Orbit of the Murchison meteorite

Abstract— Because the path of the Murchison meteorite fortuitously intersected the Earth's path at an angle of only a few degrees, the visual observations establish the entry velocity to be within 2 km/s of 13 km/s and determine the nature of its orbit with a precision quite unusual for visually sighted falls.     

Magnetic remanence in the Murchison meteorite

Abstract— The Murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules, various inclusions, and matrix with occasional porphyroblasts of olivine and/or pyroxene. It also contains amino acids that may have served as the necessary components for the origin of life. Magnetic analyses of Murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence. The rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids. The level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition. We support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination. When using this approach, 1–2% of the natural remanence left in terrestrial rocks with TRM and/or CRM determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral (with the exception of hematite). The same method is applied to the Murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200–2000 nT during and/or after the formation of the parent body.     

Trace-element concentrations in single circumstellar silicon carbide grains from the Murchison meteorite

Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < ¹²C/¹³C < 120; 200 < ¹⁴N/¹⁵N; Si isotopes on slope 1.34 line), grains A (¹²C/¹³C < 3.5), grains B (3.5 < ¹²C/¹³C < 10), grains X (¹⁵N excesses, large ²⁸Si excesses) and grains Y (150 < ¹²C/¹³C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements.     

Isotopic compositions of different presolar silicon carbide size fractions from the Murchison meteorite

Abstract— We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr, and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of silicon carbide (SiC), ranging from 0.38 to 3.0μm in diameter, from the Murchison CM2 carbonaceous chondrite. Ratios of ¹²C/¹³C range from 37 to 42 and ¹⁴N/¹⁵N ratios from 370 to 520, within the range of single‐grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains. Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material. Silicon isotopic ratios vary only little and, with one exception, lie to the right of the singlegrain mainstream correlation line. This might indicate a higher percentage of the minor populations Y and Z among finer grain‐size fractions. All bulk samples have large ²⁶Mg excesses attributed to the presence of short‐lived ²⁶Al at the time of grain formation. Inferred ²⁶Al/²⁷Al ratios are much larger than those measured in single larger mainstream grains. This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their ²⁶Al/²⁷Al ratio. Our Ca‐isotopic measurements, the first made on presolar SiC grains, show excesses in ⁴²Ca and ⁴³Ca, which is in general agreement with theoretical expectations for AGB stars. Calcium‐44 excesses are much larger than expected and are probably because of X grains, which have high⁴⁴Ca excesses because of the decay of short‐lived ⁴⁴Ti produced in supernova explosions. We arrive at an estimate of 0.014 for the initial ⁴⁴Ti/⁴⁸Ti ratio of the X grains, within the range obtained from previous single X grain measurements. The Ti‐isotopic ratios of the bulk samples show a V‐shaped pattern with excesses of all isotopes relative to ⁴⁸Ti. Isotopes ⁴⁶Ti, ⁴⁷Ti, and ⁵⁰Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s‐process nucleosynthesis in AGB stars. In contrast, ⁴⁹Ti does not show any excess relative to the singlegrain data; it also fails to agree with theory, which predicts much larger excesses than observed. Measured ⁵³Cr/⁵²Cr and ⁵⁷Fe/⁵⁶Fe ratios are normal within errors. The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination. We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.     

Model of molecular structure of the insoluble organic matter isolated from Murchison meteorite

Abstract– The molecular structure of the insoluble organic matter (IOM) from Murchison meteorite has been investigated by our group for several years using a large set of analytical methods including various spectroscopies (Fourier transform infrared spectroscopy, nuclear magnetic resonance, electron paramagnetic resonance, X‐ray absorption near‐edge spectroscopy), high resolution electron microscopy, and thermal (pyrolyses in the presence or not of tetramethylammonium hydroxide) and chemical (RuO₄ oxidation) degradations. Taken together, these techniques provided a wealth of qualitative and quantitative information, from which we derived 11 elemental and molecular parameters on the same IOM residue. In addition to the basic elemental composition, these parameters describe the distribution of the different types of carbon, nitrogen, and sulfur atoms as well as the size of the polyaromatic units. For this molecular structure, we therefore propose a model which fits with these 11 molecular quantitative parameters. Several cosmochemical implications are derived from this structure. Based on the fact that aromatic moieties are highly substituted and aliphatic chains highly branched, it can be anticipated that the synthesis of this IOM occurred through successive additions of single carbon units in the gas‐phase ending by a spontaneous cyclization for chain length ≥7 C. As a whole, these observations favor an organosynthesis in the solar T‐Tauri disk.     

