By investigating the petrology and chemical composition of type II (FeO-rich) chondrules in the Mighei-like carbonaceous (CM) chondrites we constrain their thermal histories and relationship to the ...Ornans-like carbonaceous (CO) chondrites. We identified FeO-rich relict grains in type II chondrules by their Fe/Mn ratios; their presence indicates chondrule recycling among type II chondrules. The majority of relict grains in type II chondrules are FeO-poor olivine grains. Consistent with previous studies, chemical similarities between CM and CO chondrite chondrules indicate that they had similar formation conditions and that their parent bodies probably formed in a common region within the protoplanetary disk. However, important differences such as mean chondrule size and the lower abundance of FeO-poor relicts in CM chondrite type II chondrules than in CO chondrites suggest CM and CO chondrules did not form together and they likely originate from distinct parent asteroids.
Despite being aqueously altered, many CM chondrites contain pre-accretionary anhydrous minerals (i.e., olivine) that are among the least thermally metamorphosed materials in chondrites according to the Cr2O3 content of their ferroan olivine. The presence of these minimally altered pre-accretionary chondrule silicates suggests that samples to be returned from aqueously altered asteroids by the Hayabusa2 and OSIRIS-REx asteroid sample return missions, even highly hydrated, may contain silicates that can provide information about the pre-accretionary histories and conditions of asteroids Ryugu and Bennu, respectively.
Here we report the relative degrees of thermal metamorphism for five Antarctic Ornans-like carbonaceous (CO) chondrites, including Dominion Range (DOM) 08006, as determined from the Cr-content of ...their FeO-rich (ferroan) olivine. These five CO3 chondrites complete the previously poorly-defined CO3.00 to 3.2 chondrite metamorphic trend. DOM 08006 appears to be a highly primitive CO chondrite of petrologic type 3.00. We report the detailed mineralogy and petrography of DOM 08006 using a coordinated, multi-technique approach.
The interchondrule matrix in DOM 08006 consists of unequilibrated mixtures of silicate, metal, and sulfide minerals and lacks Fe-rich rims on silicates indicating that DOM 08006 has only experienced minimal, if any, thermal metamorphism. This is also reflected by the Co/Ni ratios of Ni-rich and Ni-poor metal, a sensitive indicator of thermal metamorphism, and the presence of euhedral chrome-spinel grains, which typically become subhedral to anhedral during progressive metamorphism. DOM 08006 matrix shows minor evidence for aqueous alteration and while the presence of magnetite surrounding metal in chondrules indicates that there has been some interaction with fluid, much metal remains and none of the sulfides analyzed show evidence of being formed by aqueous alteration. Furthermore, the plagioclase of ∼50% of chondrules analyzed show resolvable excess silica indicating that these chondrules have experienced minimal, if any, reprocessing in the CO parent body.
Noble gas data for DOM 08006 show that it contains the highest concentrations of trapped 36Ar and 132Xe of all CO chondrites analyzed to date, further indicating that DOM 08006 is the most primitive CO chondrite known. The cosmic ray exposure age of DOM 08006 is estimated to be ∼19 Ma.
The minimally altered nature of DOM 08006 demonstrates that it is an extremely important sample for providing valuable insight into early Solar System conditions. At a total weight of 667 g, a significant amount of material is available for a wide array of future studies.
