Chondrites Research Articles

Reactivity of chondritic meteorites under H2-rich atmospheres: Formation of H2S.

Abstract Current models of chemical evolution during star and planetary formation rely on the presence of dust grains to act as a third body. However, they generally ignore the reactivity of the dust grains themselves. Dust grains present in the protoplanetary phase will evolve as the solar system forms and, after protoplanets have appeared, they will be constantly delivered to their surfaces in the form of large aggregates or meteorites. Chondritic meteorites are mostly unaltered samples of the dust present in the first stages of the Solar System formation, that still arrive nowadays to the surface of Earth and allow us to study the properties of the materials forming the early Solar System. These materials contain, amongst others, transition metals that can potentially act as catalysts, as well as other phases that can potentially react in different astrophysical conditions, such as FeS. In this work, we present the reactivity of chondritic meteorites under H2-rich atmospheres, particularly towards the reduction of FeS for the formation of H2S and metallic Fe during the early phases of the planetary formation. We present the obtained results on the reaction rates and the percentage of FeS available to react in the materials. Additionally, we include a computational study of the reaction mechanism and the energetics. Finally, we discuss the implications of an early formation of H2S in planetary surfaces.

The physical properties of two potential targets for space missions: (155140) 2005 UD and (612267) 2001 SG286

Abstract The ground-based characterization of asteroids is a key step for planning their exploration. The near-earth asteroid 155140 (2005 UD) is a potential flyby target of JAXA’s DESTINIY+ mission, while (612267) 2001 SG286 has been considered as a possible target for in-situ exploration. We aim to determine their physical properties using the observations obtained with various telescopes from Canary Islands Observatory. For 2005 UD we confirmed the two peak lightcurve, a rotation period of 5.224 ± 0.003 h and an amplitude of 0.34 mag. However, a three peak solution seemed also to fit the light-curve, but this was discarded as implausible. Using the obtained visible to near-infrared spectrum we classified it as a Cb-type, and we found a spectral matching with heated carbonaceous chondrite meteorites of CM2 type. The thermal emission flux at 2.2 μm points to an albedo of pV = 0.06 ± 0.02. There are significant differences in the spectrum of 2005 UD compared to that of (3200) Phaeton, hypothesized as its parent body. The accurate visible spectrum obtained with the GTC indicate that 2001 SG286 is an S-type asteroid. The photometric data obtained with INT suggests a rotation period of 12.30 ± 0.01 h and an amplitude of 0.64 mag. With these observations we found its absolute magnitude H = 21.4 ± 0.3, and estimate its size as 160 ± 45 m.

The non-carbonaceous nature of Earth’s late-stage accretion

Constraining the origin of Earth’s building blocks requires knowledge of the chemical and isotopic characteristics of the source region(s) where these materials accreted. The siderophile elements Mo and Ru are well suited to investigating the mass-independent nucleosynthetic (i.e., “genetic”) signatures of material that contributed to the latter stages of Earth’s formation. Studies contrasting the Mo and Ru isotopic compositions of the bulk silicate Earth (BSE) to genetic signatures of meteorites, however, have reported conflicting estimates of the proportions of the non-carbonaceous type or NC (presumptive inner Solar System origin) and carbonaceous chondrite type or CC (presumptive outer Solar System origin) materials delivered to Earth during late-stage accretion (likely including the Moon-forming event and onwards). The present study reports new mass-independent isotopic data for Mo, which are presumed to reflect the composition of the BSE. A comparison of the new estimate for the BSE composition with new data for a select suite of NC iron meteorites is used to constrain the genetic characteristics of materials accreted to Earth during late-stage accretion. Results indicate that the final 10 to 20 wt% of Earth’s accretion was dominated by NC materials that were likely sourced from the inner Solar System, although the addition of minor proportions of CC materials, as has been suggested to occur during accretion of the final 0.5 to 1 wt% of Earth’s mass, remains possible. If this interpretation is correct, it brings estimates of the genetic signatures of Mo and Ru during the final 10 to 20 wt% of Earth accretion into concordance.

