Search for purines and pyrimidines in the Murchison meteorite

A 1 g interior sample of the Murchison CII meteorite was examined for the presence of purines and pyrimidines by dual-column, ion-exclusion chromatography and ultraviolet spectroscopy. Xanthine, not previously reported in meteorites, was found to be the major purine liberated by extraction with formic acid, with a concentration corresponding to 2.3 μg/g of meteorite. Guanine (0.1 ppm) and hypoxanthine (0.04 ppm) were also tentatively identified. The presence of adenine could not be confirmed. No pyrimidines were detected at concentrations higher than the background level (0.01 ppm) in water, formic acid or strong acid extracts. Silylation of the water extract, however, resulted in the appearance of 4-hydroxypyrimidine, 4-hydroxy-2-methylpyrimidine and 4-hydroxy-6-methylpyrimidine. These compounds are-though to be formed during the silylation procedure from contaminants present in the reagent.     

The deuterium enrichment of individual amino acids in carbonaceous meteorites: A case for the presolar distribution of biomolecule precursors

The δD values of over 40 amino acids and two pyridine carboxylic acids of the Murchison and Murray meteorites have been obtained by compound-specific isotopic analyses. For compounds with no known terrestrial distribution, these values range from approximately +330‰ (for cyclic leucine) to +3600‰ (for 2-amino-2,3-dimethylbutyric acid). The latter value is the highest ever recorded for a soluble organic compound in meteorites and nears deuterium to hydrogen ratios observed remotely in interstellar molecules. Deuterium content varies significantly between molecular species and is markedly higher for amino acids having a branched alkyl chain. The δD value of Murray l-isovaline, with an enantiomeric excess of ∼ 6% in the meteorite, was within experimental error of that determined for the combined dl-isovaline enantiomers. Overall, the hydrogen isotope composition of meteoritic amino acids is relatively simple and their δD values appear to vary more with the structure of their carbon chains than with the number and relative distribution of their functionalities or ¹³C content. The magnitude and extent of deuterium enrichment shared by many and varied amino acids in meteorites indicate that cosmic regimes such as those found in the interstellar medium were capable of producing, if not all the amino acids directly, at least a suite of their direct precursors that was abundant, varied, and considerably saturated.     

Basic nitrogen-heterocyclic compounds in the Murchison meteorite

A fragment of the Murchison (C2) carbonaceous meteorite was analyzed for basic, $\text{N-}\text{heterocyclic}$ compounds, by dual detector capillary gas chromatography as well as capillary gas chromatography/mass spectrometry, using two columns of different polarity. In the formic acid extract 2,4,6-trimethylpyridine, quinoline, isoquinoline, 2-methylquinoline and 4-methylquinoline were positively identified. In addition, a suite of alkylpyridines and quinolines and/or isoquinolines was tentatively identified from their mass spectra. The (iso)quinolines were found to contain methyl substituents exclusively. The distribution of the pyridines observed reveals a similarity to that observed from catalytic reactions of ammonia and simple aldehydes under conditions similar to those applied in Fischer-Tropsch type reactions.     

Carbon,hydrogen and nitrogen isotopes in solvent-extractable organic matter from carbonaceous chondrites

Solvent extractions were done on the carbonaceous chondrites Murray, Murchison, Orgueil and Renazzo, using CCl₄ and CH₃OH. Between 2 and 10% of the total carbon in these meteorites is extractable by ordinary techniques, most of it in CH₃OH. After demineralization with HF, perhaps as much as 30% of the total carbon in Murray may be extractable with CH₃OH. The extracts from Renazzo have isotopic ratios which suggest that they are mainly terrestrial organic matter, with lesser contributions from indigenous organics. The CH₃OH-soluble organic matter from Murchison and both untreated and HF-treated Murray has δ¹³C values of about +5 to + 10%. and δ¹⁵N values of about +90 to +100%., both of which are significantly higher than the bulk meteorite values. The Orgueil CH₃OH-extract also has a δ¹⁵N value well above the value in residual organic matter. Values for δD of +300 to +500%. are found for the CH₃OH-soluble organic matter. The combined data for C, H and N isotopes makes it highly unlikely that the CH₃OH-soluble components are derivable from, or simply related to, the insoluble organic polymer found in the same meteorites. A relationship is suggested between the event that formed hydrous minerals in CI1 and CM2 meteorites and the introduction of water-soluble (methanol-soluble) organic compounds. Organic matter soluble in CCl₄ has essentially no nitrogen, and δ³C and δD values are lower than for CH₃OH-soluble phases. Either there are large isotopic fractionations for carbon and hydrogen between different soluble organic phases, or the less polar components are partially of terrestrial origin.     

