Exposure history of the Sutter's Mill carbonaceous chondrite

The Sutter's Mill (SM) carbonaceous chondrite fell in California on April 22, 2012. The cosmogenic radionuclide data indicate that Sutter's Mill was exposed to cosmic rays for 0.082 ± 0.008 Myr, which is one of the shortest ages for C chondrites, but overlaps with a small cluster at approximately 0.1 Myr. The age is significantly longer than proposed ages that were obtained from cosmogenic noble gas concentrations, which have large uncertainties due to trapped noble gas corrections. The presence of neutron‐capture ⁶⁰Co and ³⁶Cl in SM indicates a minimum preatmospheric radius of approximately 50 cm, and is consistent with a radius of 1–2 m, as derived from the fireball observations. Although a large preatmospheric size was proposed, one fragment (SM18) contains solar cosmic ray–produced short‐lived radionuclides, such as ⁵⁶Co and ⁵¹Cr. This implies that this specimen was less than 2 cm from the preatmospheric surface of Sutter's Mill. Although this conclusion seems surprising, it is consistent with the observation that the meteoroid fragmented high in the atmosphere. The presence of SCR‐produced nuclides is consistent with the high SCR fluxes observed during the last few months before the meteorite's fall, when its orbit was less than 1 AU from the Sun.     

The amino acid composition of the Sutter's Mill CM2 carbonaceous chondrite

We determined the abundances and enantiomeric compositions of amino acids in Sutter's Mill fragment #2 (designated SM2) recovered prior to heavy rains that fell April 25–26, 2012, and two other meteorite fragments, SM12 and SM51, that were recovered postrain. We also determined the abundance, enantiomeric, and isotopic compositions of amino acids in soil from the recovery site of fragment SM51. The three meteorite stones experienced terrestrial amino acid contamination, as evidenced by the low d/l ratios of several proteinogenic amino acids. The d/l ratios were higher in SM2 than in SM12 and SM51, consistent with rain introducing additional l‐ amino acid contaminants to SM12 and SM51. Higher percentages of glycine, β‐alanine, and γ‐amino‐n‐butyric acid were observed in free form in SM2 and SM51 compared with the soil, suggesting that these free amino acids may be indigenous. Trace levels of d +l‐ β‐aminoisobutyric acid (β‐AIB) observed in all three meteorites are not easily explained as terrestrial contamination, as β‐AIB is rare on Earth and was not detected in the soil. Bulk carbon and nitrogen and isotopic ratios of the SM samples and the soil also indicate terrestrial contamination, as does compound‐specific isotopic analysis of the amino acids in the soil. The amino acid abundances in SM2, the most pristine SM meteorite analyzed here, are approximately 20‐fold lower than in the Murchison CM2 carbonaceous chondrite. This may be due to thermal metamorphism in the Sutter's Mill parent body at temperatures greater than observed for other aqueously altered CM2 meteorites.     

Primordial and cosmogenic noble gases in the Sutter's Mill CM chondrite

The Sutter's Mill (SM) CM chondrite fell in California in 2012. The CM chondrite group is one of the most primitive, consisting of unequilibrated minerals, but some of them have experienced complex processes occurring on their parent body, such as aqueous alteration, thermal metamorphism, brecciation, and solar wind implantation. We have determined noble gas concentrations and isotopic compositions for SM samples using a stepped heating gas extraction method, in addition to mineralogical observation of the specimens. The primordial noble gas abundances, especially the P3 component trapped in presolar diamonds, confirm the classification of SM as a CM chondrite. The mineralogical features of SM indicate that it experienced mild thermal alteration after aqueous alteration. The heating temperature is estimated to be <350 °C based on the release profile of primordial ³⁶Ar. The presence of a Ni‐rich Fe‐Ni metal suggests that a minor part of SM has experienced heating at >500 °C. The variation in the heating temperature of thermal alteration is consistent with the texture as a breccia. The heterogeneous distribution of solar wind noble gases is also consistent with it. The cosmic‐ray exposure (CRE) age for SM is calculated to be 0.059 ± 0.023 Myr based on cosmogenic ²¹Ne by considering trapped noble gases as solar wind, the terrestrial atmosphere, P1 (or Q), P3, A2, and G components. The CRE age lies at the shorter end of the CRE age distribution of the CM chondrite group.     

