at least two distinct locations must sufficiently increase to explain the early formation of the parent bodies of non-carbonaceous and carbonaceous iron meteorites; and (3) dust particles, which have condensed at high temperatures (i.e. calcium–aluminium-rich inclusions), must be transported to the outer disk. Though current protoplanetary disk models are able to satisfy one or two of these criteria, none have been successful in recreating all three. We aim to find scenarios that satisfy all three. Methods. In this study we used a 1D disk model that tracks the evolution of the gas and dust disks. Planetesimals are formed within the disk at locations where the streaming instability can be triggered. We explored a large parameter space to study the effect of the disk viscosity, the timescale of infall of material into the disk, the distance within which material is deposited into the disk, and the fragmentation threshold of dust particles. Results. We find that scenarios with a large initial disk viscosity (α &gt; 0.05), a relatively short infall timescale (Tinfall &lt; 100–200kyr), and a small centrifugal radius (RC ~ 0.4 au; i.e. the distance within which material falls into the disk) result in disks that satisfy all three criteria needed to represent the protoplanetary disk of the Solar System. The large initial viscosity and short infall timescale result in a rapid initial expansion of the disk, which we dub the ‘inflationary phase’ of the disk. Furthermore, a temperature-dependent fragmentation threshold, which accounts for cold icy particles breaking more easily, results in larger and more massive disks. This, in turn, results in more ‘icy’ than ‘rocky’ planetesimals. Such scenarios are also better in line with our Solar System, which has small terrestrial planets and massive giant planet cores. Finally, we find that scenarios with large RC cannot transport calcium–aluminium-rich inclusions to the outer disk and do not produce planetesimals at two locations within the disk.

Origin of Low-26Al/27Al Corundum/Hibonite Inclusions in Meteorites

Most meteoritic calcium-rich, aluminum-rich inclusions formed from a reservoir with 26Al/27Al ≈ 5 × 10−5, but some record lower , demanding they sampled a reservoir without live 26Al. This has been interpreted as evidence for “late injection” of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them “low-26Al/27Al corundum/hibonite inclusions” (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel, or hibonite grains 0.5–2 μm in size with no live 26Al; live 26Al is carried on smaller (<50 nm) presolar chromium spinel grains from recent nearby Wolf–Rayet stars or supernovae. In hot (≈1350–1425 K) regions of the disk, these grains and perovskite grains would be the only survivors. These negatively charged grains would grow to sizes 1–103 μm, even incorporating positively charged perovskite grains, but not the small, negatively charged 26Al-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated 48Ca and 50Ti anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no 26Al in a solar nebula with uniform, canonical 26Al/27Al. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.

Age, genetics, and crystallization sequence of the group IIIE iron meteorites

Chemical and isotopic data were obtained for ten iron meteorites classified as members of the IIIE group. Nine of the IIIE irons exhibit broadly similar bulk siderophile element characteristics. Modeling of highly siderophile element abundances suggests that they can be related to one another through simple crystal-liquid fractionation of a parent melt. Our preferred model suggests initial S, P, and C concentrations of approximately 12 wt%, 0.8 wt%, and 0.08 wt%, respectively. The modeled IIIE parent melt composition is ∼4 times more enriched in highly siderophile elements than a non-carbonaceous (NC) chondrite-like parent body, suggesting a core comprising ∼22% of the mass of the parent body. Although chemically distinct from the other IIIE irons, formation of the anomalous IIIE iron Aletai can potentially be accounted for under the conditions of this model through the non-equilibrium mixing of an evolved liquid and early formed solid. Cosmic ray exposure-corrected nucleosynthetic Mo, Ru, and W isotopic compositions of four of the bona fide IIIE irons and Aletai indicate that they originated from the non-carbonaceous (NC) isotopic domain. Tungsten-182 isotopic data for the IIIE irons and Aletai yield similar model metal-silicate segregation ages of 1.6 ± 0.8 Myr and 1.2 ± 0.8 Myr, respectively, after calcium aluminum-rich inclusion (CAI) formation. These ages are consistent with those reported for other NC-type iron meteorite parent bodies. The IIIE irons are chemically and isotopically similar to the much larger IIIAB group. Despite some textural, mineralogical, and chemical differences, such as higher C content, the new results suggest they may have originated from a different crystallization sequence on the same or closely-related parent body.