Determining the origins of chondrule precursors is key to constraining how material migrated in the early Solar System. Chondrules that were only partially melted during their formation retain ...portions of their solid precursors, termed relict grains. By measuring the chemical and O-isotopic compositions of relict grains in chondrules from an unequilibrated ordinary chondrite (UOC), and Renazzo-like carbonaceous (CR) and Mighei-like carbonaceous (CM) chondrites we constrain their origins and discuss implications for disk transport within the first 4 million years of the Solar System. For all three chondrite groups, the chemical and O-isotopic compositions of dusty olivine grains are sometimes consistent with the reduction of type I (FeO-poor) and/or type II (FeO-rich) chondrules from the same meteorite group. However, other dusty olivine grains from the CM chondrites and the UOC are found to be xenocrysts that require an origin from a source distinct from the host meteorite. This material plausibly originated as fragments of earlier-formed chondrules from another chondrite group or of partially or fully differentiated planetesimals that migrated into an active chondrule-forming region. Multiple CM chondrite dusty olivine chondrules have O-isotope compositions that match those of UOC chondrule olivine (Δ17O ∼ 0‰), suggesting an origin from an UOC source. This implies that UOC chondrules and/or chondrule fragments migrated from the inner Solar System outwards to CM chondrite chondrule-forming region, likely beyond the orbit of Jupiter. These UOC chondrules or chondrule fragments could have migrated outwards in the protoplanetary disk before the formation of the Jupiter Gap, or <300 μm diameter fragments could have migrated outwards after Gap formation as CM chondrite chondrule dusty olivine grains with Δ17O ∼ 0‰ were small enough to pass through Jupiter Gap. The identification of xenocrysts in each meteorite group studied here argues for widespread migration of material in the early Solar System, potentially crossing the Jupiter Gap.
By investigating the in situ chemical and O-isotope compositions of olivine in lightly sintered dust agglomerates from the early Solar System, we constrain their origins and the retention of dust in ...the protoplanetary disk. The grain sizes of silicates in these agglomeratic olivine (AO) chondrules indicate that the grain sizes of chondrule precursors in the Renazzo-like carbonaceous (CR) chondrites ranged from <1 to 80 µm. We infer this grain size range to be equivalent to the size range for dust in the early Solar System. AO chondrules may contain, but are not solely composed of, recycled fragments of earlier formed chondrules. They also contain 16O-rich olivine related to amoeboid olivine aggregates and represent the best record of chondrule-precursor materials.
AO chondrules contain one or more large grains, sometimes similar to FeO-poor (type I) and/or FeO-rich (type II) chondrules, while others contain a type II chondrule core. These morphologies are consistent with particle agglomeration by electrostatic charging of grains during collision, a process that may explain solid agglomeration in the protoplanetary disk in the micrometer size regime. The petrographic, isotopic, and chemical compositions of AO chondrules are consistent with chondrule formation by large-scale shocks, bow shocks, and current sheets.
The petrographic, isotopic, and chemical similarities between AO chondrules in CR chondrites and chondrule-like objects from comet 81P/Wild 2 indicate that comets contain AO chondrules. We infer that these AO chondrules likely formed in the inner Solar System and migrated to the comet forming region at least 3 Ma after the formation of the first Solar System solids. Observations made in this study imply that the protoplanetary disk retained a dusty disk at least ∼3.7 Ma after the formation of the first Solar System solids, longer than half of the dusty accretion disks observed around other stars.
We re-examine the Renazzo-like (CR) chondrite metamorphic trend based on Cr2O3 contents of FeO-rich olivine, indicating that it is only appropriate to use such analyses to identify the endmembers of ...this group (i.e., those that have experienced either no detectable heating or significant heating). As such Miller Range (MIL) 090657 appears to have experienced very minimal (if any) thermal processing and is one of the most pristine CR chondrites analyzed to date, while Graves Nunataks 06100 is the most shock-heated CR chondrite studied.
On the basis of bulk H-C-N isotopic compositions, MIL 090657 appears to be of petrological type 2.7. We also report the H-C-N isotopic compositions of extracted insoluble organic matter, in situ chemical compositional data, presolar grain abundances, and a petrologic description of MIL 090657. As a minimally altered CR chondrite of relatively high mass (133.1 g), MIL 090657 provides an invaluable opportunity to perform coordinated, often destructive, analyses on pristine CR chondrite material.
By combining a number of petrographic characteristics (Cr2O3-content of ferroan olivine, Co/Ni ratios of Fe,Ni metal, ratios of Fe# in chondrule olivine and low-Ca pyroxene, and the presence of excess silica in chondrule plagioclase) with bulk isotopic compositions, we demonstrate their utility as indicators for determining the relative pristinity/heating of low petrographic (type 1–3) chondrites.
