Jessica J. Barnes

CI chondrite water‐leaching experiments are commonly used to investigate soluble inventories formed during parent‐body aqueous alteration, but terrestrial weathering may modify them. We performed water‐leaching experiments on pristine Bennu samples together with Orgueil (CI) and Murchison (CM). The pristine Bennu aggregate sample OREX‐800107‐127 released only 0.23 wt% cations, dominated by Na. In contrast, Bennu aggregate sample OREX‐800107‐0 (−114, −115, and −116), flash‐heated to 600 °C for 20 s under continuous helium flow, and a curated Orgueil sample released 2.0 and 2.2 wt% cations, respectively. Both thermally treated Bennu and Orgueil released much more Mg and Ca than untreated Bennu, while also releasing more Na in absolute terms. The thermally treated Bennu leachate was dominated by Mg (38 wt%), Na (23 wt%), Fe (17 wt%), and Ca (14 wt%), broadly resembling the Orgueil leachate, except for its higher Fe fraction than Orgueil (3.6 wt%). Mass‐balance models indicate that carbonate dissolution supplied mainly Ca and Mn, Mg came from carbonates and phyllosilicate‐associated reservoirs, and the increases in Fe and Ni record substantial sulfide mobilization. Broadly CI‐like materials can possess markedly different soluble inventories. Returned samples retain labile alteration products when stored under anaerobic conditions, whereas meteorites are unavoidably modified in oxidizing terrestrial environments.

We report on the petrography and oxygen isotopic compositions from in situ analyses of carbonates (Ca‐carbonate, dolomite, and Fe‐rich magnesite), magnetite, and apatite in particles from asteroid Bennu. Using oxygen isotope thermometry of Ca‐carbonate, dolomite, and magnetite, we estimated crystallization temperatures and the oxygen isotopic compositions of the aqueous fluids from which they precipitated. The Ca‐carbonate–magnetite pairs formed at lower temperatures (−5 to 20°C) than the dolomite–magnetite pairs (5–89°C); the uncertainties are ±10–16°C. The inferred fluid compositions for Ca‐carbonate–magnetite pairs are δ ¹⁸ O = 0.6‰–9‰ and Δ ¹⁷ O = 1.8‰–2.7‰, whereas for dolomite–magnetite pairs, they are δ ¹⁸ O = −2.2‰ to 7.6‰ and Δ ¹⁷ O = −0.1‰ to 0.9‰. On a three‐isotope oxygen diagram (δ ¹⁷ O vs. δ ¹⁸ O), 11 out of 13 of our inferred aqueous fluid compositions plot along a single line with a slope of ~0.8, potentially indicating precipitation from a single isotopically evolving fluid. Two dolomite–magnetite pairs appear to be outliers and may have formed from fluids with different oxygen isotopic compositions. The oxygen isotope data presented herein overlap with those reported from samples of asteroid Ryugu, supporting a possible close genetic relationship between these asteroids.

Analyses of returned samples from asteroids Bennu and Ryugu reveal notable similarities in chemical and isotopic compositions to Ivuna-type (CI) carbonaceous chondrites, prompting reassessment of their origin. We present nucleosynthetic iron (Fe), titanium (Ti), and chromium (Cr) isotope data for Bennu. Minor Cr isotope variations among aliquots likely reflect parent body aqueous alteration, whereas Fe and Ti isotope data are homogeneous and indistinguishable from those of Ryugu and CI chondrites at bulk (>20 milligrams) scale. These results demonstrate that CI-like bodies accreted from a common dust reservoir with affinities to both inner (noncarbonaceous) and outer (carbonaceous) Solar System materials. Combined isotopic, petrographic, and chemical evidence suggests formation near the water ice line inside proto-Jupiter’s orbit, where Jupiter’s growth restricted the inward drift of larger particles, whereas efficient mixing of fine-grained dust produced bodies with near-solar compositions.

Carbonaceous asteroids preserve presolar grains—refractory stardust that predates the Solar System—recording both their stellar origins and subsequent alteration on their parent asteroids. Here we examine the distribution of presolar grains and isotopically anomalous organic matter in mottled particles from samples returned by the OSIRIS-REx mission from asteroid Bennu. Although the mottled samples contain organic matter with carbon and nitrogen isotopic compositions similar to those of other Bennu samples, they lack presolar silicate or oxide grains and show a markedly reduced abundance of presolar silicon carbide. In this work, we show that these findings are consistent with the slow destruction of silicon carbide grains during late-stage alteration by a low-temperature, oxidizing, alkaline brine. These brines, coinciding with evaporite salt formation, selectively erased chemically robust presolar phases while preserving isotopically anomalous organic matter. This shows that evolved aqueous fluids can fundamentally alter the presolar record of primitive asteroids.