53Mn‐53Cr dating of aqueously formed carbonates in the CM2 lithology of the Sutter's Mill carbonaceous chondrite

Radiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies. Dolomite is a well‐documented secondary mineral in CM chondrites, and is thought to have formed by precipitation from an aqueous fluid on the CM parent body within several million years of accretion. The petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ, rather than in the nebular setting. Here, we present ⁵³Mn‐⁵³Cr systematics for dolomite grains in Sutter's Mill section SM51‐1. The Mn‐Cr isotope data show well‐resolved excesses of ⁵³Cr correlated with ⁵⁵Mn/⁵²Cr ratio, which we interpret as evidence for the in situ decay of radioactive ⁵³Mn. After correcting for the relative sensitivities of Mn and Cr using a synthetic Mn‐ and Cr‐bearing calcite standard, the data yield an isochron with slope corresponding to an initial ⁵³Mn/⁵⁵Mn ratio of 3.42 ± 0.86 × 10^?6. The reported error includes systematic uncertainty from the relative sensitivity factor. When calculated relative to the U‐corrected Pb‐Pb absolute age of the D'Orbigny angrite, Sutter's Mill dolomites give a formation age between 4564.8 and 4562.2 Ma (2.4–5.0 Myr after the birth of the solar system). This age is contemporaneous with previously reported ages for secondary carbonates in CM and CI chondrites. Consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation, probably occurring via heating from internal ²⁶Al decay. The high‐precision isochron for Sutter's Mill dolomite indicates that late‐stage processing did not reach temperatures that were high enough to further disturb the Mn‐Cr isochron.     

Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon

Abstract The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine-pyroxene-phyllosilicate-sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel-phyllosilicate-diopside calcium-aluminum-rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine-grained rims of material resembling matrix. The opaque, fine-grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material. The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix-supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low-Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca-carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine-grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly-crystalline material with the same bulk composition as matrix phyllosilicate. There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C. We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM-like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type-2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.     

Molecular study of insoluble organic matter in Kainsaz CO3 carbonaceous chondrite: Comparison with CI and CM IOM

Abstract— Kainsaz CO3 insoluble organic matter (IOM) was studied using Curie point pyrolysis, electronic paramagnetic resonance (EPR), and high‐resolution transmission electron microscopy (HRTEM) to determine the effect of thermal metamorphism on molecular chondritic fingerprints. Pyrolysis released a very low amount of products that consist of one‐ and two‐ring aromatic units with methyl, dimethyl, and ethyl substituents. Moreover, Kainsaz IOM contains two orders of magnitude fewer radicals than Orgueil, Murchison, and Tagish Lake IOM. In addition, no diradicaloids were found in Kainsaz, although they are thought to constitute a specific signature for weakly organized extraterrestrial organic compounds in contrast to terrestrial ones. HRTEM reveals a very heterogeneous structure, with microporous disordered carbon, mesoporous graphitic carbons and graphite. Graphitization likely occurs and explains the differences between Kainsaz and CI or CM IOM. Heating stress experienced by Kainsaz IOM, on the parent body and/or prior its accretion, is likely responsible for the differences in molecular and structural organizations compared with those of CI and CM IOM.     

Chromium isotopic systematics of the Sutter's Mill carbonaceous chondrite: Implications for isotopic heterogeneities of the early solar system

Recent studies have shown that major meteorite groups possess their own characteristic ⁵⁴Cr values, demonstrating the utility of Cr isotopes for identifying genetic relationships between the planetary materials in conjunction with other classical tools, such as oxygen isotopes. In this study, we performed Cr isotope analyses for whole rocks and chemically separated phases of the new CM2 chondrite, Sutter's Mill (SM 43 and 51). The two whole rocks of Sutter's Mill show essentially identical ε⁵⁴Cr excesses (SM 43 = +0.95 ± 0.09ε, SM 51 = +0.88 ± 0.07ε), relative to the Earth. These values are the same within error with that of the CM2‐type Murchison (+0.89 ± 0.08ε), suggesting that parent bodies of Sutter's Mill and Murchison were formed from the same precursor materials in the solar nebula. Large ε⁵⁴Cr excess of up to 29.40ε is observed in the silicate phase of Sutter's Mill, while that of Murchison shows 15.74ε. Importantly, the leachate fractions of both Sutter's Mill and Murchison form a steep linear anticorrelation between ε⁵⁴Cr and ε⁵³Cr, cross‐cutting the positive correlation previously observed in carbonaceous chondrites. The fact that L4 acid leachate fraction contains higher ⁵⁴Cr excesses than that of L5 step designed to dissolve refractory minerals suggests that spinel is not a major ⁵⁴Cr carrier. We also note that L5 contains ⁵³Cr anomalies lower than the solar initial value, suggesting it carries a component of nucleosynthetic anomaly unrelated to the ⁵³Mn decay. We have identified five endmember components of nucleosynthetic origin among the early solar system materials.     