Hf–W isotope systematics of bulk chondrites: Implications for early Solar System evolution

The short-lived 182Hf–182W system is widely used for constraining the chronology of the early Solar System, including the timing of the formation, thermal evolution, and differentiation of planetary bodies. Utilizing the full potential of the Hf–W system requires knowledge of the Hf/W ratio and W isotopic composition of primitive chondritic material. However, metal-silicate heterogeneity among chondritic samples can complicate accurately determining the Hf–W systematics of bulk chondrite parent bodies. Moreover, interpreting Hf–W data for chondrites may be complicated by potential nucleosynthetic W isotope anomalies. To this end, we report Hf/W ratios and W isotope compositions for bulk ordinary and enstatite chondrites, as well as the first such data for Rumuruti chondrites. We find that ordinary and Rumuruti chondrites show no resolvable nucleosynthetic anomalies, whereas resolved ε183W (i.e., 0.01 % deviation in 183W/184W from terrestrial standard) excesses in individual enstatite chondrites suggest the presence of nucleosynthetic W isotope anomalies in bulk meteorite samples originating in the inner Solar System. These anomalies necessitate corrections when accurately quantifying radiogenic 182W variations. Furthermore, several ordinary chondrites deviate in Hf/W ratios and W composition from the parent body compositions previously obtained from internal 182Hf–182W isochrons, indicating variations in the abundance of metal across different chondrite samples. Similarly, the Hf–W systematics of some enstatite chondrites also deviate from the parent body values, which can be attributed to the heterogeneous distribution of Hf carrier phases. The new observations highlight the challenges in obtaining Hf-W data that are representative of the chondrite parent bodies from individual chondrites, especially from metal-rich samples. By contrast, Rumuruti chondrites of variable petrologic types exhibit uniform Hf/W and 182W/184W ratios, suggesting that these samples are representative of their parent body. Whereas their Hf/W ratio is similar to that of carbonaceous chondrites, their W isotope composition is less radiogenic. This indicates that the Rumuruti precursor reservoir most likely had a significantly lower Hf/W ratio than the ratio measured in Rumuruti chondrites today. These findings underscore the importance of understanding the likely variations in Hf-W isotope systematics of iron meteorite parent bodies for accurately determining the timing of core formation.

Radial transport and nebular thermal processing of millimeter‐sized solids in the Solar protoplanetary disk inferred from Cr‐Ti‐O isotope systematics of chondrules

AbstractUnderstanding the material transport and mixing processes in the Solar protoplanetary disk provides important constraints on the origin of chemical and isotopic diversities of our planets. The limited extent of radial transport and mixing between the inner and outer Solar System has been suggested based on a fundamental isotopic dichotomy between non‐carbonaceous (NC) and carbonaceous (CC) meteorite groups. The limited transport and mixing could be further tested by tracing the formation regions of individual meteoritic components, such as Ca‐Al‐rich inclusions (CAIs) and chondrules. Here, we show further evidence for the outward transport of CAIs and chondrules from the inner and subsequent thermal processing in the outer region of the protoplanetary disk based on the petrography and combined Cr‐Ti‐O isotope systematics of chondrules from the Vigarano‐like (CV) carbonaceous chondrite Allende. One chondrule studied consists of an olivine core that exhibits NC‐like Ti and O, but CC‐like Cr isotopic signatures, which is enclosed by a pyroxene igneous rim with CC‐like O isotope ratios. These observations indicate that the olivine core formed in the inner Solar System. The olivine core then migrated into the outer Solar System and experienced nebular thermal processing that generated the pyroxene igneous rim. The nebular thermal processing would result in Cr isotope exchange between the olivine core and CC‐like materials, but secondary alteration effects on the parent body are also responsible for the CC‐like Cr isotope signature. By combining previously reported Cr‐Ti‐O isotope systematics of CV chondrules, we show that some CV chondrules larger than ~1 mm would have formed in the inner Solar System. The accretion of the millimeter‐sized, inner Solar System solids onto the CV carbonaceous chondrite parent body would require their very early migration into the outer Solar System within the first 1 million years after the Solar System formation.