Correlated isotopic and microstructural studies of turbostratic presolar graphites from the Murchison meteorite

Abstract— –We present data from TEM and NanoSIMS investigations of Murchison (CM2) KFC1 presolar graphites. TEM examinations of graphite ultramicrotome sections reveal varying degrees of graphite disorder, leading to distinctions between well‐graphitized onions, more turbostratic platy graphites, and the most disordered cauliflower graphites. Aside from their larger size, platy graphites are roughly similar in isotopic composition and in internal grain properties to the well‐graphitized onions. Most carbide‐containing platy graphites exhibit large s‐process element enrichments (∼200× solar Mo/Ti ratios), suggesting origins predominantly in AGB carbon stars. The C isotopic distribution of platy graphites is similar to onions, with representatives in both ¹²C‐depleted (5 < ¹²C/¹³C < 40) and ¹²C‐enriched groups (100 < ¹²C/¹³C < 350) and a pronounced gap in the 40 < ¹²C/¹³C < 75 region that contains 75% of mainstream SiCs. The large ¹²C enrichments combined with the extreme s‐process element enrichments suggest formation in an environment inhomogeneously enriched in the nucleosynthetic products of thermal pulses in AGB stars. In contrast, numerous scaly cauliflower graphites show ¹⁸O enrichments and lack s‐process‐enriched carbides, suggesting a SN origin, as was the case for many Murchison KE3 SN graphites. The more turbostratic graphites (platy and scaly) are on average larger than onions, likely resulting from formation in a gas with higher C number density. Oxygen content increases progressively with increasing degree of graphite disorder, which can stabilize these grains against further graphitization and may be a reflection of higher O/C ratios in their formation environments.     

The Murchison meteorite: Circumstances of its fall

Abstract— Visual reports of the fall of the Murchison meteorite constrain the time of arrival to within approximately one minute of 00:58 UT 28 September 1969 and indicate an azimuth close to south east. On the basis of recent orbital and radiant determinations, reference is made to the apparent similarities with the theoretical radiant for meteors associated with Comet P/Finlay and orbital similarities with Apollo asteroid 1979 VA.     

A hibonite‐corundum inclusion from Murchison: A first‐generation condensate from the solar nebula

Abstract— Through freeze‐thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ?90 times 75 μm refractory inclusion that consists of corundum and hibonite with minor perovskite. Corundum occurs as small (?10 μm), rounded grains enclosed in hibonite laths (?10 μm wide and 30–40 μm long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically‐corundum followed by hibonite followed by perovskite‐is that predicted for the first phases to form by equilibrium condensation from a solar gas for P^tot ≤5 times 10^?3 atm. In addition, the texture of the inclusion, with angular voids between subhedral hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ?40 × CI chondrites, light REE abundances ?20 × CI chondrites, and negative Eu anomalies. The chondrite‐normalized abundance patterns, especially one for a hibonite‐perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for hibonite and perovskite at 10^?3 atm and are not consistent with formation of the inclusion by closed‐system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 times 10^?3 atm, possibly with enrichments (<1000x) in CI dust relative to gas at low pressures (below 10^?4 atm). Both hibonite and corundum have δ17O ? δ18O ? ?50%, indicating formation from an 16O‐rich source. The inclusion does not contain radiogenic ²⁶Mg and apparently did not contain live ²⁶Al when it formed. If the short‐lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that ²⁶Al‐free refractory inclusions such as this one formed within the first ?6 times 10⁵ years of nebular collapse.     

Laboratory comparisons of organic materials to interstellar dust and the Murchison meteorite

Spectra of objects which lie along several lines of sight through the diffuse interstellar medium (DISM) reveal an absorption feature near 3.4 micrometers, which has been attributed to saturated aliphatic hydrocarbons on interstellar grains. The similarity of the absorption bands near 3.4 micrometers (2950 cm-1) along different lines of sight indicates that the carrier of this band lies in the diffuse dust. Several materials have been proposed as "fits" to the 3.4 micrometers feature over the years. A comparison of these identifications is presented. These comparisons illustrate the need for high resolution, high signal-to-noise observational data as a means of distinguishing between laboratory organics as matches to the interstellar material. Although any material containing hydrocarbons will produce features in the 3.4 micrometers region, the proposed "matches" to the DISM do differ in detail. These differences may help in the analyses of the chemical composition and physical processes which led to the production of the DISM organics, although ISO Observations through the 5-8 micrometers spectral region are essential for a definitive identification. A remarkable similarity between the spectrum of the diffuse dust and an organic extract from the Murchison meteorite suggests that some of the interstellar organic material may be preserved in primitive solar system bodies. The 3.4 micrometers absorption feature (in the rest frame) has recently been detected in external galaxies, indicating the widespread availability of organic material for incorporation into planetary systems.     