Mineralogy and petrography of C asteroid regolith: The Sutter's Mill CM meteorite

Based upon our characterization of three separate stones by electron and X‐ray beam analyses, computed X‐ray microtomography, Raman microspectrometry, and visible‐IR spectrometry, Sutter's Mill is a unique regolith breccia consisting mainly of various CM lithologies. Most samples resemble existing available CM2 chondrites, consisting of chondrules and calcium‐aluminum‐rich inclusion (CAI) set within phyllosilicate‐dominated matrix (mainly serpentine), pyrrhotite, pentlandite, tochilinite, and variable amounts of Ca‐Mg‐Fe carbonates. Some lithologies have witnessed sufficient thermal metamorphism to transform phyllosilicates into fine‐grained olivine, tochilinite into troilite, and destroy carbonates. One finely comminuted lithology contains xenolithic materials (enstatite, Fe‐Cr phosphides) suggesting impact of a reduced asteroid (E or M class) onto the main Sutter's Mill parent asteroid, which was probably a C class asteroid. One can use Sutter's Mill to help predict what will be found on the surfaces of C class asteroids such as Ceres and the target asteroids of the OSIRIS‐REx and Hayabusa 2 sample return missions (which will visit predominantly primitive asteroids). C class asteroid regolith may well contain a mixture of hydrated and thermally dehydrated indigenous materials as well as a significant admixture of exogenous material would be essential to the successful interpretation of mineralogical and bulk compositional data.     

The water content of CM carbonaceous chondrite falls and finds,and their susceptibility to terrestrial contamination

CM carbonaceous chondrites can be used to constrain the abundance and H isotopic composition of water and OH in C-complex asteroids. Previous measurements of the water/OH content of the CMs are at the higher end of the compositional range of asteroids as determined by remote sensing. One possible explanation is that the indigenous water/OH content of meteorites has been overestimated due to contamination during their time on Earth. Here we have sought to better understand the magnitude and rate of terrestrial contamination through quantifying the concentration and H isotopic composition of telluric and indigenous water in CM falls by stepwise pyrolysis. These measurements have been integrated with published pyrolysis data from CM falls and finds. Once exposed to Earth's atmosphere CM falls are contaminated rapidly, with some acquiring weight percent concentrations of water within days. The amount of water added does not progressively increase with time because CM falls have a similar range of adsorbed water contents to finds. Instead, the petrologic types of CMs strongly influence the amount of terrestrial water that they can acquire. This relationship is probably controlled by mineralogical and/or petrophysical properties of the meteorites that affect their hygroscopicity. Irrespective of the quantity of water that a sample adsorbs or its terrestrial age, there is minimal exchange of H in indigenous phyllosilicates with the terrestrial environment. The falls and finds discussed here contain 1.9–10.5 wt% indigenous water (average 7.0 wt%) that is consistent with recent measurements of C-complex asteroids including Bennu.     

The Sutter's Mill meteorite: Thermoluminescence data on thermal and metamorphic history

A piece of the Sutter's Mill meteorite, fragment SM2‐1d, has been examined using thermoluminescence techniques to better understand its thermal and metamorphic history. The sample had very weak but easily measureable natural and induced thermoluminescence (TL) signals; the signal‐to‐noise ratio was better than 10. The natural TL was restricted to the high‐temperature regions of the glow curve suggesting that the meteorite had been heated to approximately 300 °C within the time it takes for the TL signal to recover from a heating event, probably within the last 10⁵ years. It is possible that this reflects heating during release from the parent body, close passage by the Sun, or heating during atmospheric passage. Of these three options, the least likely is the first, but the other possibilities are equally likely. It seems that temperatures of approximately 300 °C reached 5 or 6 mm into the meteorite, so that all but one of the small Sutter's Mill stones have been heated. The Dhajala normalized induced TL signal for SM2‐1d is comparable to that of type 3.0 chondrites and is unlike normal CM chondrites, the class it most closely resembles, which do not have detectable TL sensitivity. The shape of the induced TL curve is comparable to other low‐type ordinary, CV, and CO chondrites, in that it has a broad hummocky structure, but does not resemble any of them in detail. This suggests that Sutter's Mill is a unique, low‐petrographic–type (3.0) chondrite.     