The background temperature of the protoplanetary disk is a fundamental but poorly constrained parameter that strongly influences a wide range of conditions and processes in the early Solar System, ...including the widespread process(es) by which chondrules originate. Chondrules, mm-scale objects composed primarily of silicate minerals, were formed in the protoplanetary disk almost entirely during the first four million years of Solar System history but their formation mechanism(s) are poorly understood. Here we present new constraints on the sub-silicate solidus cooling rates of chondrules at <873 K (600 °C) using the compositions of sulfide minerals. We show that chondrule cooling rates remained relatively rapid (∼100 to 101 K/hr) between 873 and 503 K, which implies a protoplanetary disk background temperature of <503 K (230 °C) and is consistent with many models of chondrule formation by shocks in the solar nebula, potentially driven by the formation of Jupiter and/or planetary embryos, as the chondrule formation mechanism. This protoplanetary disk background temperature rules out current sheets and resulting short-circuit instabilities as the chondrule formation mechanism. More detailed modeling of chondrule cooling histories in impacts is required to fully evaluate impacts as a chondrule formation model. These results motivate further theoretical work to understand the expected thermal evolution of chondrules at ≤873 K under a variety of chondrule formation scenarios.
•We present cooling rates of chondrules at <873 K (600 °C) using sulfide minerals.•These are the lowest temperature cooling rates determined for chondrules to date.•These cooling rates are relatively rapid (∼100 to 101 K/hr) between 873 and 503 K.•The rates imply a protoplanetary disk background temperature of <503 K (230 °C).•These results motivate further theoretical work for chondrule formation models.
To better understand the effects of aqueous alteration in the Renazzo-like carbonaceous (CR) chondrite parent asteroid, a minor body in the early Solar System, we studied the petrology and O-isotope ...compositions of fine-grained matrix from 14 different CR chondrites. The O-isotope compositions of matrix from Queen Alexandra Range 99177 confirm that this sample is the least aqueously altered CR chondrite, provides the best approximation of the primary anhydrous matrix, and suggests matrix is not a byproduct of chondrule formation. Matrix O-isotope compositions within individual CR chondrites are heterogeneous, varying up to ∼5‰ in both δO18 and δO17, as a result of the heterogeneous nature of the matrix and diverse range of aqueous alteration recorded by each sample. Aqueous alteration resulted in matrix that is progressively more 16O-depleted and Ca-carbonate rich. Due to the fine-grained nature of matrix its O-isotope composition is a more sensitive indicator of a chondrite's overall degree of aqueous alteration than whole-rock O-isotope compositions, which are typically dominated by the compositions of type I (FeO-poor) chondrule phenocrysts. Petrographic signatures correlate with the degree of aqueous alteration and the wide range of matrix O-isotope compositions indicate that some regions of the CR chondrite parent asteroid were relatively dry, while others were heavily hydrated with water. The O-isotope composition of aqueously altered matrix is consistent with asteroidal water being near ΔO17∼0‰, which suggests an inner Solar System origin for the water. The diverse range of aqueous alteration recorded by a single asteroid has a range of implications for spectral studies of the asteroid belt, and the arrival of Dawn at 1 Ceres, Hayabusa-2 at 162173 1999 JU3, and OSIRIS-REx at 101955 Bennu.
•We present the O-isotope compositions and petrology of CR chondrite matrix.•CR chondrite matrix is a distinct component, not a byproduct of chondrule formation.•Fine-grained matrix is a sensitive indicator of aqueous alteration.•More aqueously altered matrix is relatively 16O-depleted and Ca-carbonate rich.•The CR chondrite parent asteroid recorded a diverse range of aqueous alteration.
