CM Chondrites Research Articles

Near-ultraviolet absorption distribution of primitive asteroids from photometric surveys

Context. Primitive asteroids consisting of mainly phyllosilicates and opaque minerals have great variation at near-ultraviolet (NUV) wavelengths (0.35–0.5 μm). The absorption in NUV could be indicative of phyllosilicates that reflect their formation environments such as the distribution of water, temperature, and pressure. The asteroid collisional families are the fragments of large primordial bodies that record the early Solar System environments. Aims. Our objective is to investigate the reflectance spectrophotometry of primitive asteroid families in NUV to visible (VIS) wavelengths to constrain the internal structure and formation of primordial bodies. Methods. The NUV-VIS reflectance spectrophotometry of 38 primitive asteroid families was investigated using two spectrophotometric surveys, the Eight Color Asteroid Survey (ECAS) and the Sloan Digital Sky Survey (SDSS). We classified the members of the primitive asteroid families based on Tholen’s taxonomy. After grouping these families into eight overarching types, we discussed the compositions of primitive asteroid families based on the NUV, 0.7 μm, 3 μm absorptions, and the near-infrared (NIR) spectral slopes. Results. We have found a correlation between the 0.7 μm absorption band and the NUV absorption among the asteroid families, suggesting that both features are caused by the charge transfer of interlayer iron in phyllosilicates. This implies that NUV absorption can be a valuable indicator of Fe-rich phyllosilicate abundance. Furthermore, we have revealed correlations between the NUV absorption, VIS slope, albedo, and the NIR slope. Primitive asteroid families with strong NUV absorption exhibit a high albedo and a low NIR slope (1.25–2.14 μm). The Pallas family deviates from this general trend due to its exceptionally high albedo. This anomaly, combined with the Pallas family’s unique density and the deep and sharp 3 μm absorption, suggests that the Pallas family could be a potential source of CR chondrites. Overall, our results demonstrate that NUV absorption aligns well with established indicators of phyllosilicate presence (0.7 μm and 3 μm absorptions). The largest bodies in the high-NUV absorption families show a sharp 3-μm feature, while the red-dominant families show a w-shaped 3-μm feature. Notably, two young endmember families (Theobalda, F-dominant; Veritas, G-dominant) highlight that NUV absorption is not solely linked to aging or space weathering, but likely reflects inherent compositional differences. The Polana–Eulalia complex family and the Theobalda family, dominated by F types (>80%), exhibit minimal to non-NUV absorption, suggesting that their primordial bodies contained little Fe-rich phyllosilicates, such as CI drated carbonaceous chondrites. Conversely, the Veritas family, with over 80% of C and G types, displays stron. indicative of an Fe-rich phyllosilicate-rich parent body, such as CM chondrites.

Observations and Quantitative Compositional Analysis of Ceres, Pallas, and Hygiea Using JWST/NIRSpec

Abstract We present JWST Near Infrared Spectrograph (NIRSpec) measurements of the three largest low-albedo main-belt asteroids: (1) Ceres, (2) Pallas, and (10) Hygiea. Their reflectance spectra all have very similar absorptions centered near 2.72 μm attributed to Mg–OH in minerals. Within this band, Pallas also shows evidence of a sharper, deeper band, also centered near 2.72 μm. These band positions are similar to those seen in the most aqueously altered carbonaceous chondrites and samples from Ryugu and Bennu. Absorptions in the 2.7–2.9 μm region due to other cation–OH combinations are weak, if present. The NIRSpec spectrum of Ceres is consistent with the global average spectrum of Dawn, and the similarity between Ceres and Hygiea seen in other wavelength regions continues into the 2.5–2.8 μm region. This similarity in spectral properties, and thus in interpretations of surface composition, implies that the two bodies may have had similar processes occur and similar histories. This suggests that Hygiea, similar to Ceres, may be associated with the “ocean worlds” despite its relatively small mass. Quantitative estimates of the hydrogen concentrations on the surfaces suggest hydrogen concentrations of roughly 0.5–1 wt%, consistent with CM chondrites. Additional absorptions attributed to ammoniated minerals are seen in Ceres’s and Hygiea’s spectra, as has been reported by others, but are not seen in Pallas’s spectrum. Absorptions are also seen in the 2.5–2.7 μm region in all three asteroids, likely due to OH combination bands, and from roughly 3.9 to 4.3 μm in Hygiea, which could be due to carbonates plus an unidentified constituent.