Hydrocarbon components in carbonaceous meteorites

Currently, the presence of free n-alkanes and isoprenoid alkanes in carbonaceous meteorites is usually explained either by microbial contamination during the period between the meteorite fall and collection or by contamination from the environment of analytical laboratories and museums. The goal of this research was to repeat analysis of hydrocarbon components in meteorites and to investigate possible meteorite contamination routes discussed in the literature. Experimental analysis of free organic constituents in five carbonaceous meteorites by infrared spectroscopy (IR) and gas chromatographic (GC) methods confirmed the presence of extractable aliphatic components, n-alkanes in the C₁₅H₃₂-C₂₇H₅₆ range and isoprenoid alkanes (phytane, pristane, and norpristane), in some of these meteorites. The contents of these compounds vary depending on the source. Insoluble organic components of two meteorites (meteorite kerogens) were isolated, and their composition was analyzed by IR and cracking/GC methods. Comparison with the data on several terrestrial contamination sources proposed in the literature shows that the presence of free saturated hydrocarbons in meteorites and the composition of the meteorite kerogen could not be explained either by microbial contamination or by contamination from the laboratory environment. The types of the hydrocarbons in meteorites resemble those typical of ancient terrestrial deposits of organic-rich sediments, except for the absence of lighter hydrocarbons, which apparently slowly evaporated in space, and multi-ring naphthenic compounds of the biologic origin, steranes, terpanes, etc. The prevailing current explanation for the presence of free linear saturated hydrocarbons in carbonaceous meteorites, apart from contamination, is the abiotic route from hydrogen and carbon monoxide. However, the data on the structure of meteorite kerogens require a search for different routes that initially produce complex polymeric structures containing n-alkyl and isoprenoid chains which are attached, via polar links (esters, salts, etc.), to a cross-linked polymer matrix. Later, the polymer slowly decomposes with the liberation of free aliphatic hydrocarbons.     

Rare-gas-rich separates from carbonaceous chondrites

Samples studied were residual, carbonaceous /Alates—a coined word to designate colloids prepared sometimes before and sometimes after acid demineralization—from Murray, Murchison, Cold Bokkeveld (type C2s) and Allende (type CV3) meteorites. Characterization: C2 /Alates, comprising 0.5% of the bulk meteorite are fine-grained (< 100 Å), amorphous, sulfide-free, oxidizable, 95% carbonaceous materials which pyrolyze bimodally at 200–700 and 800–1200°C. Allende /Alates are similar but with traces of inferred spinel and chromite and of sulfur, Rare gas results: Elemental: Release from stepwise heated Murray is bimodal with maximum release and upper temperature peak at 1000°C, probably accompanying chemical reaction. All /Alates studied had very nearly the same elemental concentrations, distinctly planetary in pattern. Isotopic: Trapped neon compositions are unprecedentedly close to Pepin's neon-A corner but nevertheless show signs of complexity, as if accompanied by neon-E. The trapped ³He/⁴He ratio is essentially constant at (1.42 ± 0.2 × 10^?4. The isotopically anomalous heavy noble gases, easily detected in the residues of oxidized /Alates, were not conspicuous in this particular study. Comparison and Chicago results: Concentrations of heavy rare gases in our /Alates agree with concentrations measured directly (as opposed to inferred by difference) in acid resistant residues at Chicago. Alone, our results support the idea of a carbonaceous gas-carrier uniformly present in meteorites of various types, but Chicago characterizations of the samples can apply to both their samples and ours provided that the right amount of gas was lost in the Berkeley procedures to make the uniform gas contents in various samples a coincidence.     

Irradiation history and atmospheric ablation of meteorites: Results from cosmic ray track studies

The dominant component of nuclear tracks observed in meteoritic minerals poor in uranium is produced by cosmic ray very heavy (vh:Z>20) nuclei. Studies of cosmic ray tracks and other cosmogenic effects in meteorites give us information on the irradiation history of these meteorites and enable us to estimate the extent of ablation during their atmospheric transit, and hence their pre-atmospheric masses. In a specific type of meteorite, known asgas-rich meteorite, one finds individual grains and xenoliths that have received solar flare and galactic cosmic ray irradiation prior to the formation of these meteorites. Detailed studies of these exotic components give insight into the accretionary processes occurring in the early history of the solar system. Some of the important results obtained from such studies and their implications to meteoritics are summarized.     