Selective release of D and 13C from insoluble organic matter of the Murchison meteorite by impact shock

Abstract— We performed shock‐recovery experiments on insoluble organic matter (IOM) purified from the Murchison meteorite, and determined the abundances and isotope ratios of hydrogen and carbon in the shocked IOM sample. We also performed shock experiments on type III kerogen and compared the results of these experiments with the experimental results regarding IOM. The shock selectively released D and ¹³C from the IOM, while it preferably released H and ¹²C from the kerogen. The release of these elements from IOM cannot be explained in terms of the isotope effect, whereas their release from kerogen can be explained by this effect. The selective release of heavier isotopes from IOM would be due to its structure, in which D and ¹³C‐enriched parts are present as an inhomogeneity and are weakly attached to the main network. Shock gave rise to a high release of D even at a lower degree of dehydrogenation compared with the stepwise heating of IOM. This effective release of D is probably an inherent result of shock, in which a dynamic high‐pressure and high‐temperature condition prevails. Thus, shock would effectively control the hydrogen isotope behavior of extraterrestrial organic matter during the evolution of the solar nebula.     

Diradicaloids in the insoluble organic matter from the Tagish Lake meteorite: Comparison with the Orgueil and Murchison meteorites

Abstract— The radicals in the insoluble organic matter (IOM) from the Tagish Lake meteorite were studied by electron paramagnetic resonance and compared to those existing in the Orgueil and Murchison meteorites. As in the Orgueil and Murchison meteorites, the radicals in the Tagish Lake meteorite are heterogeneously distributed and comprise a substantial amount (?42%) of species with a thermally accessible triplet state and with the same singlet‐triplet gap, ΔE ?0.1 eV, as in the Orgueil and Murchison meteorites. These species were identified as diradicaloid moieties. The existence of similar diradicaloid moieties in three different carbonaceous chondrites but not in terrestrial IOM strongly suggests that these moieties could be “fingerprints” of the extraterrestrial origin of meteoritic IOM and markers of its synthetic pathway before its inclusion into a parent body.     

Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite

Abstract— Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the ¹³C‐ and D‐enriched signature of water‐extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270–330 °C, monocarboxylic acids with carbon numbers ranging from 2 (C₂) to 5 (C₅) were detected, acetic acid (CH₃COOH; C₂) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 °C. This result indicates that the Murchison macromolecule has a potential to generate at least ?0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water‐extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ?‐27‰ (versus PDB), which is much more depleted in ¹³C than the water‐extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH₃‐)‐C is more enriched in ¹³C relative to carboxyl (‐COOH)‐C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270–330 °C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.     

Compound‐specific carbon isotope compositions of aldehydes and ketones in the Murchison meteorite

Compound‐specific carbon isotope analysis (δ¹³C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker‐cyanohydrin synthesis in the early solar system. Previous δ¹³C investigations have targeted α‐amino acid and α‐hydroxy acid Strecker products and reactant HCN; however, δ¹³C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O‐(2,3,4,5,6‐pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification, and δ¹³C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde, and acetone, were detected in the sample. δ¹³C values, ranging from ?10.0‰ to +66.4‰, were more ¹³C‐depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar ¹²C/¹³C ratios. These relatively ¹³C‐depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g., oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between δ¹³C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration‐driven chemical pathways.     

Martian meteorite Tissint records unique petrogenesis among the depleted shergottites