NASA’s OSIRIS-REx mission returned material from the carbonaceous asteroid Bennu, which is dominated by aqueous-alteration products but preserves rare high-temperature components. Here we present an integrated petrographic, oxygen-isotopic and ²⁶Al–²⁶Mg chronological characterization of refractory inclusions in Bennu samples. All identified inclusions are small (<~100 µm) and commonly preserve mineralogical evidence of in situ aqueous alteration, indicating incorporation into Bennu’s parent body prior to hydration. Their mineralogical characteristics and oxygen-isotopic and ²⁶Al–²⁶Mg systematics demonstrate formation during the earliest stage of Solar System evolution and genetic affinity with refractory inclusions in asteroid Ryugu samples and chondritic meteorites. The shared refractory-inclusion signatures across diverse chondritic materials are consistent with widespread outward transport and mixing of early-formed inner-disk solids. In Bennu samples analyzed thus far, large (>submillimeter) refractory inclusions are absent, in contrast to most carbonaceous chondrites. This size distribution implies that Bennu’s constituents accreted in the outer Solar System beyond the pressure bump associated with proto-Jupiter, which effectively filtered distinct dust populations.

Remote spectroscopy is used to characterize the mineralogy and infer the history of planetary bodies. Carbonaceous asteroids, such as B‐type (101955) Bennu, represent the earliest stages of planet formation. B types have a blue (negative) spectral slope and comprise <5% of asteroids. Samples from Bennu returned by the OSIRIS‐REx spacecraft complement remote observations of this rare population. We show here, using laboratory spectra that are directly comparable to spacecraft data, that OSIRIS‐REx accurately determined Bennu's dust content and most of its surface composition. However, spectra of the asteroid exhibit stronger water absorptions than those of bulk samples, possibly due to hydrous, Mg‐rich phosphate or solar wind implantation at Bennu's uppermost surface. Bennu samples spectrally resemble the most aqueously altered carbonaceous meteorites and samples of (162173) Ryugu, indicating similarly pervasive aqueous alteration. However, one carbon‐enriched Bennu stone does not appear to have a spectral analog among Ryugu samples or meteorites. Our findings demonstrate the leverage obtained using a wide range of wavelengths and that sample analysis anchors the interpretations of remote sensing, leading to more robust characterization of planetary surface composition and evolution.

We performed an in‐depth study of the mineralogy, petrology, chemical composition, and presolar grain abundance of the C3.00‐ungrouped chondrite Chwichiya 002 using a combination of in situ and bulk analytical methods. Chwichiya 002's bulk composition is significantly enriched compared to CI chondrites (over 20 × CI for Sr, Ba, U) in rare earth and trace elements like Li, Sr, Ba, U, and Pb, indicating effects of terrestrial weathering. Concentrations of Ti and Al are similar to CI chondrites, but notably depleted relative to other carbonaceous chondrite groupings, suggesting that the parent body of Chwichiya 002 accreted from a source with a near CI composition of refractory elements, and with a low quantity of calcium–aluminum‐rich inclusions. A slight reduction in Fe is linked to the predominance of FeO‐poor chondrules and olivine, likely a remnant of Chwichiya 002's formation history. Our findings uncovered phyllosilicates in the matrix and combinations of tochilinite–cronstedtite, confirming that Chwichiya 002 underwent more extensive alteration on its parent body than previously believed. We identified 12 O‐anomalous presolar grains (8 Group 1, 4 Group 4) and nine presolar SiC grains, with abundances of 12.1 + 4.6/−3.5 ppm for O‐rich grains and 7.8 + 3.6/−2.6 ppm for SiC grains. The relatively low occurrence of O‐rich presolar grains is similar to what is seen in CM2 chondrites and samples from asteroid 162173 Ryugu, despite Chwichiya 002 being classified as type 3.00 petrologically. We conclude that Chwichiya 002 formed from a refractory‐poor source similar to CM and CO chondrites, primarily consisting of FeO‐poor chondrules and olivine, and underwent a moderate level of aqueous alteration on its parent body. Furthermore, the mineralogy, petrology, and compositional data reported in this study, combined with previous data on the O‐isotopes of Chwichiya 002, suggest that the sample may better be classified as a CM2.8 or CM 2.9 chondrite.

NASA's OSIRIS‐REx mission delivered pristine regolith samples from the near‐Earth carbonaceous asteroid (101955) Bennu, enabling direct assessment of primitive asteroid material free from terrestrial alteration. We report comprehensive analyses of H, C, O, S, Cl, and Hg in Bennu samples using X‐ray fluorescence (XRF), thermogravimetric analysis coupled with mass spectrometry (TG‐MS), combination analyses of pyrolysis and combustion (EMIA‐Step), and direct Hg measurements. Quantification of O, Si, S, Cl, and Ge by XRF is reported for Bennu samples for the first time. TG‐MS reveals ~15 mass% total mass loss, with H ₂ O and CO ₂ release patterns attributed to phyllosilicates, carbonates, and organic matter. Bennu samples contain substantially higher interlayer H ₂ O than those from asteroid Ryugu, consistent with subsurface deeper sampling by OSIRIS‐REx. Total carbon abundance is ~4.1 mass%, dominated by organic carbon (~2.9 mass%), comparable to Ryugu samples. Mercury abundances (82–141 ng g ⁻¹ ) are far lower than those reported for CI chondrites, indicating pervasive terrestrial contamination in meteorite samples. We recommend a Hg abundance of 98 ± 2 ng g ⁻¹ for Bennu samples and pristine CI‐like material, demonstrating that returned asteroid samples provide the most reliable record of primitive carbonaceous matter.