Insights on the Sun Birth Environment in the Context of Star Cluster Formation in Hub–Filament Systems

Cylindrical molecular filaments are observed to be the main sites of Sunlike star formation, while massive stars form in dense hubs at the junction of multiple filaments. The role of hub–filament configurations has not been discussed yet in relation to the birth environment of the solar system and to infer the origin of isotopic ratios of short-lived radionuclides (SLR, such as 26Al) of calcium–aluminum-rich inclusions (CAIs) observed in meteorites. In this work, we present simple analytical estimates of the impact of stellar feedback on the young solar system forming along a filament of a hub–filament system. We find that the host filament can shield the young solar system from stellar feedback, both during the formation and evolution of stars (stellar outflow, wind, and radiation) and at the end of their lives (supernovae). We show that a young solar system formed along a dense filament can be enriched with supernova ejecta (e.g., 26Al) during the formation timescale of CAIs. We also propose that the streamers recently observed around protostars may be channeling the SLR-rich material onto the young solar system. We conclude that considering hub–filament configurations as the birth environment of the Sun is important when deriving theoretical models explaining the observed properties of the solar system.

Protracted Timescales for Nebular Processing of First-formed Solids in the Solar System

The calcium–aluminum-rich inclusions (CAIs) from chondritic meteorites are the first solids formed in the solar system. Rim formation around CAIs marks a time period in early solar system history when CAIs existed as free-floating objects and had not yet been incorporated into their chondritic parent bodies. The chronological data on these rims are limited. As seen in the limited number of analyzed inclusions, the rims formed nearly contemporaneously (i.e., <300,000 yr after CAI formation) with the host CAIs. Here we present the relative ages of rims around two type B CAIs from NWA 8323 CV3 (oxidized) carbonaceous chondrite using the 26Al–26Mg chronometer. Our data indicate that these rims formed ∼2–3 Ma after their host CAIs, most likely as a result of thermal processing in the solar nebula at that time. Our results imply that these CAIs remained as free-floating objects in the solar nebula for this duration. The formation of these rims coincides with the time interval during which the majority of chondrules formed, suggesting that some rims may have formed in transient heating events similar to those that produced most chondrules in the solar nebula. The results reported here additionally bolster recent evidence suggesting that chondritic materials accreted to form chondrite parent bodies later than the early-formed planetary embryos, and after the primary heat source, most likely 26Al, had mostly decayed away.

Late accretion of Ceres-like asteroids and their implantation into the outer main belt

Low-albedo asteroids preserve a record of the primordial Solar System planetesimals and the conditions in which the solar nebula was active. However, the origin and evolution of these asteroids are not well constrained. Here we measured visible and near-infrared (about 0.5–4.0 μm) spectra of low-albedo asteroids in the mid-outer main belt. We show that numerous large (diameter >100 km) and dark (geometric albedo <0.09) asteroids exterior to the dwarf planet Ceres’ orbit share the same spectral features, and presumably compositions, as Ceres. We also developed a thermal evolution model that demonstrates that these Ceres-like asteroids have highly porous interiors, accreted relatively late at 1.5–3.5 Myr after the formation of calcium–aluminium-rich inclusions, and experienced maximum interior temperatures of <900 K. Ceres-like asteroids are localized in a confined heliocentric region between about 3.0 au and 3.4 au, but were probably implanted from more distant regions of the Solar System during the giant planet’s dynamical instability. The large, low-albedo asteroids in the main belt between 3.0 au and 3.4 au share spectral characteristics and history with Ceres. Accreted in different parts of the outer Solar System, they might have been implanted into the main belt by the dynamic upheaval created by the giant planets’ instability.

Number of stars in the Sun’s birth cluster revisited

The Sun is thought to have been formed within a star cluster. The coexistence of 26Al-rich and 26Al-poor calcium–aluminum-rich inclusions indicates that a direct injection of 26Al-rich materials from a nearby core-collapse supernova would be expected to occur in the first 105 years of the existence of the Solar System. Therefore, at least one core-collapse supernova ought to occur within the duration of star formation in the Sun’s birth cluster. Here, we revisit the number of stars in the Sun’s birth cluster from the point of view of the probability of experiencing at least one core-collapse supernova within the finite duration of star formation within the birth cluster. We find that the number of stars in the birth cluster may be significantly greater than that previously considered, depending on the duration of star formation.