A high-resolution in situ X-ray diffraction study of mineral transitions due to post-hydration heating in CM chondrite meteorites.

The effects of post-hydration heating over a broad range of temperatures are evident in many Mighei-like carbonaceous (CM) chondrites as a variety of mineral transitions. To better understand these processes and how a CM chondrite's starting composition may have affected them, we experimentally heated two meteorites with different degrees of aqueous alteration, Allan Hills 83100 and Murchison, at 25°C temperature steps from 200°C to 950°C and 300°C to 750°C, respectively. During heating, synchrotron in situ X-ray diffraction patterns were collected. With the exception of calcite decomposition and its products, most mineral transitions were unaffected by starting composition. Key observations include: (1) partial decomposition of tochilinite at 200°C, which indicates that tochilinite breakdown might be a two-stage process due to its intergrown layers of brucite/amakinite and mackinawite; (2) the breakdown of serpentine occurring at 300°C with transitional phases appearing at 525°C and 575-600°C, while secondary olivine formed at 600°C; (3) cronstedtite decomposing faster than lizardite, (4) the formation of secondary enstatite at 750°C, and (5) calcite decomposition temperature differing significantly between meteorites, occurring at 725°C and 575°C in ALH 83100 and Murchison, respectively. The results for calcite are likely controlled by differences in its microstructure and chemical composition, related to the meteorite's impact history and degree of aqueous alteration. The difference in calcite decomposition temperature also explains the contrasts in the observed breakdown products, with clinopyroxene occurring in both meteorites, and oldhamite only in ALH 83100. Mineral transitions due to post-hydration heating have been characterized with a high resolution XRD method, enabling a better understanding of processes occurring on the parent asteroids of CM chondrites. The online version contains supplementary material available at 10.1186/s40623-024-02116-2.

Identification of Hydroandradite in CM Carbonaceous Chondrites: A Product of Calc-Silicate Alteration on C-Complex Asteroids

A hydrous Ca-Fe-rich silicate identified as hydroandradite was observed in the ‘Mighei19 type’ carbonaceous (CM) chondrite falls, Shidian and Kolang. This is the first report of hydroandradite occurring within meteorites. Hydroandradite forms through aqueous calc-silicate alteration under specific fluid conditions. Its presence within Shidian and Kolang has implications for interpreting alteration processes within the C-complex asteroid parent bodies of the CM chondrites. To better understand its occurrence, the meteoritic hydroandradite was studied with scanning electron microscopy, electron probe microanalysis, transmission electron microscopy, and Raman spectroscopy. It occurs in four petrographic contexts: layered, perovskite-associated, sulphide-associated, and spheroidal. Kolang has all four morphologies, while only the sulphide-associated occurs in Shidian. In Kolang, hydroandradite was likely produced by replacement of kamacite, Ti-bearing clinopyroxene in calcium- and aluminium-rich inclusions, and secondary magnetite in three distinct alteration events. The formation temperature of meteoritic hydroandradite was estimated to be 100–245°C, based on the mineralogy of the lithologies within which it occurs as well as on its degree of hydration relative to synthetic and terrestrial hydroandradites. Because Kolang and Shidian are the only reported meteorites with hydroandradite to date, they may be from the same parent body.

