Fractionated sulfur isotopes in sulfides of the Kaidun meteorite

Abstract— The Kaidun meteorite contains carbonaceous chondrite (CM1) clasts that have been highly altered by reactions with hydrothermal fluids. Pyrrhotite in these clasts occurs as unusual needles wrapped by sheaths of phyllosilicate, and pentlandite forms veins that crosscut aggregates of phyllosilicate and garnet but not pyrrhotite. The isotopic compositions of S (δ³⁴S_CDT) in individual sulfide grains, measured by ion micro-probe, are fractionated compared to troilite in ordinary chondrites. The S in Kaidun sulfides is isotopically light (as much as ?4.2% for pyrrhotite and ?5.7%0 for pentlandite), unlike sulfides in other carbonaceous chondrites, which are enriched in ³⁴S. The unusual S-isotopic composition of these texturally unique sulfides supports the hypothesis that Kaidun CM1 clasts were pervasively altered under extreme thermal conditions, possibly by fluids that had lost isotopically heavy SO₂.     

P‐O‐rich sulfide phase in CM chondrites: Constraints on its origin on the CM parent body

CM chondrites are a group of primitive meteorites that have recorded the alteration history of the early solar system. We report the occurrence, chemistry, and oxygen isotopic compositions of P‐O‐rich sulfide phase in two CM chondrites (Grove Mountains [GRV] 021536 and Murchison). This P‐O‐rich sulfide is a polycrystalline aggregate of nanometer‐size grains. It occurs as isolated particles or aggregates in both CM chondrites. These grains, in the matrix and in type‐I chondrules from Murchison, were partially altered into tochilinite; however, grains enclosed by Ca‐carbonate are much less altered. This P‐O‐rich sulfide in Murchison is closely associated with magnetite, FeNi phosphide, brezinaite (Cr₃S₄), and eskolaite (Cr₂O₃). In addition to sulfur as the major component, this sulfide contains ~6.3 wt% O, ~5.4 wt% P, and minor amounts of hydrogen. Analyses of oxygen isotopes by SIMS resulted in an average δ¹⁸O value of ?22.5 ‰ and an average Δ¹⁷O value of 0.2 ± 9.2 ‰ (2σ). Limited variations in both chemical compositions and electron‐diffraction patterns imply that the P‐O‐rich sulfide may be a single phase rather than a polyphase mixture. Several features indicate that this P‐O‐rich sulfide phase formed at low temperature on the parent body, most likely through the alteration of FeNi metal (a) close association with other low‐temperature alteration products, (b) the presence of hydrogen, (c) high Δ¹⁷O values and the presence in altered mesostasis of type‐I chondrules and absence in type‐II chondrules. The textural relations of the P‐O‐rich sulfide and other low‐temperature minerals reveal at least three episodic‐alteration events on the parent body of CM chondrites (1) formation of P‐O‐rich sulfide during sulfur‐rich aqueous alteration of P‐rich FeNi metal, (2) formation of Ca‐carbonate during local carbonation, and (3) alteration of P‐O‐rich sulfide and formation of tochilinite during a period of late‐stage intensive aqueous alteration.     