Nonracemic isovaline in the Murchison meteorite: chiral distribution and mineral association

The enantiomeric and carbon-isotopic composition of the amino acid isovaline have been analyzed in several samples of the Murchison meteorite and one sample of the Murray meteorite. l-Enantiomeric excesses of the amino acid were found to range from 0 to 15.2%, varying significantly both between meteorite stones and at short distances within a single stone. The upper limit of this range is the largest enantiomeric excess measured to date for a biologically rare meteoritic amino acid and raises doubts that circularly polarized light irradiation could have been the sole cause of amino acids chiral asymmetry in meteorites. Individual d- and l-isovaline δ¹³C values ware found to be about +18‰, with no significant differences between the two enantiomers to suggest terrestrial contamination. The amino acid relative abundance also varied between samples, with isovaline/alanine ratios of 0.5 to 6.5. X-ray diffraction analyses of contiguous meteorite fragments suggest a possible correlation between isovaline and hydrous silicates abundances.     

Purines and triazines in the Murchison meteorite

Two samples of the Murchison C2 chondrite were examined for organic nitrogen compounds, using mass spectrometry as well as paper and thin-layer chromatography. Under mild extraction conditions (water or formic acid) only aliphatic amines and C₂-C₆ alkylpyridines were seen; the latter may be contaminants. Under drastic extraction conditions (hot, 3–6 M HCl or CF₃COOH), a variety of basic nitrogen compounds appeared, in the following amounts (ppm): adenine (15), guanine (5), melamine (20), cyanuric acid (20–30), guanylurea (30–45), urea (25), etc. Apparently these compounds are present mainly in macromolecular material, and are released only upon acid hydrolysis.These findings support our earlier identifications of these compounds in the Orgueil meteorite. They also suggest that the recent failure by Folsomeet al. (Nature232, 108–109, 1971; Geochim. Cosmochim. Acta37, 455–465, 1973) to find purines or triazines in carbonaceous chondrites was due to inadequate extraction conditions: water and formic acid, rather than HCl. Conversely, we were unable to detect the principal compound class reported by Folsome et al.: 4-hydroxypyrimidines.     

Diverse nucleosynthetic components in barium isotopes of carbonaceous chondrites: Incomplete mixing of s- and r-process isotopes and extinct Cs in the early solar system

Barium isotopic compositions of chemical leachates from six carbonaceous chondrites, Orgueil (CI), Mighei (CM2), Murray (CM2), Efremovka (CV3), Kainsaz (CO3), and Karoonda (CK4), were determined using thermal ionization mass spectrometry in order to assess the chemical evolution in the early solar system.The Ba isotopic data from most of the leachates show variable ¹³⁵Ba excesses correlated with ¹³⁷Ba excesses, suggesting the presence and heterogeneity of additional nucleosynthetic components for s- and r-processes in the solar system. The isotopic deviations observed in this study were generally small (−1 < ε < +1) except in the case of the acid residues of CI and CM meteorites. Large deviations of ¹³⁵Ba (ε = −13.5 to −5.0) and ¹³⁷Ba (ε = −6.2∼−1.2) observed in the acid residues from one CI and two CM meteorites show significant evidence for the enrichment of s-process isotopes derived from presolar grains. Two models were proposed to estimate the ¹³⁵Cs isotopic abundances by subtraction of the s- and r-isotopic components from the total Ba isotopic abundances in the three CM meteorites, Mighei, Murchison (measured in a previous study), and Murray. The data points show individual linear trends between ¹³⁵Cs/¹³⁶Ba ratios and ¹³⁵Ba isotopic deviations for the three samples. Considering the different trends observed in the three CM meteorites, the Ba isotopic composition of the CM meteorite parent body was heterogeneous at its formation. Chronological information is unclear in the data for Murchison and Murray because of large analytical uncertainties imposed by error propagation. Only the Mighei meteorite data indicate the possible existence of presently extinct ¹³⁵Cs (¹³⁵Cs/¹³³Cs = (2.7 ± 1.6) × 10⁻⁴) in the early solar system. Another explanation of the data for the three CM meteorite is mixing of at least three components with different Ba isotopic compositions, although this is model-dependent.     