The Dehydration of Serpentines and Extraction of Water

Some groups of carbonaceous chondrites are dominated by phyllosilicates, specifically the serpentine group, which are characterized as alternating layers of silicates, metal cations, and hydroxyl (OH). Both antigorite, an Mg-rich member, and cronstedtite, an Fe-rich member, can be found in hydrated carbonaceous chondrites. Cronstedtite is found in substantial amounts in CM chondrites. The hydroxyl can make up to a quarter of the molecular mass, and part of it can turn into molecular water through thermal-induced crystal structure breakup. Understanding how the water is released through thermal effects is important for describing the history of certain near-Earth asteroids, as well as for ascertaining the viability of in situ resource utilization (ISRU) for a given object. Here we investigate the temperature of dehydration of the main serpentine constituents of carbonaceous chondrites—antigorite, lizardite, and cronstedtite—under both atmospheric and vacuum conditions. Our results show that the mass loss for cronstedtite starts at about 150 °C lower temperature than for the Mg serpentines. Additionally, vacuum does not affect significantly the onset of the mass loss, but it affects its dynamics. Finally, our observations suggest that cronstedtite loses additional oxygen molecules besides the ones contained within the hydroxyl. These results provide an important new perspective on the orbital history of certain asteroids and the required proximity to the Sun in order for them to lose water. Additionally, it puts the CM-type asteroids on the center stage for ISRU.

Accretion and Core Formation of Earth-like Planets: Insights from Metal–Silicate Partitioning of Siderophile and Volatile Elements

The origin of volatile elements, the timing of their accretion and their distribution during Earth’s differentiation are fundamental aspects of Earth’s early evolution. Here, we present the result of a newly developed accretion and core formation model, which features the results of high P–T metal–silicate partitioning experiments. The model includes well-studied reference elements (Fe, Ni, Ca, Al, Mg, Si) as well as trace elements (V, Ga, Ag, Au, S) covering a wide range from refractory to volatile behavior. The accretion model simulates the different steps of planet formation, such as the effects of continuous, heterogenous core formation at high P–T, the effect of the Moon-forming giant impact and the addition of matter after the core formation was completed, the so-called “late veneer”. To explore the “core formation signature” of the volatile depletion patterns and the quantitative influence of a late veneer, we modeled planets that would have formed from known materials, such as CI, CM, CV, CO, EH and EL meteorites, and from a hypothetical volatile depleted material, CI*. Some of the resulting planets are Earth-like in key properties, such as overall core size, major element composition, oxygen fugacity and trace element composition. The model predicts the chemical signatures of the main planetary reservoirs, the metallic core and bulk silicate planet (BSP) of the modeled planets, which we compare with the chemical signature of Earth derived previously from core formation models and mass balance-based approaches. We show that planets accreted from volatile depleted carbonaceous chondrites (CM, CV, CO and CI*) are closest in terms of major element (Si, Mg, Fe, Ca, Al, Ni) and also siderophile volatile element (Ge, Ga, Au) concentrations to the components from which Earth accreted. Chalcophile volatile elements (S, Ag), instead, require an additional process to lower their concentrations in the BSP to Earth-like concentrations, perhaps the late segregation of a sulfide melt.

The secondary classification of unequilibrated chondrites

AbstractThe multiplication of decimal petrologic schemes for different or the same chondrite groups evinces a lack of unified guiding principle in the secondary classification of type 1–3 chondrites. We show that the current OC, R and CO classifications can be a posteriori unified, with only minor reclassifications, if the decimal part of the subtype is defined as the ratio m = FaI/FaII of the mean fayalite contents of type I and type II chondrules, rounded to the nearest tenth (with adaptations from Cr systematics for the lowest subtypes following the past literature). This parameter is more efficiently evaluable than the oft‐used relative standard deviation of fayalite contents and defines a general metamorphic scale from M0.0 to M1, where the suffixed number is the rounded m. Type 3 chondrites thus span the range M0.0–M0.9 (i.e. subtypes 3.0–3.9) and M1 designates type 4. Corresponding applications are then proposed for other chondrite groups (with, e.g., CV secondary classification reduced to essentially three grades from M0.0 to M0.2, that is, subtypes 3.0–3.2). Known type 1 and 2 chondrites are at M0.0 (i.e. the metamorphic grade of type 3.0 chondrites), even so‐called “CY” chondrites, since our metamorphic scale is insensitive to brief heating. Independently, we define an aqueous alteration scale from A0.0 to A1.0, where the suffixed number is the (rounded) phyllosilicate fraction (PSF). For CM and CR chondrites, the alteration degrees can be characterized in terms of the thin‐section‐based criteria of previous schemes which are thus incorporated in the present framework, if in a coarser, but hereby more robust form. We propose their corresponding petrologic subtype to be 3‐PSF, rounded to the nearest tenth (so that type 1 would correspond to subtypes 2.0 and 2.1). Since nonzero alteration and metamorphic degrees remain mutually exclusive at the level of precision chosen, a single petrologic subtype ≈3+m‐PSF indeed remains a good descriptor of secondary processes for all unequilibrated chondrites, obviating the explicit mention of our separate scales unless finer subdivisions are adopted for the most primitive chondrites.