The timing and extent to which the initial interstellar material was thermally processed provide fundamental constraints for models of the formation and early evolution of the solar protoplanetary ...disk. We argue that the nonsolar (solar Δ17O ≈ −29‰) and near‐terrestrial (Δ17O ≈ 0‰) O‐isotopic compositions of the Earth and most extraterrestrial materials (Moon, Mars, asteroids, and comet dust) were established very early by heating of regions of the disk that were modestly enriched (dust/gas ≥ 5–10 times solar) in primordial silicates (Δ17O ≈ −29‰) and water‐dominated ice (Δ17O ≈ 24‰) relative to the gas. Such modest enrichments could be achieved by grain growth and settling of dust to the midplane in regions where the levels of turbulence were modest. The episodic heating of the disk associated with FU Orionis outbursts were the likely causes of this early thermal processing of dust. We also estimate that at the time of accretion the CI chondrite and interplanetary dust particle parent bodies were composed of ~5–10% of pristine interstellar material. The matrices of all chondrites included roughly similar interstellar fractions. Whether this interstellar material avoided the thermal processing experienced by most dust during FU Orionis outbursts or was accreted by the disk after the outbursts ceased to be important remains to be established.
To better understand the formation conditions of type-I and type-II chondrules in the Renazzo-like carbonaceous (CR) chondrites, an in situ major- and minor-element and O-isotope study was conducted. ...Twenty-one ferromagnesian chondrules from three CR chondrites (GRA 95229, GRA 06100, and QUE 99177) were analyzed to establish an internally-consistent data set. From this study we infer that type-II chondrule precursors contained enhanced S-bearing dust and ice abundances relative to type-I chondrules. There is a relationship between the O-isotope composition and oxidation state of olivine, which may be related to the amount of 16O-poor ice and reduced carbon accreted by chondrule precursors before melting. Type-II chondrules formed under H2O/H2 ratios of ∼230–740 times solar. In contrast, type-I chondrules formed under more reducing conditions with lower H2O/H2 ratios of ∼10–100 times solar. We find a relationship between type-II chondrule petrology (relict free vs. relict grain-bearing) and O-isotope composition, which is due to degree of melting and exchange with a 16O-poor gas reservoir. The 16O-poor gas that interacted with both type-I and type-II chondrules is estimated to have an isotopic composition between ∼δ18Og=13–27‰ and δ17Og=10–22‰, different from the O-isotope composition of the water accreted by the CR chondrite parent body. Due to partial melting, type-I chondrules and relict grain-bearing type-II chondrules exchanged with the 16O-poor gas to a lower degree than relict-free type-II chondrules.
To better understand the formation conditions of ferromagnesian chondrules from the Renazzo‐like carbonaceous (CR) chondrites, a systematic study of 210 chondrules from 15 CR chondrites was ...conducted. The texture and composition of silicate and opaque minerals from each observed FeO‐rich (type II) chondrule, and a representative number of FeO‐poor (type I) chondrules, were studied to build a substantial and self‐consistent data set. The average abundances and standard deviations of Cr2O3 in FeO‐rich olivine phenocrysts are consistent with previous work that the CR chondrites are among the least thermally altered samples from the early solar system. Type II chondrules from the CR chondrites formed under highly variable conditions (e.g., precursor composition, redox conditions, cooling rate), with each chondrule recording a distinct igneous history. The opaque minerals within type II chondrules are consistent with formation during chondrule melting and cooling, starting as S‐ and Ni‐rich liquids at 988–1350 °C, then cooling to form monosulfide solid solution (mss) that crystallized around olivine/pyroxene phenocrysts. During cooling, Fe,Ni‐metal crystallized from the S‐ and Ni‐rich liquid, and upon further cooling mss decomposed into pentlandite and pyrrhotite, with pentlandite exsolving from mss at 400–600 °C. The composition, texture, and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration. However, some opaque minerals (Fe,Ni‐metal versus magnetite and panethite) present in type II chondrules are a proxy for the degree of whole‐rock aqueous alteration. The texture and composition of sulfide‐bearing opaque minerals in Graves Nunataks 06100 and Grosvenor Mountains 03116 suggest that they are the most thermally altered CR chondrites.
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