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.

Effect of a giant meteorite impact on Paleoarchean surface environments and life

Large meteorite impacts must have strongly affected the habitability of the early Earth. Rocks of the Archean Eon record at least 16 major impact events, involving bolides larger than 10 km in diameter. These impacts probably had severe, albeit temporary, consequences for surface environments. However, their effect on early life is not well understood. Here, we analyze the sedimentology, petrography, and carbon isotope geochemistry of sedimentary rocks across the S2 impact event (37 to 58 km carbonaceous chondrite) forming part of the 3.26 Ga Fig Tree Group, South Africa, to evaluate its environmental effects and biological consequences. The impact initiated 1) a giant tsunami that mixed Fe2+-rich deep waters into the Fe2+-poor shallow waters and washed debris into coastal areas, 2) heating that caused partial evaporation of surface ocean waters and likely a short-term increase in weathering and erosion on land, and 3) injection of P from vaporization of the S2 bolide. Strata immediately above the S2 impact event contain abundant siderites, which are associated with organic matter and exhibit light and variable δ13Ccarb values. This is consistent with microbial iron cycling in the wake of the impact event. Thus, the S2 impact likely had regional, if not global, positive and negative effects on life. The tsunami, atmospheric heating, and darkness would likely have decimated phototrophic microbes in the shallow water column. However, the biosphere likely recovered rapidly, and, in the medium term, the increase in nutrients and iron likely facilitated microbial blooms, especially of iron-cycling microbes.

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).

Nitrogen in the Orgueil meteorite: Abundant ammonium among other reservoirs of variable isotopic compositions

Nitrogen, because of its abundance and variety of carrier phases, is a unique tracer of physico-chemical processes occurring throughout star and planet formations. The refractory organic matter is commonly considered as the main carrier of nitrogen in the most primitive objects of our Solar System. However, nitrogen in the form of ammonium (NH4+) was observed in the Ivuna-type carbonaceous (CI) chondrites Alais in 1834, and Orgueil just after its fall in 1864, as well as more recently on Ceres, comet 67P/Churyumov-Gerasimenko, and possibly on some asteroids. In the present study, we have measured the nitrogen content and isotopic composition in various nitrogen-bearing phases of several samples of the Orgueil meteorite, with different degrees of terrestrial weathering. Water-soluble NH4+ is present in Orgueil at a mean concentration of 0.07 ± 0.01 wt%, with a mean isotopic composition of δ15N = +72 ± 9 ‰ (14N/15N = 254 ± 2), confirming its extra-terrestrial origin. In the most terrestrially altered sample of Orgueil that we analysed, the isotopic composition is δ15N = +50 ± 12 ‰ (14N/15N = 259 ± 3). NH4+ is in species that are thermally stable up to 383 K, possibly ammonium inorganic/organic salts and ammoniated phyllosilicates. We also show that the nitrogen in Orgueil is distributed among the insoluble organic matter (IOM) (35 ± 5 %), ammonium (27 ± 5 %), and other minor water-soluble species (e.g., nitrate, amines etc.: < 6 %). The remaining nitrogen (34 ± 14 %) is mainly in an unidentified organic matter (UOM), which may be IOM lost during its extraction and/or acid hydrolysable functional groups bounded to the IOM and/or organic nitrogen trapped within minerals. The three main carriers of nitrogen in Orgueil have δ15N (and 14N/15N) values of + 32 ± 1 ‰ (264 ± 0.3) for IOM, +39 ± 16 ‰ (262 ± 4) for UOM, and + 72 ± 9 ‰ (254 ± 2) for NH4+.Although IOM and NH4+ have significantly different δ15N, we cannot exclude that these phases could be compositionally related because IOM is heterogeneous in 15N. Ammonium could have been produced via heating and/or aqueous alteration processes of organic matter in the CI parent body. Alternatively, or additionnally, ammonium could be a tracer of the accretion and/or later deposit of NH3 ice, NH3 hydrates, and/or NH4+ salts on the CI parent body.As shown by previous studies, Ryugu grains sampled by the Hayabusa2 mission (JAXA) have heterogeneous compositions at the millimeter scale, with nitrogen concentrations and δ15N similar or lower than Orgueil, possibly because of different parent body processing. The present study suggests that the lack or loss of 15N-rich NH4+ in some Ryugu grains may explain some of these differences with Orgueil.