The evolution of amino acids under asteroidal aqueous alteration

Carbonaceous chondrites contain amino acids, with variable abundances and isotope compositions between and within carbonaceous chondrites. The parent body processes, and the presence of clay minerals may explain those differences. Here, we experimentally investigate the evolution of 6 amino acids (glycine, β-alanine, α-alanine, 2-aminoisobutyric acid, γ-aminobutyric acid, and isovaline) exposed to hydrothermal conditions in the presence or absence of silicates. We determined the chemical nature and isotopic composition of the organic compounds of the soluble and solid fractions of the residues using X-ray diffraction, spectroscopy, and mass-spectrometry methods. Glycine and α-alanine exhibit a rather high stability, which is consistent with the measured abundances of α-alanine and glycine in chondrites having experienced various degrees of aqueous alteration. In the meantime, the evolution of β-alanine under hydrothermal conditions leads to the formation of a new compound, which likely results from the decarboxylation and deamination of β-alanine followed by recombination. More than 95 % of γ-ABA was transformed into 2-pyrrolidione though self-cyclization during the aqueous alteration. The solid residues of the experiments conducted in the presence of clay minerals contain organic material, with abundances varying depending on the amino acid used for the experiments (TOC isovaline > 2-aminoisobutyric acid > γ-aminobutyric acid > glycine > α-alanine > β-alanine). Clay minerals thus preferentially trap branched amino acids over chained amino acids, likely within their interlayer spaces as suggested by XRD data. The δ13C values of amino acids have not changed significantly during the experiments, even with the presence of silicates. Thus, the δ13C values of amino acids reported in CR and CM chondrites likely relate to synthetic conditions or the origin of their precursors (i.e. inherited from the pre-accretion processes).

Finely layered CM2 carbonaceous chondrites may be analogs for layered boulders on asteroid (101955) Bennu

AbstractOrbital observations of Bennu revealed a surface covered in boulders that are most similar among meteorites in our collections to aqueously altered carbonaceous chondrites, and initial analyses of the returned Bennu sample have begun to reveal insights into Bennu's origins. We identified a suite of paired CM2 chondrite meteorites that have a finely layered texture and bear a striking similarity, although at a different scale, to rugged, layered boulders on Bennu. We investigated the nature and potential origin of this layered texture by performing a petrofabric analysis on samples MET 00431, 00434, and 00435. We developed a micro‐geospatial mapping framework that is more commonly used for landscape‐scale investigations. Our results reveal a pervasive fracture network that exhibits a similar orientation to flattened particles dominated by tochilinite–cronstedtite intergrowths (TCI). We propose that their petrofabrics originated from a low‐energy impact on the parent body that occurred after the main period of aqueous alteration halted. The impactdeformed TCI (which formed during earlier aqueous alteration) and generated the fractures. We propose that the sample from Bennu may contain particles with similar layered textures to these meteorites which, if present, would likewise indicate the dominant role of impacts and aqueous alteration on Bennu's parent body.