Geochemistry of the ungrouped carbonaceous chondrite Tagish Lake,the anomalous CM chondrite Bells,and comparison with CI and CM chondrites

Abstract— I have determined the composition via instrumental neutron activation analysis of a bulk pristine sample of the Tagish Lake carbonaceous chondrite fall, along with bulk samples of the CI chondrite Orgueil and of several CM chondrites. Tagish Lake has a mean of refractory lithophile element/Cr ratios like those of CM chondrites, and distinctly higher than the CI chondrite mean. Tagish Lake exhibits abundances of the moderately volatile lithophile elements Na and K that are slightly higher than those of mean CM chondrites. Refractory through moderately volatile siderophile element abundances in Tagish Lake are like those of CM chondrites. Tagish Lake is distinct from CM chondrites in abundances of the most volatile elements. Mean CI‐normalized Se/Co, Zn/Co and Cs/Co for Tagish Lake are 0.68 ± 0.01, 0.71 ± 0.07 and 0.76 ± 0.02, while for all available CM chondrite determinations, these ratios lie between 0.31 and 0.61, between 0.32 and 0.58, and between 0.39 and 0.74, respectively. Considering petrography, and oxygen isotopic and elemental compositions, Tagish Lake is an ungrouped member of the carbonaceous chondrite clan. The overall abundance pattern is similar to those of CM chondrites, indicating that Tagish Lake and CMs experienced very similar nebular fractionations. Bells is a CM chondrite with unusual petrologic characteristics. Bells has a mean CI‐normalized refractory lithophile element/Cr ratio of 0.96, lower than for any other CM chondrite, but shows CI‐normalized moderately volatile lithophile element/Cr ratios within the ranges of other CM chondrites, except for Na which is low. Iridium, Co, Ni and Fe abundances are like those of CM chondrites, but the moderately volatile siderophile elements, Au, As and Sb, have abundances below the ranges for CM chondrites. Abundances of the moderately volatile elements Se and Zn of Bells are within the CM ranges. Bells is best classified as an anomalous CM chondrite.     

Molecular distribution and 13C isotope composition of volatile organic compounds in the Murchison and Sutter's Mill carbonaceous chondrites

Volatile organic compounds (VOCs) are carbon-containing chemicals that may evaporate rapidly at room temperature and standard pressure. Such organic compounds can be preserved inside carbonaceous chondrite matrices. However, unlike meteoritic soluble organic matter (SOM) and insoluble organic matter (IOM), VOCs are typically lost (at least in part) during sample processing (meteorite crushing) and exposure to terrestrial atmosphere and/or solvents. Like SOM and IOM, VOCs can provide valuable insights into the chemical inventory of the meteorite parent body and even the presolar cloud from which our solar system formed, as well as the composition and processes that occurred during the early formation of our solar system and the asteroidal stage. Thus, in this work, we designed and built an instrument that allowed us to access the VOCs present in samples of the carbonaceous chondrites Murchison and Sutter's Mill after mineral disaggregation by means of freeze–thaw cycling. We simultaneously evaluated the abundances and compound-specific ¹³C-distributions of the volatiles evolving after meteorite powdering at ~20, 60, and 100°C. Carbon monoxide (CO) and methane (CH₄) were released from these meteorites as the most abundant VOCs. They were combusted together for analysis and showed positive δ¹³C values, indicative of their extraterrestrial origins. Carbon dioxide (CO₂) was also an abundant VOC in both meteorites, and its isotopic values suggest that it was mainly formed from dissolved carbonates in the samples. We also detected aldehydes, ketones, and aromatic compounds in low amounts. Contrary to Murchison, which mostly yielded VOCs with positive δ¹³C values, Sutter's Mill yielded VOCs with negative δ¹³C values. The less enriched ¹³C isotope composition of the VOCs detected in Sutter's Mill suggest that they are either terrestrial contaminants, such as VOCs in compressed gas dusters and common laboratory solvents, or compounds disconnected from interstellar sources and/or formed through parent body processing. Understanding the relative abundances and determining the molecular distributions and isotopic compositions of free meteoritic VOCs are key in assessing their extraterrestrial origins and those of chondritic SOM and IOM. Our newly developed technique will be valuable in the study of the samples brought to the Earth from carbonaceous asteroid Bennu by NASA's OSIRIS-REx mission.     