Unique achondritic impact debris in the CH3 chondrite Acfer 182

The metal-rich CH carbonaceous chondrites contain abundant xenolithic clasts originating from different regions of the Solar System. In the CH3 chondrite Acfer 182, we identified two phosphide spherules (one 95-μm in diameter and the other 50μm×60μm) of schreibersite ((Fe,Ni)3P) and barringerite ((Fe,Ni)2P) with kamacite eutectic structures. These objects are likely to have formed during an impact between planetesimals during the debris-disk phase of the protoplanetary disk before being incorporated into the CH chondrite parent body. In the same sample we identified a 130μm×60μm heideite grain (iron‑titanium sulfide: (Fe,Cr)1.15(Ti,Fe)2S4) with exsolution lamellae of calcium-rich titanium oxide. Thin veins of shock-induced kamacite cross-cut the oxide lamellae, suggesting that it was ejected into the protoplanetary debris disk during an impact event before eventually being accreted by the CH chondrite parent body. This assemblage is distinct from heideite grains found in enstatite chondrites, aubrites, and the Kaidun meteorite. We propose that this object originated from a highly-reduced planetesimal in the inner Solar System that may have been similar to proto-Mercury.

The Ni isotopic composition of Ryugu reveals a common accretion region for carbonaceous chondrites.

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu/CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk's lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites/Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites.

Recent replenishment of aliphatic organics on Ceres from a large subsurface reservoir.

Ceres hosts notable aliphatic-organic concentrations, ranging from approximately 5 to >30 weight % in specific surface areas. The origins and persistence of these organics are under debate due to the intense aliphatic organic signature and radiation levels in Ceres' orbit, which would typically lead to their destruction, hindering detection. To investigate this, we conducted laboratory experiments to replicate how the signature of the organic-rich regions would degrade due to radiation. Our findings indicate a fast degradation rate, implying the exposure of buried organics within the past few million years. This degradation rate, coupled with observed quantities, implies that the aliphatics must be present in substantial quantities within the shallow subsurface. Our estimates suggest an initial aliphatic abundance 2 to 30 times greater than currently observed, surpassing significantly the levels found in carbonaceous chondrites, indicating either a significant concentration or remarkable purity.

Fe, Zn, and Mg stable isotope systematics of acapulcoite lodranite clan meteorites

AbstractThe processes of planetary accretion and differentiation, whereby an unsorted mass of primitive solar system material evolves into a body composed of a silicate mantle and metallic core, remain poorly understood. Mass‐dependent variations of the isotope ratios of non‐traditional stable isotope systems in meteorites are known to record events in the nebula and planetary evolution processes. Partial melting and melt separation, evaporation and condensation, diffusion, and thermal equilibration between minerals at the parent body (PB) scale can be recorded in the isotopic signatures of meteorites. In this context, the acapulcoite–lodranite meteorite clan (ALC), which represents the products of thermal metamorphism and low‐degree partial melting of a primitive asteroid, is an attractive target to study the processes of early planetary differentiation. Here, we present a comprehensive data set of mass‐dependent Fe, Zn, and Mg isotope ratio variations in bulk ALC species, their separated silicate and metal phases, and in handpicked mineral fractions. These non‐traditional stable isotope ratios are governed by mass‐dependent isotope fractionation and provide a state‐of‐the‐art perspective on the evolution of the ALC PB, which is complementary to interpretations based on the petrology, trace element composition, and isotope geochemistry of the ALC. None of the isotopic signatures of ALC species show convincing co‐variation with the oxygen isotope ratios, which are considered to record nebular processes occurring prior to the PB formation. Iron isotopic compositions of ALC metal and silicate phases broadly fall on the isotherms within the temperature ranges predicted by pyroxene thermometry. The isotope ratios of Mg in ALC meteorites and their silicate minerals are within the range of chondritic meteorites, with only accessory spinel group minerals having significantly different compositions. Overall, the Mg and Fe isotopic signatures of the ALC species analyzed are in line with their formation as products of high‐degree thermal metamorphism and low‐degree partial melting of primitive precursors. The δ66/64Zn values of the ALC meteorites demonstrate a range of ~3.5‰ and the Zn is overall isotopically heavier than in chondrites. The superchondritic Zn isotopic signatures have possibly resulted from evaporative Zn losses, as observed for other meteorite parent bodies. This is unlikely to be the result of PB differentiation processes, as the Zn isotope ratio data show no covariation with the proxies of partial melting, such as the mass fractions of the platinum group and rare earth elements.