Early fluid migration and alteration fronts in the CM chondrite Reckling Peak 17085

AbstractReckling Peak (RKP) 17085 is a newly classified Antarctic CM chondrite that preserves a complex alteration history characterized by mild aqueous alteration (CM2.7), overprinted by a short‐lived thermal metamorphic event (heating stage III [<750°C]), and affected by low‐grade terrestrial weathering. This meteorite contains abundant Fe‐rich bands within the fine‐grained matrix, composed of micron‐scale Fe‐oxyhydroxide minerals. They are interpreted as “alteration fronts” arising due to the dissolution and transport of Fe (typically <500 μm) before being abruptly deposited. This alteration texture is relatively rare among hydrated carbonaceous chondrites, with only five reported instances to date (Murchison, Murray, Allan Hills 81002, Miller Range 07687, and Northwest Africa 5958). Evidence from RKP 17085 suggests that early aqueous alteration operated as multiple geochemically isolated microenvironments, which moved outwards from local point sources within the matrix. Low permeability fine‐grained rims on chondrules appear to have acted as barriers to fluid flow, controlling the migration of fluid across the parent body. Furthermore, the higher porosity regions within the altered fine‐grained matrix represent either void space generated by the dehydration of hydrated minerals during post‐hydration metamorphism and/or sites of ice accretion (water‐ice or C‐bearing ices) preserved within a mildly altered primitive matrix.

Testing the Bus–DeMeo Asteroid Taxonomy Using Meteorite Spectra

The most widely used method to spectrally classify asteroids is the Bus–DeMeo taxonomy. To test how well the Bus–DeMeo taxonomy groups asteroids on the basis of their mineralogy, we have classified ∼1500 meteorite spectra using this Bus–DeMeo system. Some asteroid classes group together meteorites with similar compositions better than others. Howardite, eucrite, and diogenite spectra tend to be classified as V-types, while ordinary chondrite spectra tend to be classified as S-complex or Q-type bodies. The relatively featureless D- and X-types tend to be dominated by CM carbonaceous chondrites but with a substantial number of matches also with iron meteorites. The large proportion of CM chondrite matches for the D- and X-classes is most likely due to the large number of CM chondrite spectra and the rarity of spectra of more fragile carbonaceous chondrites in our data set. A number of relatively featureless asteroid classes like the C-, B-, L-, and Xc-types group meteorite types together with a wide variety of mineralogies and thermal histories. Visual albedos are vital for distinguishing between many of these assemblages. The Bus–DeMeo taxonomy does have trouble classifying olivine-dominated meteorites that do not have red-sloped spectra because this type of spectrum is rare among asteroids. For many asteroid classes, care must be used when making mineralogical interpretations based solely on spectral type.

On the origin and evolution of deuterium enrichment in type 1 and 2 chondritic organic solids

Rotationally resonant Deuterium Nuclear Magnetic Resonance spectroscopy (D MAS NMR) was applied to IOM isolated from a CR1 chondrite Grosvenor Mountains (GRO) 95577 and a CM2 chondrite (Murchison). It is shown that in IOM D strongly prefers the aliphatic hydrogen reservoir over the aromatic hydrogen reservoir. For GRO 95577, that has a bulk δD of 3303 ‰ (Alexander et al., 2010), the average δD value of the aromatic reservoir is 1740 ± 128 ‰ and the aliphatic reservoir is 4477 ± 105 ‰, i.e., D/H enrichments of 1.27 and 0.64, respectively, relative to the bulk. For Murchison IOM, that has a bulk δD of 811 ‰ (Alexander et al., 2010), the average δD of the aromatic reservoir is 512 ± 88 ‰ and the aliphatic reservoir is 1033 ± 64 ‰ i.e., D/H enrichments of 1.12 and 0.82, respectively, relative to the bulk. D-H exchange between D-enriched water and a type III kerogen reveals nearly equivalent D up take by both aromatics and aliphatics. Laboratory synthesis of IOM-like material in the presence of D2O reveals a high degree of deuteration with a strong preferential deuteration of the aliphatic hydrogen reservoir indicating that the δD of the water during IOM synthesis is the primary determinant of syn-IOM’s δD. The IOM in GRO 95577 and Murchison (FA and H/C × 100) lie on the molecular evolution line as defined by the IOM of the Tagish Lake clasts and Murchison IOM has experienced more molecular evolution relative to that exhibited by GRO 95577 IOM. A forward prediction derived from the D/H ratios for the aliphatic and aromatic hydrogen reservoirs in Murchison and GRO 95577, relative to their bulk D/H ratios, derived from D MAS NMR, is applied to explain the origin of the Tagish Lake trend of δD vs molecular evolution (H/C × 100). The results of this forward prediction suggest that the Tagish Lake isotopic trend results from a combination of molecular evolution (loss of predominantly aliphatic H and D) and partial D-H exchange with D depleted chondritic water during a short-term hydrothermal alteration event. Such events may be faithfully identified in chondritic organic solids and be a common occurrence, but not necessarily revealed in the mineralogy of type 1 and 2 carbonaceous chondrites.