Carbonate abundances and isotopic compositions in chondrites

We report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba (CV3), and Semarkona (LL3.0). For the unheated CMs, the total ranges of carbonate isotopic compositions are δ¹³C ≈ 25–75‰ and δ¹⁸O ≈ 15–35‰, and bulk carbonate C contents range from 0.03 to 0.60 wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0–130 °C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (δ¹⁸O ≈ 5‰) and the C isotopes were controlled by equilibrium with CO and/or CH₄ (δ¹³C ≈ ?33‰ or ?20‰ for CO‐ or CH₄‐dominated systems, respectively). However, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. A quite similar fluid composition (δ¹⁸O ≈ ?5.5‰, and δ¹³C ≈ ?31‰ or ?17‰ for CO‐ or CH₄‐dominated systems, respectively) can explain the carbonate compositions of the CIs, although the formation temperatures would have been lower (~10–40 °C) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the CMs. The CR carbonates exhibit a similar range of O isotopes, but an almost bimodal distribution of C isotopes between more (δ¹³C ≈ 65–80‰) and less altered samples (δ¹³C ≈ 30–40‰). This bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ¹⁸O ≈ ?9.25‰, and δ¹³C ≈ ?21‰ or ?8‰ for CO‐ or CH₄‐dominated systems, respectively) if the less altered CRs had higher mole fractions of CO₂ in their fluids. Semarkona and Kaba carbonates have some of the lightest C isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more CO₂‐rich fluids. The fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. By analogy with cometary ices, CO₂ and/or CO would have dominated the trapped volatile species in the ices. The chondrites studied are too oxidized for CO‐dominated fluids to have formed in their parent bodies. If CH₄ was the dominant C species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from CO and/or CO₂ when oxidation of metal by water created high partial pressures of H₂. The fact that the chondrite carbonate C/H₂O mole ratios are of the order predicted for CO/CO₂‐H₂O ices that experienced temperatures of >50–100 K suggests that the chondrites formed at radial distances of <4–15 AU.     

Light dement geochemistry of the Tagish Lake CI2 chondrite: Comparison with CI1 and CM2 meteorites

Abstract— We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high‐resolution stepped‐combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), Elephant Moraine (EET) 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (?1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole‐rock carbon isotopic composition (δ¹³C ? +24.4%_o) of the chip was dominated by ¹³C‐enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; δ¹³C ? +67%_o and δ¹⁸O ? +35%_o, in the proportions ?4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (?2.6 wt%, δ¹³C ? ?9%_o; 1033 ppm N, δ¹⁵N ? +77%_o), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further ?18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (approximately 3650–4330 ppm) than other carbonaceous chondrites, along with ?8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.     

Magnesium isotopic fractionation in chondrules from the Murchison and Murray CM2 carbonaceous chondrites

We present high‐precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine‐ and pyroxene‐rich and have low ²⁷Al/²⁴Mg ratios (0.012–0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (δ²⁶Mg ranging from ?0.95‰ to ?0.15‰ relative to the DSM‐3 standard) than those of Murchison (δ²⁶Mg ranging from ?1.27‰ to +0.77‰). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least‐altered CM2 chondrules are on average lighter (average δ²⁶Mg = ?0.39 ± 0.30‰, 2SE) than the moderately to heavily altered CM2 chondrules (average δ²⁶Mg = ?0.11 ± 0.21‰, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low‐Al CM2 chondrules analyzed here do not exhibit any mass‐independent variations in ²⁶Mg from the decay of ²⁶Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable ²⁶Mg excess suggests that it either formed >1 Ma after calcium‐aluminum‐rich inclusions, or that its Al‐Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of ²⁶Al.     

Oxygen isotopic and chemical composition of chromites in micrometeorites: Evidence of ordinary chondrite precursors

We identified 66 chromite grains from 42 of ~5000 micrometeorites collected from Indian Ocean deep‐sea sediments and the South Pole water well. To determine the chromite grains precursors and their contribution to the micrometeorite flux, we combined quantitative electron microprobe analyses and oxygen isotopic analyses by high‐resolution secondary ion mass spectrometry. Micrometeorite chromite grains show variable O isotopic compositions with δ¹⁸O values ranging from ?0.8 to 6.0‰, δ¹⁷O values from 0.3 to 3.6‰, and Δ¹⁷O values from ?0.9 to 1.6‰, most of them being similar to those of chromites from ordinary chondrites. The oxygen isotopic compositions of olivine, considered as a proxy of chromite in chromite‐bearing micrometeorites where chromite is too small to be measured in ion microprobe have Δ¹⁷O values suggesting a principal relationship to ordinary chondrites with some having carbonaceous chondrite precursors. Furthermore, the chemical compositions of chromites in micrometeorites are close to those reported for ordinary chondrite chromites, but some contribution from carbonaceous chondrites cannot be ruled out. Consequently, carbonaceous chondrites cannot be a major contributor of chromite‐bearing micrometeorites. Based on their oxygen isotopic and elemental compositions, we thus conclude with no ambiguity that chromite‐bearing micrometeorites are largely related to fragments of ordinary chondrites with a small fraction from carbonaceous chondrites, unlike other micrometeorites deriving largely from carbonaceous chondrites.     