Turbulent Transport of Dust Particles in Protostellar Disks: The Effect of Upstream Diffusion

We study the long-term radial transport of micron to millimeter-size grains in protostellar disks (PSDs) based on diffusion and viscosity coefficients measured from 3D global stratified-disk simulations with a Lagrangian hydrodynamic method. While gas drag tends to transport dust species radially inwards, stochastic diffusion can spread a considerable fraction of dust radially outwards (upstream) depending on the nature of turbulence. In gravitationally unstable disks, we measure a high radial diffusion coefficient D r ∼ H 2Ω with little dependence on altitude. This leads to strong and vertically homogeneous upstream diffusion in early PSDs. In the solar nebula, the robust upstream diffusion of micron to millimeter-size grains not only efficiently transports highly refractory micron-size grains (such as those identified in the samples of comet 81P/Wild 2) from their regions of formation inside the snow line out to the Kuiper Belt, but can also spread millimeter-size calcium–aluminum-rich inclusions formed close to the Sun to distances where they can be assimilated into chondritic meteorites. In disks dominated by magnetorotational instability, the upstream diffusion effect is generally milder, with a separating feature due to diffusion being stronger in the surface layer than in the midplane. This variation becomes much more pronounced if we additionally consider a quiescent midplane with lower turbulence and larger characteristic dust size due to nonideal MHD effects. This segregation scenario helps to account for the dichotomy of the spatial distribution of two dust populations as observed in scattered light and Atacama Large Millimeter/submillimeter Array images.

Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples.

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide-bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu's parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu's parent body formed ~2 million years after the beginning of Solar System formation.

Compositions of carbonaceous-type asteroidal cores in the early solar system.

The parent cores of iron meteorites belong to the earliest accreted bodies in the solar system. These cores formed in two isotopically distinct reservoirs: noncarbonaceous (NC) type and carbonaceous (CC) type in the inner and outer solar system, respectively. We measured elemental compositions of CC-iron groups and used fractional crystallization modeling to reconstruct the bulk compositions and crystallization processes of their parent asteroidal cores. We found generally lower S and higher P in CC-iron cores than in NC-iron cores and higher HSE (highly siderophile element) abundances in some CC-iron cores than in NC-iron cores. We suggest that the different HSE abundances among the CC-iron cores are related to the spatial distribution of refractory metal nugget–bearing calcium aluminum–rich inclusions (CAIs) in the protoplanetary disk. CAIs may have been transported to the outer solar system and distributed heterogeneously within the first million years of solar system history.

Rethinking the role of the giant planet instability in terrestrial planet formation models

Advances in computing power and numerical methodologies over the past several decades sparked a prolific output of dynamical investigations of the late stages of terrestrial planet formation. Among other peculiar inner solar system qualities, the ability of simulations to reproduce the small mass of Mars within the planets’ geochemically inferred accretion timescale of ≲10 Myr after the appearance of calcium aluminum-rich inclusions (CAIs) is arguably considered the gold standard for judging evolutionary hypotheses. At present, a number of independent models are capable of consistently generating Mars-like planets and simultaneously satisfying various important observational and geochemical constraints. However, all models must still account for the effects of the epoch of giant planet migration and orbital instability; an event which dynamical and cosmochemical constraints indicate occurred within the first 100 Myr after nebular gas dispersal. If the instability occurred in the first few Myr of this window, the disturbance might have affected the bulk of Mars’ growth. In this manuscript, we turn our attention to a scenario where the instability took place after t≃ 50 Myr. Specifically, we simulate the instability’s effects on three nearly-assembled terrestrial systems that were generated via previous embryo accretion models and contain three large proto-planets (i.e. Earth, Venus and Theia) with 0.5 <m<1.0M⊕ and orbits interior to a collection of ∼Mars-mass embryos (a>1.3 au and m<0.2M⊕) and debris. While the instability consistently triggers a Moon-forming impact and efficiently removes excessive material from the Mars-region in our models, we find that our final systems are too dynamically excited and devoid of Mars and Mercury analogs. Thus, we conclude that, while possible, our scenario is far more improbable than one where the instability either occurred earlier, or at a time where Earth and Venus’ orbits were far less dynamically excited than considered here.

The dissipation of the solar nebula constrained by impacts and core cooling in planetesimals

Rapid cooling of planetesimal cores has been inferred for several iron meteorite parent bodies based on metallographic cooling rates, and linked to the loss of their insulating mantles during impacts. However, the timing of these disruptive events is poorly constrained. Here, we used the short-lived 107Pd / 107Ag decay system to date rapid core cooling by determining Pd-Ag ages for iron meteorites. We show closure times for the iron meteorites equate to cooling in the timeframe ~7.8 to 11.7 Myr after CAI, and indicate that an energetic inner Solar System persisted at this time. This likely results from the dissipation of gas in the protoplanetary disk, after which the damping effect of gas drag ceases. An early giant planet instability between 5 and 14 Myr after CAI could have reinforced this effect. This correlates well with the timing of impacts recorded by the Pd Ag system for iron meteorites.