Shock melt in the Cold Bokkeveld CM2 carbonaceous chondrite and the response of C‐complex asteroids to hypervelocity impacts

AbstractMany of the CM carbonaceous chondrites are regolith breccias and so should have abundant evidence for collisional processing. The constituent clasts of these fragmental rocks frequently display compactional petrofabrics; yet, olivine microstructures show that most CMs are unshocked. To better understand the reasons for this contradiction, we have sought other evidence for hypervelocity impact processing of CM chondrites using the Cold Bokkeveld meteorite. We find that this regolith breccia contains rare particles of vesicular shock melt that are close in chemical composition to bulk CM chondrite. Transmission electron microscopy of a melt bead shows that it is composed of silicate glass with inclusions of pentlandite, pyrrhotite, and wüstite. Characterization of shards of another bead by atom probe tomography reveals nanoscale clusters of sulfur that represent sulfide inclusions arrested at an early stage of growth. These glass particles are mineralogically comparable to micrometeoroid impact melt described from the Cb‐type asteroid Ryugu and melt that has been experimentally produced by pulsed laser irradiation of CM targets. The glass could have formed by in situ shock‐melting, but petrographic evidence is more consistent with an origin as ballistic ejecta from a distal impact. The scarcity of melt in this meteorite, and CM chondrites more broadly, is consistent with the explosive fragmentation of hydrous asteroids following energetic collisions. Cold Bokkeveld's parent body is likely to be a second‐generation asteroid that was constructed from the debris of one or more earlier bodies, and only a small proportion of the reaccreted material had been highly shocked and melted.

Kinetic analysis of dehydration/dehydroxylation from carbonaceous chondrites by in situ heating experiments under an infrared microscope

AbstractCI, CM, and CR carbonaceous chondrites contain hydrous minerals, indicating that their parent bodies underwent aqueous alteration at low temperatures. Some of these chondrites, such as heated CM, CI, and CY chondrites, experienced thermal dehydration by impacts or solar radiation after aqueous alteration. This study conducted heating experiments on carbonaceous chondrites and evaluated their dehydration/dehydroxylation kinetics in an effort to explain the thermal history of the parent asteroids of heated carbonaceous chondrites using their degrees of dehydration/dehydroxylation of hydrous minerals. Murchison (CM2.5) and Ivuna (CI1), relatively primitive (having not undergone thermal alteration) carbonaceous chondrites, were used as starting materials. Weakening in the OH band at ~3680 cm−1 (2.72 μm) with isothermal heating at 350–500°C (Murchison) and 450–525°C (Ivuna) were observed under in situ infrared spectroscopy (FT‐IR) equipped with a heating stage. To determine the rate constants, the decrease in the OH band was fitted using kinetic models such as first‐order reactions, two‐dimensional diffusion, and three‐dimensional diffusion. The apparent activation energies and frequency factors were determined using the Arrhenius equation. Time–temperature transformation diagrams were drawn to represent the decrease in the OH‐band intensity as a function of temperature and heating duration. Such kinetic approaches can provide constraints on the temperature and time of the dehydration/dehydroxylation processes and enable us to estimate long‐term effects from experiments in the laboratory within a short time.