Tissint, a new unaltered piece of Martian volcanic materials, is the most silica‐poor and Mg‐Fe‐rich igneous rock among the “depleted” olivine‐phyric shergottites. Fe‐Mg zoning of olivine suggests equilibrium growth (<0.1 °C h^?1) in the range of Fo_80–56 and olivine overgrowth (Fo_55–18) through a process of rapid disequilibrium (~1.0–5.0 °C h^?1). The spatially extended (up to 600 μm) flat‐top Fe‐Mg profiles of olivine indicates that the early‐stage cooling rate of Tissint was slower than the other shergottites. The chemically metastable outer rim of olivine (55) consists of oscillatory phosphorus zoning at the impact‐induced melt domains and grew rapidly compared to the early to intermediate‐stage crystallization of the Tissint bulk. High‐Ca pyroxene to low‐Ca pyroxene and high‐Ca pyroxene to plagioclase ratios of Tissint are more comparable to the enriched basaltic and enriched olivine‐phyric shergottites. Dominance of augite over plagioclase induced augite to control the Ca‐buffer in the residual melt suppressing the plagioclase crystallization, which also caused a profound effect on the Al‐content in the late‐crystallized pyroxenes. Mineral chemical stability, phase‐assemblage saturation, and pressure–temperature path of evolution indicates that the parent magma entered the solidus and left the liquidus field at a depth of 40–80 km in the upper mantle. Petrogenesis of Tissint appears to be similar to LAR 06319, an enriched olivine‐phyric shergottite, during the early to intermediate stage of crystallization. A severe shock‐induced deformation resulted in remelting (10–15 vol%), recrystallization (most Fe‐rich phases), and exhumation of Tissint in a time scale of 1–8 yr. Tissint possesses some distinct characteristics, e.g., impact‐induced melting and deformation, forming phosphorus‐rich recrystallization rims of olivine, and shock‐induced melt domains without relative enrichment of LREEs compared to the bulk; and shared characteristics, e.g., modal composition and magmatic evolution with the enriched basaltic shergottites, evidently reflecting unique mantle source in comparison to the clan of the depleted members.     

Noble gases and strontium isotopes in the unique meteorite Divnoe

Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67^+0.20_–0.40), together with clear excess of ¹²⁹Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ ³He ≤ 29.0 × 10^?8; ²⁰Ne = 6.22 × 10^?8; 2.44 ≤ ³⁶Ar ≤ 5.10 × 10^?8; ¹³⁰Xe = 41.3 × 10^?12 cm³/g; 0.079 ≤ ³He/⁴He ≤ 0.193; ²⁰Ne/²²Ne = 0.860; ²¹Ne/²²Ne = 0.927; 3.47 ≤ ⁴⁰Ar/³⁶Ar ≤ 9.47; 2.22 ≤ 36Ar/³⁸Ar ≤ 3.27; ¹²⁹Xe/¹³²Xe = 1.09. The exposure age calculated from cosmogenic ³He, ²¹Ne, and ³⁸Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of ¹³⁶Xe vs. ¹²⁹Xe/¹³⁰Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the ⁴⁰Ar/³⁸Ar ratio, and the ⁴⁰Ar, ³⁸Ar, ³He, and ⁴He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.     

A unique type B inclusion from Allende with evidence for multiple stages of melting

Abstract— A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (?1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 μm long that occur at the inclusion rim. The core consists of blocky melilite grains 20–50 μm across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ?1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine‐grained (1–10 μm) spinel. The rim laths are normally zoned (Åk_30–70) inward from the rim of the inclusion with reverse zoning over the last ?200 μm to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk_5–12) occur. These grains are enclosed in strongly zoned (Åk_15–70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na₂O‐free, but the outer parts of the overgrowths are not. In the laths at the rim, Na₂O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 2–9 wt% TiO₂ + Ti₂O₃. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ?1400 °C and melts except for gehlenitic (Åk_5–12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na₂O‐free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ?1260 °C. Melilite more gehlenitic than Åk₄₀ does not melt. During cooling, Na₂O‐bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co‐crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na‐free and Na‐bearing melilite, plus a positive correlation between Na₂O and åkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass‐fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse‐grained inclusions also have features consistent with a sequence of early, high‐temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.     

High‐mass resolution molecular imaging of organic compounds on the surface of Murchison meteorite