Early generation of a refractory inclusions-enriched H-chondritic parent body: A safe harbor for Ca, Al-rich inclusions

Calcium-aluminum-rich inclusions (CAIs) commonly observed in chondritic meteorites are the oldest dated solids formed in the Solar System. Short-lived isotope chronologies (26Al-26Mg, 182Hf-182W) suggest a ∼2 Ma gap between the formation of CAIs and the accretion of the final chondrite parent bodies. One thin section, 3.27 cm2 in size, of an ordinary chondrite NWA 3358 (H3.1) studied contains 52 refractory inclusions (CAIs and amoeboid olivine aggregates (AOAs)) comprising 0.14 % of its area, which is the highest abundance of refractory inclusions among non-carbonaceous chondrites containing on average ∼0.009 area % of CAIs and AOAs. In combination with a low chondrule/matrix ratio of ∼1.5, this makes NWA 3358 a unique ordinary chondrite. The aqueously-formed fayalites (Fa>99) in NWA 3358 have the inferred initial 53Mn/55Mn ratio of (5.56 ± 0.44) × 10−6 which is the highest measured value for secondary minerals in chondrites and corresponds to the formation time of ∼1.0–1.5 Ma after CAIs. Based on the 53Mn-53Cr chronology of fayalite formation and the thermal modeling, we infer that the first-generation of an H chondrite parent body, ∼6–12 km in diameter, accreted within 1.0 Ma after formation of CAIs, filling the gap of ∼2 Ma between CAIs and the earliest chondrite parent bodies. This early accretion provides a possible mechanism of CAIs/AOAs storage in the inner solar nebula and could explain the high amount of refractory inclusions in NWA 3358. A later destruction of these first-generation bodies may also explain the presence of CAIs and chondrules of different ages within later formed chondrite parent bodies.

Coordinated microanalysis of volatiles in apatite and silicate minerals in ancient lunar basalts

Despite being essentially water-free, nominally anhydrous minerals such as plagioclase and pyroxene represent the biggest reservoir of water in most lunar rocks due to their sheer abundance. Apatite, which incorporates F, Cl, and OH into its mineral structure as essential crystal components, on the other hand, is the only other volatile-bearing phase common in lunar samples. Here, we present the first coordinated study of volatiles (e.g., H2O, Cl, F, and S) in nominally anhydrous minerals combined with isotopic measurements in apatite from the ancient lunar basalt fragments from meteorite Miller Range (MIL) 13317. Apatite in MIL 13317 basalt contains ∼ 2000 ppm H2O and has an elevated δD values (+ 523–737 ‰), similar to Apollo mare basalts, but has high δ37Cl values (+ 29–36 ‰), similar to apatite found in several KREEP-rich samples. MIL 13317 is unique compared with other lunar basalts; it has both elevated δD and δ37Cl values currently only observed in highlands sample 79215 (a granulitic impactite). Based on measurements of H2O in nominally anhydrous minerals and in apatite, the source magma of MIL 13317 basalt is estimated to contain ∼ 130–330 ppm H2O. Assuming reasonable levels of partial melting of the lunar mantle and magmatic degassing during eruption of the basalt, the Moon contained at least one reservoir with < 100 ppm H2O, a δD value of < 0 ‰ similar to carbonaceous chondrites, and extensively fractionated Cl isotopes prior to 4.332 Gyr, the crystallization age of the MIL 13317 basalt.