Rapid heating rates define the volatile emission and regolith composition of (3200) Phaethon

Asteroid (3200) Phaethon experiences extreme solar radiant heating ( ~ 750 °C) during perihelion (0.14 au), leading to comet-like activity. The regolith composition and mechanism of volatile emission are unknown but key to understanding JAXA’s DESTINY+ mission data (fly-by in 2029) and the fate of near-Sun asteroids more generally. By subjecting CM chondrite fragments to fast, open system, cyclic heating (2-20 °C/min), simulating conditions on Phaethon we demonstrate that rapid heating rates combine with the low permeability, resulting in reactions between volatile gases and decomposing minerals. The retention of S-bearing gas limits the thermal decomposition of Fe-sulphides, allowing these minerals to survive repeated heating cycles. Slow escape of S-bearing gases provides a mechanism for repeated gas release from a thermally processed surface and, therefore the comet-like activity without requiring surface renewal to expose fresh material each perihelion cycle. We predict Phaethon regolith is composed of olivine, Fe-sulphides, Ca-sulphates and hematite.

Chondrule sizes within the CM carbonaceous chondrites and measurement methodologies

AbstractThe sizes of chondrules are a valuable tool for understanding relationships between meteorite groups and the affinity of ungrouped chondrites, documenting temporal/spatial variability in the solar nebula, and exploring the effects of parent body processing. Many of the recently reported sizes of chondrules within the CM carbonaceous chondrites differ significantly from the established literature average and are more closely comparable to those of chondrules within CO chondrites. Here, we report an updated analysis of chondrule dimensions within the CM group based on data from 1937 chondrules, obtained across a suite of CM lithologies ranging from petrologic subtypes CM2.2 to CM2.7. Our revised average CM chondrule size is 194 μm. Among the samples examined, a relationship was observed between petrologic subtype and chondrule size such that chondrule long‐axis lengths are greater in the more highly aqueously altered lithologies. These findings suggest a greater similarity between the CM and CO chondrites than previously thought and support arguments for a genetic link between the two groups (i.e., the CM‐CO clan). Using the 2‐D and 3‐D data gathered, we also apply numerous stereological corrections to examine their usefulness in correcting 2‐D chondrule measurements within the CM chondrites. Alongside this analysis, we present the details of a standardized methodology for 2‐D chondrule size measurement to facilitate more reliable inter‐study comparisons.

Extent of alteration, paleomagnetic history, and infrared spectral properties of the Tarda ungrouped carbonaceous chondrite

AbstractTarda is an ungrouped, hydrated carbonaceous chondrite (C2‐ung) that was seen to fall in Morocco in 2020. Early studies showed that Tarda chemically resembles another ungrouped chondrite, Tagish Lake (C2‐ung), which has previously been linked to the dark D‐type asteroids. Samples of D‐type asteroids provide an important opportunity to investigate primitive conditions in the outer solar system. We show that Tarda contains few intact chondrules and refractory inclusions and that its composition is dominated by secondary Mg‐rich phyllosilicates (>70 vol%), carbonates, oxides, and Fe‐sulfides that formed during extensive water–rock reactions. Quantitative assessment of first‐order reversal curve (FORC) diagrams shows that Tarda's magnetic mineralogy (i.e., framboidal magnetite) is comparable to that of the CI chondrites and differs notably from that of most CM chondrites. These traits support a common formation process for magnetite in Tarda and the CI chondrites. Furthermore, Tarda's pre‐terrestrial paleomagnetic remanence is similar to that of Tagish Lake and samples returned from asteroid Ryugu, with a very weak paleointensity (<0.6 μT) suggesting that Tarda's parent body accreted more distally than that of the CM chondrites, possibly at a distance of >5.4–8.3 AU. An origin in the cold, outer regions of the solar system is further supported by the presence of distinct, porous clasts enriched in aliphatic‐rich organics that potentially retain a pristine interstellar composition. Together, our observations support a genetic relationship between Tarda and Tagish Lake.