Diamond xenolith and matrix organic matter in the Sutter's Mill meteorite measured by C‐XANES

The Sutter's Mill (SM) meteorite fell in El Dorado County, California, on April 22, 2012. This meteorite is a regolith breccia composed of CM chondrite material and at least one xenolithic phase: oldhamite. The meteorite studied here, SM2 (subsample 5), was one of three meteorites collected before it rained extensively on the debris site, thus preserving the original asteroid regolith mineralogy. Two relatively large (10 μm sized) possible diamond grains were observed in SM2‐5 surrounded by fine‐grained matrix. In the present work, we analyzed a focused ion beam (FIB) milled thin section that transected a region containing these two potential diamond grains as well as the surrounding fine‐grained matrix employing carbon and nitrogen X‐ray absorption near‐edge structure (C‐XANES and N‐XANES) spectroscopy using a scanning transmission X‐ray microscope (STXM) (Beamline 5.3.2 at the Advanced Light Source, Lawrence Berkeley National Laboratory). The STXM analysis revealed that the matrix of SM2‐5 contains C‐rich grains, possibly organic nanoglobules. A single carbonate grain was also detected. The C‐XANES spectrum of the matrix is similar to that of insoluble organic matter (IOM) found in other CM chondrites. However, no significant nitrogen‐bearing functional groups were observed with N‐XANES. One of the possible diamond grains contains a Ca‐bearing inclusion that is not carbonate. C‐XANES features of the diamond‐edges suggest that the diamond might have formed by the CVD process, or in a high‐temperature and ‐pressure environment in the interior of a much larger parent body.     

The alteration history of the Jbilet Winselwan CM carbonaceous chondrite: An analog for C‐type asteroid sample return

Jbilet Winselwan is one of the largest CM carbonaceous chondrites available for study. Its light, major, and trace elemental compositions are within the range of other CM chondrites. Chondrules are surrounded by dusty rims and set within a matrix of phyllosilicates, oxides, and sulfides. Calcium‐ and aluminum‐rich inclusions (CAIs) are present at ≤1 vol% and at least one contains melilite. Jbilet Winselwan is a breccia containing diverse lithologies that experienced varying degrees of aqueous alteration. In most lithologies, the chondrules and CAIs are partially altered and the metal abundance is low (<1 vol%), consistent with petrologic subtypes 2.7–2.4 on the Rubin et al. ( 2007 ) scale. However, chondrules and CAIs in some lithologies are completely altered suggesting more extensive hydration to petrologic subtypes ≤2.3. Following hydration, some lithologies suffered thermal metamorphism at 400–500 °C. Bulk X‐ray diffraction shows that Jbilet Winselwan consists of a highly disordered and/or very fine‐grained phase (73 vol%), which we infer was originally phyllosilicates prior to dehydration during a thermal metamorphic event(s). Some aliquots of Jbilet Winselwan also show significant depletions in volatile elements such as He and Cd. The heating was probably short‐lived and caused by impacts. Jbilet Winselwan samples a mixture of hydrated and dehydrated materials from a primitive water‐rich asteroid. It may therefore be a good analog for the types of materials that will be encountered by the Hayabusa‐2 and OSIRIS‐REx asteroid sample‐return missions.     

On the origin of shocked and unshocked CM clasts in H‐chondrite regolith breccias

Abstract— CM chondrite clasts that have experienced different degrees of aqueous alteration occur in H‐chondrite and HED meteorite breccias. Many clasts are fragments of essentially unshocked CM projectiles that accreted at low relative velocities to the regoliths of these parent bodies. A few clasts were heated and dehydrated upon impact; these objects most likely accreted at higher relative velocities. We examined three clasts and explored alternative scenarios for their formation. In the first scenario, we assumed that the H and HED parent bodies had diameters of a few hundred kilometers, so that their high escape velocities would effectively prevent soft landings of small CM projectiles. This would imply that weakly shocked CM clasts formed on asteroidal fragments (family members) associated with the H and HED parent bodies. In the second scenario, we assumed that weakly shocked CM clasts were spall products ejected at low velocities from larger CM projectiles when they slammed into the H and HED parent bodies. In both cases, if most CM clasts turn out to have ancient ages (e.g., ?3.4‐4.1 Ga), a plausible source for their progenitors would be outer main belt objects, some which may have been dynamically implanted 3.9 Ga ago by the events described in the so‐called “Nice model.” On the other hand, if most CM clasts have recent ages (<200 Ma), a plausible source location for their parent body would be the inner main belt between 2.1–2.2 AU. In that case, the possible source of the CM‐clasts' progenitors' parent fragments would be the breakup ?160 Ma ago of the parent body 170 km in diameter of the Baptistina asteroid family (BAF).     