Barium stable isotopic composition of chondrites and its implication for the Earth

Barium is a refractory and fluid mobile element that is highly incompatible in basaltic systems. The elemental and isotopic compositions of Ba are used to trace fluid processes or sediment recycling during subduction. However, the Ba isotopic composition of primitive meteorites (chondrites), which represents the possible building blocks of terrestrial planets, is presently poorly known. Here, we report high precision Ba isotopic compositions of major types of chondrites using a double-spike method. Chondritic meteorites without clear terrestrial weathering show relatively homogeneous Ba isotopic distributions and have an average δ137/134Ba (the permil deviation of the 137Ba/134Ba ratio from the SRM3104a standard) value of 0.09 ± 0.09‰ (2SD, N = 14). Given the refractory and lithophile behavior of Ba and the limited variations of Ba isotope composition between various chondrite groups, this value can represent the Ba isotopic composition of the bulk Earth and the Bulk Silicate Earth (BSE). It is actually consistent with the present estimate of BSE (0.05 ± 0.06‰, Li et al., 2020; Nan et al., 2022) based on the measurements of mid-ocean ridge basalts and carbonatites. Small Ba isotope variations are observed between carbonaceous chondrite groups where CO and CV show slightly heavier (by 0.09‰) Ba isotopic compositions than CI and CM. Significant light stable Ba isotopes enrichment has previously been documented in calcium, aluminum-rich inclusions (CAIs) in the Allende CV chondrite, and the Ba isotopic differences between CO and CV might result from the various proportions of CAIs based on a mass balance calculation. However, the parent body of CI and CM might have experienced aqueous alteration and secondary carbonates are most likely responsible for their light Ba isotopic signatures.

Uniform initial 10Be/9Be inferred from refractory inclusions in CV3, CO3, CR2, and CH/CB chondrites

Short-lived radionuclides (SLRs) once present in the solar nebula can be used as probes of the formation environment of our Solar System within the Milky Way Galaxy. The first-formed solids in the Solar System, calcium-, aluminum-rich inclusions (CAIs) in meteorites, record the one-time existence of SLRs such as 10Be and 26Al in the solar nebula. We measured the 10Be–10B isotope systematics in 29 CAIs from several CV3, CO3, CR2, and CH/CB chondrites and show that all except for a FUN CAI record a homogeneous initial 10Be/9Be with a single probability density peak at 10Be/9Be = 7.4 × 10–4. Integrating these data with those of previous studies, we find that most CAIs (81%) for which 10Be–10B isotope systematics have been determined, record a homogeneous initial 10Be/9Be ratio in the early Solar System with a weighted mean 10Be/9Be = (7.1 ± 0.2) × 10–4. This uniform distribution provides evidence that 10Be was predominantly formed in the parent molecular cloud and inherited by the solar nebula. Possible explanations for why unusual CAIs (FUNs, PLACs, those from CH/CBs, and those irradiated on the parent body) recorded a 10Be/9Be ratio outside of 7.1 × 10−4 include the following: 1) They incorporated a component of 10Be that was produced in the nebula by irradiation; 2) they formed after normal CAIs; and 3) they were processed (post-formation) in a way that affected their original 10Be signatures. Given the rarity of these examples, the overall uniformity of initial 10Be/9Be suggests that Solar System 10Be was predominantly inherited from the molecular cloud.

Determination of the initial hydrogen isotopic composition of the solar system

The initial isotopic composition of water in the Solar System is of paramount importance to understanding the origin of water on planetary bodies but remains unknown, despite numerous studies1–5. Here we use the isotopic composition of hydrogen in calcium–aluminium-rich inclusions (CAIs) from primitive meteorites, the oldest Solar System rocks, to establish the hydrogen isotopic composition of water at the onset of Solar System formation. We report the hydrogen isotopic composition of nominally anhydrous minerals from CAI fragments trapped in a once-melted host CAI. Primary minerals have extremely low D/H ratios, with δD values down to −850‰, recording the trapping of nebular hydrogen. Minerals rich in oxidised iron formed before the capture of the fragments record the existence of a nebular gas reservoir with an oxygen fugacity substantially above the solar value and a D/H ratio within 20% of that of the Earth’s oceans. Hydrogen isotopes also correlate with oxygen and nitrogen isotopes, indicating that planetary reservoirs of volatile elements formed within the first 2 × 105 years of the Solar System, during the main CAI formation epoch. We propose that the isotopic composition of inner Solar System water was established during the collapse of the protosolar cloud core owing to a massive admixture of interstellar water. The hydrogen isotopic composition of the oldest Solar System rocks demonstrates that a gaseous reservoir of terrestrial isotopic composition existed as early as the onset of Solar System formation and coexisted with the solar gas.