Identification of a primordial high D/H component in the matrix of unequilibrated ordinary chondrites

Deuterium to hydrogen isotope ratios in unequilibrated ordinary chondrites (UOCs) which have undergone little-to-no thermal metamorphism pose an interesting problem when looking at water in the early Solar System. Bulk chondrite studies have shown that UOCs of the lowest subtypes have D/H ratios as high as comets from the outer Solar System, which, along with bulk UOC water abundances, decrease with thermal metamorphism. Since bulk UOC analyses represent a complex mixture of organic and hydrated phases, it is not clear what phase(s) is responsible for the high bulk D/H values. In this study, we report in situ secondary ion mass spectrometry (SIMS) measurements of the H isotope composition of the fine-grained matrix of UOCs with petrological subtypes ranging from 3.00 to 3.9. We find that for matrix areas in UOCs of petrologic subtype ≥3.2, correlations between D-rich organic material and D-poor phyllosilicates give relatively D-poor intrinsic water isotopic compositions, with δD values between −320 ± 91 ‰ and −71 ± 71 ‰, which are inherited from parent body accretion. Therefore, we conclude that OC parent bodies accreted D-poor water ice that had an H isotopic composition similar to that of CM and CV chondrite parent bodies. We find that matrix in UOCs of the lowest subtypes (Semarkona, Bishunpur, and Ngawi) show similar water and organic H isotope compositions to higher type UOCs. Our in situ analyses also show that matrix areas in these pristine UOCs contain a third, thus far unidentified, component that carries the high D/H signature, with δD values up to ∼6000 ‰. We propose that this component is pristine amorphous silicates preserved from the molecular cloud or early protoplanetary disc that is extremely sensitive to thermal and aqueous alteration on asteroidal parent bodies.

High‐resolution cathodoluminescence of calcites from the Cold Bokkeveld chondrite: New insights on carbonatation processes in CM parent bodies

AbstractCarbonates, as secondary minerals found in CM chondrites, have been widely employed for reconstructing the composition of the fluids from which they precipitated. They also offer valuable insights into the hydrothermal evolution of their parent bodies. In this study, we demonstrate that high‐resolution cathodoluminescence (HR‐CL) analyses of calcites derived from the brecciated Cold Bokkeveld CM2 chondrite can effectively reveal subtle compositional features and intricate zoning patterns. We have identified two distinct types of cathodoluminescence (CL) centers: a blue emission band (approximately 375–425 nm), associated with intrinsic structural defects, and a lower energy orange extrinsic emission (around 620 ± 10 nm), indicating the presence of Mn cations. These compositional variations enable discrimination between the calcite grain types previously designated as T1 and T2 in studies of CM chondrites. T1 calcites exhibit variable CL and peripheral Mn enrichments, consistently surrounded by a rim composed of Fe‐S‐rich serpentine–tochilinite assemblage. Conversely, T2 calcites display homogeneous CL and more abundant lattice defects. These polycrystalline aggregates of calcite grains, devoid of serpentine, contain Fe‐Ni sulfide inclusions and directly interface with the matrix. We propose that changes in the Mn content of calcite (indicated by the intensity of orange CL emission) are influenced by variations in redox potential (Eh) and pH of the fluid phase. This proposed hydrothermal evolution establishes a parallel between terrestrial serpentinization followed by carbonation processes and the aqueous alteration of CM chondrites, warranting further exploration and investigation of this intriguing similarity.

Abundance, sizes, and major element compositions of components in CR and LL chondrites: Formation from single reservoirs

AbstractAbundances, apparent sizes, and individual chemical compositions of chondrules, refractory inclusions, other objects, and surrounding matrix have been determined for Semarkona (LL3.00) and Renazzo (CR2) using consistent methods and criteria on X‐ray element intensity maps. These represent the non‐carbonaceous (NC, Semarkona) and carbonaceous chondrite (CC, Renazzo) superclans of chondrite types. We compare object and matrix abundances with similar data for CM, CO, K, and CV chondrites. We assess, pixel‐by‐pixel, the major element abundance in each object and in the entire matrix. We determine the abundance of “metallic chondrules” in LL chondrites. Chondrules with high Mg/Si and low Fe/Si and matrix carrying opposing ratios complement each other to make whole rocks with near‐solar major element ratios in Renazzo. Similar Mg/Si and Fe/Si chondrule–matrix relationships are seen in Semarkona, which is within 11% of solar Mg/Si but significantly Fe‐depleted. These results provide a robust constraint in support of single‐reservoir models for chondrule formation and accretion, ruling out whole classes of astrophysical models and constraining processes of chondrite component formation and accretion into chondrite parent bodies.