The petrography,mineralogy and origins of calcium sulphate within the Cold Bokkeveld CM carbonaceous chondrite

Abstract— A detailed scanning and transmission electron microscopy study of Cold Bokkeveld has shown that calcium sulphate is widespread, occluding ～30 μm wide matrix-cutting fractures in addition to μm-sized veins and irregular dissolution pores within calcitized chondrules, matrix calcite grains and Ca- and Al-rich inclusions (CAI). The majority of calcium sulphate crystals have fibrous habits, especially those which have grown within straight-sided fractures and veins. Mineralogically, the calcium sulphate is composed of a fine-scale mixture of hemihydrate and anhydrite which have probably formed by the dehydration of primary gypsum during sample preparation. In all contexts, calcium sulphate precipitation was the last identifiable diagenetic event in the pre-terrestrial history of Cold Bokkeveld and followed pervasive aqueous alteration of the meteorite matrix to phyllosilicates. The fibrous fracture- and vein-filling calcium sulphates are morphologically similar to a terrestrial form of gypsum termed “satinspar” and so may have formed in a similar manner by precipitation from supersaturated aqueous solutions during fracture and vein dilation. The aqueous solutions were most probably generated by melting of water ice due to internal heating and/or post-accretional impact heating of the parent body. Although impact-produced fractures and veins may have provided conduits for fluid advection, the dissolution of calcite, alteration of metal sulphides and precipitation of gypsum probably took place when aqueous solutions were more-or-less static. Despite their similarity to late-stage mineralized fractures in CI meteorites, the calcium sulphate-filled fractures in Cold Bokkeveld probably do not have any significance regarding a shared CM-CI parent body.     

The amino acid and polycyclic aromatic hydrocarbon compositions of the promptly recovered CM2 Winchcombe carbonaceous chondrite

The rapid recovery of the Winchcombe meteorite offers a valuable opportunity to study the soluble organic matter (SOM) profile in pristine carbonaceous astromaterials. Our interests in the biologically relevant molecules, amino acids—monomers of protein, and the most prevalent meteoritic organics—polycyclic aromatic hydrocarbons (PAHs) are addressed by analyzing the solvent extracts of a Winchcombe meteorite stone using gas chromatography mass spectrometry. The Winchcombe sample contains an amino acid abundance of ~1132 parts-per-billion that is about 10 times lower than other CM2 meteorites. The detection of terrestrially rare amino acids, including α-aminoisobutyric acid (AIB); isovaline; β-alanine; α-, β-, and γ-amino-n-butyric acids; and 5-aminopentanoic acid, and the racemic enantiomeric ratios (D/L = 1) observed for alanine and isovaline indicate that these amino acids are indigenous to the meteorite and not terrestrial contaminants. The presence of predominantly α-AIB and isovaline is consistent with their formation via the Strecker-cyanohydrin synthetic pathway. The L-enantiomeric excesses in isovaline previously observed for aqueously altered meteorites were viewed as an indicator of parent body aqueous processing; thus, the racemic ratio of isovaline observed for Winchcombe, alongside the overall high free:total amino acid ratio, and the low amino acid concentration suggest that the analyzed stone is derived from a lithology that has experienced brief episode(s) of aqueous alteration. Winchcombe also contains 2- to 6-ring alkylated and nonalkylated PAHs. The low total PAHs abundance (6177 ppb) and high nonalkylated:alkylated ratio are distinct from that observed for heavily aqueously altered CMs. The weak petrographic properties of Winchcombe, as well as the discrepancies observed for the Winchcombe SOM content—a low total amino acid abundance comparable to heavily altered CMs, and yet the high free:total amino acid and nonalkylated:alkylated PAH ratios are on par with the less altered CMs—suggest that Winchcombe could represent a class of weak, poorly lithified meteorite not been previously studied.     