Molecular distribution, 13C‐isotope,and enantiomeric compositions of carbonaceous chondrite monocarboxylic acids

The water‐soluble organic compounds in carbonaceous chondrite meteorites constitute a record of the synthetic reactions occurring at the birth of the solar system and those taking place during parent body alteration and may have been important for the later origins and development of life on Earth. In this present work, we have developed a novel methodology for the simultaneous analysis of the molecular distribution, compound‐specific δ¹³C, and enantiomeric compositions of aliphatic monocarboxylic acids (MCA) extracted from the hot‐water extracts of 16 carbonaceous chondrites from CM, CR, CO, CV, and CK groups. We observed high concentrations of meteoritic MCAs, with total carbon weight percentages which in some cases approached those of carbonates and insoluble organic matter. Moreover, we found that the concentration of MCAs in CR chondrites is higher than in the other meteorite groups, with acetic acid exhibiting the highest concentration in all samples. The abundance of MCAs decreased with increasing molecular weight and with increasing aqueous and/or thermal alteration experienced by the meteorite sample. The δ¹³C isotopic values of MCAs ranged from ?52 to +27‰, and aside from an inverse relationship between δ¹³C value and carbon straight‐chain length for C₃–C₆ MCAs in Murchison, the ¹³C‐isotopic values did not correlate with the number of carbon atoms per molecule. We also observed racemic compositions of 2‐methylbutanoic acid in CM and CR chondrites. We used this novel analytical protocol and collective data to shed new light on the prebiotic origins of chondritic MCAs.     

Origin of a metamorphosed lithic clast in CM chondrite Grove Mountains 021536

Abstract– A metamorphosed lithic clast was discovered in the CM chondrite Grove Mountains 021536, which was collected in the Antarctica by the Chinese Antarctic Research Exploration team. The lithic clast is composed mainly of Fe‐rich olivine (Fo₆₂) with minor diopside (Fs_9.7–11.1Wo_48.3–51.6), plagioclase (An_43–46.5), nepheline, merrillite, Al‐rich chromite (21.8 wt% Al₂O₃; 4.43 wt% TiO₂), and pentlandite. Δ¹⁷O values of olivine in the lithic clast vary from ?3.9‰ to ?0.8‰. Mineral compositions and oxygen isotopic compositions of olivine suggest that the lithic clast has an exotic source different from the CM chondrite parent body. The clast could be derived from strong thermal metamorphism of pre‐existing chondrule that has experienced low‐temperature anhydrous alteration. The lithic clast is similar in mineral assemblage and chemistry to a few clasts observed in oxidized CV3 chondrites (Mokoia and Yamato‐86009) and might have been derived from the interior of the primitive CV asteroid. The apparent lack of hydration in the lithic clast indicates that the clast accreted into the CM chondrite after hydration of the CM components.     

Silicon and iron isotopes in components of enstatite chondrites: Implications for metal–silicate–sulfide fractionation in the solar nebula