Complex irradiation history of chondrules and matrix – A study of CR2 and some other meteorites

Excesses of cosmic-ray produced nuclei in individual components of meteorites indicate “pre-irradiation”, either in the surface region of their parent bodies or as free-floating small particles in the early Solar System. We expand on our earlier work (Beyersdorf-Kuis et al., 2015) and report a study of cosmic-ray produced He and Ne in chondrules and “matrix” (i.e., matrix-dominated) material of several CR2 and CV meteorites as well as the highly primitive, unique, carbonaceous chondrite Acfer 094. In accordance with previous work, no evidence for pre-irradiation was found for CV3 Allende, while for CV3 Vigarano evidence for pre-irradiation is marginal at best. Also, the single chondrule from unique Acfer 094 that we studied has a cosmic ray exposure indistinguishable from the one we found for “matrix” material. Chondrules from Acfer 082 (CV) exhibit both excesses and deficits relative to “matrix”, which points to pre-irradiation of not only chondrules, but also matrix material. A similar case may be Renazzo (CR2), where, however, the identification is complicated by the presence of abundant pre-solar Ne-E. A large number of chondrules (ten) were studied from CR2 El Djouf 001, which yielded slightly variable, small but consistent, excesses relative to “matrix”, corresponding to “nominal” (i.e., irradiation by galactic cosmic rays in 4π geometry) excess ages of 1–2 Ma. Modelling suggests contributions from irradiation in the parent body regolith by solar cosmic rays (SCR) as well as galactic cosmic rays (GCR), where the latter dominates. Reevaluating the large variations previously identified in chondrules from QUE 99177, we suggest either a very different regolith history compared to that of El Djouf 001 or, more likely, pre-irradiation by, primarily, GCR in the early solar system as suggested previously. The case of solar-wind-rich NWA 852 (CR2) shows similarity to El Djouf 001 except for a much larger size of the effects. We suggest that the situation may be common among meteorites with a regolith origin. With independent information on the cosmic ray exposure age, which could be obtained by the study of cosmic-ray produced radionuclides, the individual parent body contributions may be disentangled, providing constraints on regolith dynamics.

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 [&lt;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 &lt;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.

Spectral properties of ablating meteorite samples for improved meteoroid composition diagnostics

Emission spectra and diagnostic spectral features of a diverse range of ablated meteorite samples with a known composition are presented. We aim to provide a reference spectral dataset to improve our abilities to classify meteoroid composition types from meteor spectra observations. The data were obtained by ablating meteorite samples in high-enthalpy plasma wind tunnel facilities recreating conditions characteristic of low-speed meteors. Near-UV to visible-range (320–800 nm) emission spectra of 22 diverse meteorites captured by a high-resolution Echelle spectrometer were analyzed to identify the characteristic spectral features of individual meteorite groups. The same dataset captured by a lower-resolution meteor spectrograph was applied to compare the meteorite data with meteor spectra observations. Spectral modeling revealed that the emitting meteorite plasma was characterized by temperatures of 3700–4800 K, similar to the main temperature component of meteors. The studied line intensity variations were found to trace the differences in the original meteorite composition and thus can be used to constrain the individual meteorite classes. We demonstrate that meteorite composition types, including ordinary chondrites, carbonaceous chondrites, various achondrites, stony-iron and iron meteorites, can be spectrally distinguished by measuring relative line intensities of Mg I, Fe I, Na I, Cr I, Mn I, Si I, H I, CN, Ni I, and Li I. Additionally, we confirm the effect of the incomplete evaporation of refractory elements Al, Ti, and Ca, and the presence of minor species Co I, Cu I, and V I.