Alteration of calcium- and aluminium-rich inclusions in the Murray (CM2) carbonaceous chondrite

Abstract— Four different types of calcium- and aluminium-rich inclusions (CAIs) have been identified in the CM2 chondrite Murray, three of which contain alteration products. Two types of altered CAIs, spinel inclusions and spinel-pyroxene inclusions, contain primary spinel (± perovskite ± hibonite ± diopside) and secondary Fe-rich serpentine phyllosilicates (± tochilinite ± calcite). Original melilite in these CAIs is inferred to have been altered during aqueous activity in the parent body and Fe-rich serpentines, tochilinite and calcite were formed in its place. The other type of altered CAI is represented by one inclusion, here called MCA-1. This CAI contains primary spinel, perovskite, fassaite and diopside with secondary calcite, paragonite, Mg-Al-Fe phyllosilicates and a Mg-Al-Fe sulphate. Importantly, MCA-1 is similar in both primary and secondary mineralogy to a small number of altered CAIs described from other CM2 meteorites including Essebi, Murchison and a CM2 clast from Plainview. Features that these CAIs have in common include an unusually large size, a CV3-like primary mineralogy and the presence of secondary aluminosilicates and calcite. The Al-rich alteration products in MCA-1 are also reminiscent of secondary minerals in refractory inclusions from CV3 meteorites, which have previously been interpreted to form by interaction of the inclusions with solar nebula gases. In common with the other types of altered CAIs in Murray, MCA-1 is inferred to have experienced its main phase of alteration in a parent body environment. The Mg-Al-Fe phyllosilicates, calcite and the Mg-Al-Fe sulphate formed following aqueous alteration of an Al-rich precursor, possibly Ca dialuminate. This episode of parent body alteration may have overprinted an earlier phase of alteration in a solar nebula environment from which only paragonite remains.     

The effects of secondary processing in the unique carbonaceous chondrite Miller Range 07687

Our detailed mineralogical, elemental, and isotopic study of the Miller Range (MIL) 07687 meteorite showed that, although this meteorite has affinities to CO chondrites, it also exhibits sufficient differences to warrant classification as an ungrouped carbonaceous chondrite. The most notable feature of MIL 07687 is the presence of two distinct matrix lithologies that result from highly localized aqueous alteration. One of these lithologies is Fe‐rich and exhibits evidence for interaction with water, including the presence of fibrous (dendritic) ferrihydrite. The other lithology, which is Fe‐poor, appears to represent relatively unaltered protolith material. MIL 07687 has presolar grain abundances consistent with those observed in other modestly altered carbonaceous chondrites: the overall abundance of O‐rich presolar grains is 137 ± 3 ppm and the overall abundance of SiC grains is 71 ± 11 ppm. However, there is a large difference in the observed O‐rich and SiC grain number densities between altered and unaltered areas, reflecting partial destruction of presolar grains (both O‐ and C‐rich grains) due to the aqueous alteration experienced by MIL 07687 under highly oxidizing conditions. Detailed coordinated NanoSIMS‐TEM analysis of a large hotspot composed of an isotopically normal core surrounded by a rim composed of ¹⁷O‐rich grains is consistent with either original condensation of the core and surrounding grains in the same parent AGB star, or with grain accretion in the ISM or solar nebula.     

Spatial distribution of organic matter in the Bells CM2 chondrite using near‐field infrared microspectroscopy

Abstract– Distributions of organic functional groups as well as inorganic features were analyzed in the Bells (CM2) carbonaceous chondrite using near‐field infrared (NFIR) spectroscopy. NFIR spectroscopy has recently been developed to enable infrared spectral mapping beyond the optical diffraction limit of conventional Fourier transform infrared microspectroscopy. NFIR spectral mapping of the Bells 300 nm thick sections on Al plates for 7.5 × 7.5 μm² areas showed some C‐H‐rich areas which were considered to represent the organic‐rich areas. Heterogeneous distributions of organic matter as well as those of inorganic phases such as silicates (Si‐O) were observed with 1 μm spatial resolution. The NFIR mappings of aliphatic C‐H (2960 and 2930 cm^?1) and structural OH (3650 cm^?1) confirm that organic matter is associated with phyllosilicates as previously suggested. The NFIR mapping method can provide 1 μm spatial distribution of organic functional groups and their association with minerals. High local sensitivity of NFIR enables us to find organic‐rich areas and to characterize them by their aliphatic CH₂/CH₃ ratios. The aliphatic CH₂/CH₃ ratio of Bells is slightly higher than Murchison, similar to Orgueil, and lower than literature values of IDPs and cometary dust particles.