Silicon and iron isotope compositions of different physically separated components of enstatite chondrites (EC) were determined in this study to understand the role of nebular and planetary scale events in fractionating Si and Fe isotopes of the terrestrial planet-forming region. We found that the metal–sulfide nodules of EC are strongly enriched in light Si isotopes (δ³⁰Si ≥ −5.61 ± 0.12‰, 2SD), whereas the δ³⁰Si values of angular metal grains, magnetic, slightly magnetic, and non-magnetic fractions become progressively heavier, correlating with their Mg# (Mg/(Mg+Fe)). White mineral phases, composed primarily of SiO₂ polymorphs, display the heaviest δ³⁰Si of up to +0.23 ± 0.10‰. The data indicate a key role of metal–silicate partitioning on the Si isotope composition of EC. The overall lighter δ³⁰Si of bulk EC compared to other planetary materials can be explained by the enrichment of light Si isotopes in EC metals along with the loss of isotopically heavier forsterite-rich silicates from the EC-forming region. In contrast to the large Si isotope heterogeneity, the average Fe isotope composition (δ⁵⁶Fe) of EC components was found to vary from −0.30 ± 0.08‰ to +0.20 ± 0.04‰. A positive correlation between δ⁵⁶Fe and Ni/S in the components suggests that the metals are enriched in heavy Fe isotopes whereas sulfides are the principal hosts of light Fe isotopes in the non-magnetic fractions of EC. Our combined Si and Fe isotope data in different EC components reflect an inverse correlation between δ³⁰Si and δ⁵⁶Fe, which illustrates that partitioning of Si and Fe among metal, silicate, and sulfidic phases has significantly fractionated Si and Fe isotopes under reduced conditions. Such isotope partitioning must have occurred before the diverse components were mixed to form the EC parent body. Evaluation of diffusion coefficients of Si and Fe in the metal and non-metallic phases suggests that the Si isotope compositions of the silicate fractions of EC largely preserve information of their nebular processing. On the other hand, the Fe isotopes might have undergone partial or complete re-equilibration during parent body metamorphism. The relatively uniform δ⁵⁶Fe among different types of bulk chondrites and the Earth, despite Fe isotope differences among their components, demonstrates that the chondrite parent bodies were not formed by random mixing of chondritic components from different locations in the disk. Instead, the chondrite components mostly originated in the same nebular reservoir and Si and Fe isotopes were fractionated either due to gas–solid interactions and associated changes in physicochemical environment of the nebular reservoir and/or during parent body processing. The heavier Si isotope composition of the bulk silicate Earth may require accretion of chondritic and/or isotopically heavier EC silicates along with cumulation of refractory forsterite-rich heavier silicates lost from the EC-forming region to form the silicate reservoir of the Earth.     

Maribo—A new CM fall from Denmark

Abstract– Maribo is a new Danish CM chondrite, which fell on January 17, 2009, at 19:08:28 CET. The fall was observed by many eye witnesses and recorded by a surveillance camera, an all sky camera, a few seismic stations, and by meteor radar observatories in Germany. A single fragment of Maribo with a dry weight of 25.8 g was found on March 4, 2009. The coarse‐grained components in Maribo include chondrules, fine‐grained olivine aggregates, large isolated lithic clasts, metals, and mineral fragments (often olivine), and rare Ca,Al‐rich inclusions. The components are typically rimmed by fine‐grained dust mantles. The matrix includes abundant dust rimmed fragments of tochilinite with a layered, fishbone‐like texture, tochilinite–cronstedtite intergrowths, sulfides, metals, and carbonates often intergrown with tochilinite. The oxygen isotopic composition: (δ¹⁷O = ?1.27‰; δ¹⁸O = 4.96‰; Δ¹⁷O = ?3.85‰) plots at the edge of the CM field, close to the CCAM line. The very low Δ¹⁷O and the presence of unaltered components suggest that Maribo is among the least altered CM chondrites. The bulk chemistry of Maribo is typical of CM chondrites. Trapped noble gases are similar in abundance and isotopic composition to other CM chondrites, stepwise heating data indicating the presence of gas components hosted by presolar diamond and silicon carbide. The organics in Maribo include components also seen in Murchison as well as nitrogen‐rich components unique to Maribo.     