High‐resolution mass spectrometry (HRMS) imaging by desorption electrospray ionization (DESI) coupled with Orbitrap MS using methanol (MeOH) spray was performed on a fragment of the Murchison (CM2) meteorite in this study. Homologues of C_nH_2n–1N₂⁺ (n = 7–9) and C_nH_2nNO⁺ (n = 9–14) were detected on the sample surface by the imaging. A high‐performance liquid chromatography (HPLC)/HRMS analysis of MeOH extracts from the sample surface after DESI/HRMS imaging indicated that the C_nH_2n–1N₂⁺ homologues corresponds to alkylimidazole, and that a few isomers of the C_nH_2nNO⁺ homologues present in the sample. The alkylimidazoles and C_nH_2nNO⁺ homologues displayed different spatial distributions on the surface of the Murchison fragment, indicating chromatographic separation effects during aqueous alteration. Moreover, the distribution pattern of compounds is also different among homologues. This is probably also resulting from the separation of isomers by similar chromatographic effects, or different synthetic pathways. Alkylimidazoles and the C_nH_2nNO⁺ homologues are mainly distributed in the matrix region of the Murchison by mineralogical observations, which is consistent with previous reports. Altered minerals (e.g., Fe‐oxide, Fe‐sulfide, and carbonates) occurred in this region. However, no clear relationship was found between these minerals and the organic compounds detected by DESI/HRMS imaging. Although this result might be due to scale differences between the spatial resolution of DESI/HRMS imaging and the grain size in the matrix of the Murchison, our results would indicate that alkylimidazoles and the C_nH_2nNO⁺ homologues in the Murchison fragment were mainly synthesized by different processes from hydrothermal alteration on the parent body.     

Transmission electron microscopy of a refractory inclusion from the Allende meteorite: Anatomy of a pyroxene

Abstract— A crystal of clinopyroxene from the coarse-grained refractory inclusion Egg 6 of the Allende meteorite has been studied in detail by transmission electron microscopy. The pyroxene crystal contains euhedral, dislocation-free inclusions of pure spinel MgAl₂O₄, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high-temperature anneal, are present in the crystal. Alteration products are occasionally observed at the spinel-pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape of tubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations. The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of metasomatism.     

Kinetics of organic matter degradation in the Murchison meteorite for the evaluation of parent‐body temperature history

Abstract– To evaluate kinetic parameters for thermal degradation of organic matter, in situ heating experiments of insoluble organic matter (IOM) and bulk of Murchison (CM2) meteorite were conducted under Fourier transform infrared micro‐spectroscopy combined with a heating stage. Decreases of aliphatic C–H band area under Ar flow were well fitted with Ginstling‐Brounshtein three‐dimensional diffusion model, and the rate constants for decreases of aliphatic C–H were determined. Activation energies E_a and frequency factors A obtained from these rate constants at different temperatures using the Arrhenius equation were E_a = 109 ± 3 kJ mol^?1 and A = 8.7 × 10⁴ s^?1 for IOM, and E_a = 61 ± 6 kJ mol^?1 and A = 3.8 s^?1 for bulk, respectively. Activation energy values of aliphatic C–H decrease are larger for IOM than bulk. Hence, the mineral assemblage of the Murchison meteorite might have catalytic effects for the organic matter degradation. Using obtained kinetic expressions, the time scale for metamorphism can be estimated for a given temperature with aliphatic C–H band area, or the temperature of metamorphism can be estimated for a given time scale. For example, using the obtained kinetic parameters of IOM, aliphatic C–H is lost approximately within 200 years at 100 °C and 100 Myr at 0 °C. Assuming alteration period of 7.5 Myr, alteration temperatures could be calculated to be <15 ± 12 °C. Aliphatic C–H decrease profiles in a parent body can be estimated using time–temperature history model. The kinetic expression obtained by the infrared spectral band of aliphatic C–H could be used as an alternative method to evaluate thermal processes of organic matter in carbonaceous chondrites.     

A refractory inclusion with solar oxygen isotopes and the rarity of such objects in the meteorite record

NASA's Genesis mission revealed that the Sun is enriched in ¹⁶O compared to the Earth and Mars (the Sun's Δ¹⁷O, defined as δ¹⁷O–0.52×δ¹⁸O, is –28.4 ± 3.6‰; McKeegan et al. 2011). Materials as ¹⁶O‐rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca‐Al‐rich inclusion (CAI) from a CM chondrite that is as ¹⁶O‐enriched as the Sun (Δ¹⁷O = –29.1 ± 0.7‰). This CAI also has large nucleosynthetic anomalies in ⁴⁸Ca and ⁵⁰Ti (δ‐values are –8.1 ± 3.3 and –11.7 ± 2.4‰, respectively) and shows no clear evidence for incorporation of live ²⁶Al; (²⁶Al/²⁷Al)₀ = (0.03 ± 0.11) × 10^–5. Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Δ¹⁷O value near –23.5‰. Combined with ²⁶Al‐²⁶Mg systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later‐formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Δ¹⁷O near 0‰. The distribution could be easier to explain if the common CAI value of –23.5‰, which is consistent with the Genesis value within 3σ, represented the average composition of the protoplanetary disk.