Oxygen isotope and chemical compositions of magnetite and olivine in the anomalous CK3 Watson 002 and ungrouped Asuka‐881595 carbonaceous chondrites: Effects of parent body metamorphism

We report in situ O isotope and chemical compositions of magnetite and olivine in chondrules of the carbonaceous chondrites Watson‐002 (anomalous CK3) and Asuka (A)‐881595 (ungrouped C3). Magnetite in Watson‐002 occurs as inclusion‐free subhedral grains and rounded inclusion‐bearing porous grains replacing Fe,Ni‐metal. In A‐881595, magnetite is almost entirely inclusion‐free and coexists with Ni‐rich sulfide and less abundant Ni‐poor metal. Oxygen isotope compositions of chondrule olivine in both meteorites plot along carbonaceous chondrite anhydrous mineral (CCAM) line with a slope of approximately 1 and show a range of Δ¹⁷O values (from approximately ?3 to ?6‰). One chondrule from each sample was found to contain O isotopically heterogeneous olivine, probably relict grains. Oxygen isotope compositions of magnetite in A‐881595 plot along a mass‐dependent fractionation line with a slope of 0.5 and show a range of Δ¹⁷O values from ?2.4‰ to ?1.1‰. Oxygen isotope compositions of magnetite in Watson‐002 cluster near the CCAM line and a Δ¹⁷O value of ?4.0‰ to ?2.9‰. These observations indicate that magnetite and chondrule olivine are in O isotope disequilibrium, and, therefore, not cogenetic. We infer that magnetite in CK chondrites formed by the oxidation of pre‐existing metal grains by an aqueous fluid during parent body alteration, in agreement with previous studies. The differences in Δ¹⁷O values of magnetite between Watson‐002 and A‐881595 can be attributed to their different thermal histories: the former experienced a higher degree of thermal metamorphism that led to the O isotope exchange between magnetite and adjacent silicates.     

A study of presolar material in hydrated lithic clasts from metal-rich carbonaceous chondrites

We report on the investigation of presolar grain inventories of hydrated lithic clasts in three metal-rich carbonaceous chondrites from the CR clan, Acfer 182 (CH3), Isheyevo (CH3/CB_b3), and Lewis Cliff (LEW) 85332 (C3-un), as well as the carbon- and nitrogen-isotopic compositions of the fine-grained clast material. Eleven presolar silicate grains as well as nine presolar silicon carbide (SiC) grains were identified in the clasts. Presolar silicate abundances range from 4 to 22 parts per million (ppm), significantly lower than in pristine meteorites and interplanetary dust particles (IDP), and comparable to recent findings for CM2s and CR2 interchondrule matrix. SiC concentrations lie between 9 and 23 ppm, and are comparable to the values for CI, CM, and CR chondrites. The results of our investigation suggest similar alteration pathways for the clast material, the interchondrule matrix of the CR2 chondrites, and the fine-grained fraction of CM2 chondrites. Fine-grained matter of all three meteorites contains moderate to high ¹⁵N-enrichments (~50‰ ≤ δ¹⁵N ≤ ~1600‰) compared to the terrestrial value, indicating the presence of primitive organic material. We observed no correlation between ¹⁵N-enrichments and presolar dust concentrations in the clasts. This is in contrast to the findings from a suite of primitive IDPs, which display in several cases enhanced bulk ¹⁵N/¹⁴N ratios and high presolar grain abundances of several hundred or even thousand ppm. The bulk ¹⁵N/¹⁴N ratios of the clasts are comparable to the range for primitive IDPs, suggesting a nitrogen carrier less susceptible to destruction by aqueous alteration than silicate stardust.     

Lithium isotopes as indicators of meteorite parent body alteration

Hydrothermal processing on planetesimals in the early solar system produced new mineral phases, including those generated by the transformation of anhydrous silicates into their hydrated counterparts. Carbonaceous chondrites represent tangible remnants of such alteration products. Lithium isotopes are known to be responsive to aqueous alteration, yet previously recognized variability within whole rock samples from the same meteorite appears to complicate the use of these isotopes as indicators of processing by water. We demonstrate a new way to use lithium isotopes that reflects aqueous alteration in carbonaceous chondrites. Temperature appears to exert a control on the production of acetic acid‐soluble phases, such as carbonates and poorly crystalline Fe‐oxyhydroxides. Temperature and degree of water‐rock interaction determines the amount of lithium isotope fractionation expressed as the difference between whole rock and acetic acid‐leachable fractions. Using these features, the type 1 chondrite Orgueil (δ⁷Li_{(whole rock)} = 4.3‰; Δ⁷Li_{(acetic‐whole)} = 1.2‰) can be distinguished from the type 2 chondrites Murchison (δ⁷Li_{(whole rock)} = 3.8; Δ⁷Li_{(acetic‐whole)} = 8.8‰) and carbonate‐poor Tagish Lake (δ⁷Li_{(whole rock)} = 4.3; Δ⁷Li_{(acetic‐whole)} = 9.4‰). This initial study suggests that lithium isotopes have the potential to reveal the role of liquid water in the early solar system.     

The CM carbonaceous chondrite regolith Diepenveen

A carbonaceous chondrite was recovered immediately after the fall near the village of Diepenveen in the Netherlands on October 27, 1873, but came to light only in 2012. Analysis of sodium and poly‐aromatic hydrocarbon content suggests little contamination from handling. Diepenveen is a regolith breccia with an overall petrology consistent with a CM classification. Unlike most other CM chondrites, the bulk oxygen isotopes are extremely ¹⁶O rich, apparently dominated by the signature of anhydrous minerals, distributed on a steep slope pointing to the domain of intrinsic CM water. A small subset plots closer to the normal CM regime, on a parallel line 2 ‰ lower in δ¹⁷O. Different lithologies in Diepenveen experienced varying levels of aqueous alteration processing, being less aqueously altered at places rather than more heated. The presence of an agglutinate grain and the properties of methanol‐soluble organic compounds point to active impact processing of some of the clasts. Diepenveen belongs to a CM clan with ~5 Ma CRE age, longer than most other CM chondrites, and has a relatively young K‐Ar resetting age of ~1.5 Ga. As a CM chondrite, Diepenveen may be representative of samples soon to be returned from the surface of asteroid (162173) Ryugu by the Hayabusa2 spacecraft.     

The origin of chondritic macromolecular organic matter: A carbon and nitrogen isotope study

Abstract— The N and C abundances and isotopic compositions of acid-insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only ～δ¹³C = 30%, the N isotopes range from δ¹⁵N ' -40 to 260%. After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3–4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M-shaped N isotope release profiles that can be explained most simply by the superposition of two components, one with a composition between δ¹⁵N = -20 and -40% and a narrow combustion interval, the other having a broader release profile and a composition of δ¹⁵N ～ 260%. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M-shaped profile. How and where the components in the acid-insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most ¹⁵N-rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being presolar.     

Oxygen isotope characteristics of chondrules from the Yamato‐82094 ungrouped carbonaceous chondrite: Further evidence for common O‐isotope environments sampled among carbonaceous chondrites

High‐precision secondary ion mass spectrometry (SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty‐one of 21 chondrules have multiple homogeneous pyroxene data (?¹⁷O 3SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty‐one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O‐isotope ratios; and (2) suggest efficient O‐isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral (PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ¹⁷O and δ¹⁸O values approaching ?40‰, similar to CAI or AOA‐like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8–99.5 and ?¹⁷O of ?3.9‰ to ?6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O‐isotope reservoir devoid of ¹⁶O‐poor H₂O. Six Y‐82094 chondrules have ?¹⁷O near ?2.5‰, with Mg#s of 64–97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules plus ¹⁶O‐poor H₂O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (?¹⁷O: ~+0.1‰). Their O‐isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite‐like chondrule precursors. Finally, three Mg# >99 chondrules have ?¹⁷O of ?6.7‰ to ?8.1‰, potentially due to ¹⁶O‐rich refractory precursor components. The predominance of Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules and a high chondrule‐to‐matrix ratio suggests bulk Y‐82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules.     

Oxygen isotopes in chondrule olivine and isolated olivine grains from the CO3 chondrite Allan Hills A77307

Abstract— We have measured O‐isotopic ratios in a variety of olivine grains in the CO3 chondrite Allan Hills (ALH) A77307 using secondary ion mass spectrometry in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites. Oxygen‐isotopic ratios of olivines in this chondrite are variable from δ¹⁷O = ?15.5 to +4.5% and δ¹⁸O = ?11.5 to +3.9%, with Δ¹⁷O varying from ?10.4 to +3.5%. Forsteritic olivines, Fa_<1, are enriched in ¹⁶O relative to the bulk chondrite, whereas more FeO‐rich olivines are more depleted in ¹⁶O. Most ratios lie close to the carbonaceous chondrite anhydrous minerals (CCAM) line with negative values of Δ¹⁷O, although one grain of composition Fa₄ has a mean Δ¹⁷O of +1.6%. Marked O‐isotopic heterogeneity within one FeO‐rich chondrule is the result of incorporation of relic, ¹⁶O‐rich, Mg‐rich grains into a more ¹⁶O‐depleted host. Isolated olivine grains, including isolated forsterites, have similar O‐isotopic ratios to olivine in chondrules of corresponding chemical composition. This is consistent with derivation of isolated olivine from chondrules, as well as the possibility that isolated grains are chondrule precursors. The high ¹⁶O in forsteritic olivine is similar to that observed in forsterite in CV and CI chondrites and the ordinary chondrite Julesburg and suggests nebula‐wide processes for the origin of forsterite that appears to be a primitive nebular component.     

Sulfide–oxide assemblages in Acfer 094—Clues to nebular metal–gas interactions

The ungrouped carbonaceous chondrite Acfer 094 is among the least altered samples of the early solar system. We have studied concentric sulfide–oxide aggregates from this meteorite by transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). The main minerals present are magnetite, pentlandite, and pyrrhotite/troilite. The outer parts of the aggregates include μm-sized olivine and pyroxenes with variable Mg/Fe ratios. One aggregate contains taenite (56.7 wt% Ni) within its central part that is surrounded by pentlandite and magnetite. We conclude that both phases have formed by oxidation and sulfidization of metal and, based on the metal and sulfide Fe/Ni ratio, a (sulfide)-formation temperature of 400–550 °C can be constrained. This temperature is higher than any temperature that could be expected to have occurred on the Acfer 094 parent body, and also textural evidence indicates that the aggregates formed before parent-body accretion. We therefore conclude that the formation of the sulfide–oxide aggregates occurred most likely in the solar nebular at highly variable H₂O and H₂S fugacities. Oxygen-isotopic compositions of magnetite within these assemblages show that they are indistinguishable from the meteorite's matrix (δ¹⁷O_SMOW ≈ 4 ± 8‰, δ¹⁸O_SMOW ≈ 10 ± 6‰, and ∆¹⁷O_SMOW ≈ −1 ± 5‰). An enrichment of ^17,18O relative to chondrules of Acfer 094 suggests a link between the formation of the sulfide–oxide aggregates and the preaccretionary processing of matrix grains in a volatile-enriched nebular environment.     

From whom Bells tolls: Reclassifying Bells among CR chondrites and implications for the formation conditions of CR parent bodies

The ungrouped carbonaceous chondrite (CC) Bells has long been considered petrographically similar to CM chondrites based on its matrix abundance and degree of aqueous alteration, but also shows significant isotopic affinities to CR chondrites. Its taxonomic status is thus important for clarifying the relationship of the CRHB (formerly “CR”) clan with other CCs. In this study, we measured the oxygen isotopic compositions of olivines in type I chondrules and isolated olivine grains in Bells. Bells olivines mostly have ∆¹⁷O > −4‰, similar to CR chondrites but unlike other CCs that are rich in refractory inclusions, in which chondrules are generally richer in ¹⁶O. Therefore, Bells is a CR chondrite (albeit an anomalous one), most similar to the rare, matrix-rich CRs like Al Rais. These chondrites (i) may not necessarily derive from the same primary parent body as mainstream CRs, (ii) bear witness to significant variations of the matrix/chondrule ratio within the CRHB clan, and (iii) may be a good analog for samples retrieved by the space mission OSIRIS-REx.