Presolar spinel grains from the Murray and Murchison carbonaceous chondrites

With a new type of ion microprobe, the NanoSIMS, we determined the oxygen isotopic compositions of small (<1μm) oxide grains in chemical separates from two CM2 carbonaceous meteorites, Murray and Murchison. Among 628 grains from Murray separate CF (mean diameter 0.15 μm) we discovered 15 presolar spinel and 3 presolar corundum grains, among 753 grains from Murray separate CG (mean diameter 0.45 μm) 9 presolar spinel grains, and among 473 grains from Murchison separate KIE (mean diameter 0.5 μm) 2 presolar spinel and 4 presolar corundum grains. The abundance of presolar spinel is highest (2.4%) in the smallest size fraction. The total abundance in the whole meteorite is at least 1 ppm, which makes spinel the third-most abundant presolar grain species after nanodiamonds (if indeed a significant fraction of them are presolar) and silicon carbide. The O-isotopic distribution of the spinel grains is very similar to that of presolar corundum, the only statistically significant difference being that there is a larger fraction of corundum grains with large ¹⁷O excesses (¹⁷O/¹⁶O > 1.5 × 10⁻³), which indicates parent stars with masses between 1.8 and 4.5 M_⊙.     

Total nitrogen in meteorites

Total N has been measured in a number of meteorites by neutron activation analysis using the reaction N¹⁴(n, p)C¹⁴. From each meteorite a number of chips have been analysed to investigate the variation of N contents in a sample. Many meteorites are found to contain a heterogeneous distribution of N. Eighteen chondrites, mostly of the classes C3, H4, H5, L4, L5, L6 and LL6, and six achondrites are found to have average N contents of 10–45 ppm. These do not show any clear-cut dependence of N on petrological group. However, the inherent heterogeneity or the fact that from most meteorite classes only single falls were studied might be responsible for this lack of correlation. In Cold Bokkeveld (C2) N is high (420 ppm). Unlike C, N content of ureilites is low (26 ppm). Nitrogen is enriched in the non-magnetic as compared to the magnetic fractions in H-group chondrites. Analyses of sieved Bjurböle phases show no enrichment of N in finer matrix material, nor any depletion in chondrules. In two gas-rich meteorites, Kapoeta and Assam, there is no excess N in the dark phases. Nine iron meteorites and three mesosiderites were analysed. Twenty analyses of Canyon Diablo and seven of Odessa establish a very heterogeneous N distribution in these meteorites.     

6块新回收沙漠陨石的矿物岩石学特征及类型划分

干旱沙漠地区与南极冰盖均有利于陨石样品保存。2013年4-5月,通过首次新疆哈密沙漠陨石考察,回收了陨石样品47块。文中报导其中6块样品的矿物岩石学特征,并划分它们的化学岩石类型。Arlatager004、006、0014、0022和TuzLeik001等5块样品主要由橄榄石、辉石、长石、铁镍金属和陨硫铁组成,具有典型的普通球粒陨石岩石结构特征,其球粒结构不明显,表明经历过较强的热变质,岩石类型划分为5型;根据样品中橄榄石Fa值,低钙辉石Fs值和样品的金属含量等,将Arlatager004、006、0014、0022划分为L5型普通球粒陨石;TuzLeik001划分为H5型普通球粒陨石。Kumtag005具有典型的球粒结构,结合橄榄石Fa值和低钙辉石Fs值以及岩石学特征,将其划分为L3型普通球粒陨石。根据橄榄石Fa值与其百分标准平均方差(PMD)之间的关系,将Kumtag005的岩石类型亚型划分为L 3.4。这6块样品代表了3个化学群,结合该地区回收到的其他陨石分析结果,表明新疆哈密沙漠是一个新的陨石富集区,这些沙漠陨石的发现和研究,必将极大促进中国陨石学和天体化学的发展。     

The study of chemical composition of 35 iron meteorites and its application in taxonomy

Reported in this paper are structural and compositional data as the basis for the classification of 35 iron meteorites. The Xingjiang iron meteorite, previously labelled IIIAB, is reclassified as IIIE on the basis of its lower Ga/Ni and Ge/Ni ratios, its wider and swollen kamacite bands and the ubiquitous presence of haxonite, (Fe, Ni)₂₃C. IIICD Dongling appears not to be a new meteorite, but to be paired with Nandan. Four Antarctic iron meteorites IAB Allan Hills A77250, A77263, A77289 and A77290 are classified as paired meteorites based on their similarities in structure, and the concentrations of Cr, Co, Ni, Cu, Ga, Ge, As, Sb, W, Re, Ir and Au. It is found that Cu shares certain properties with Ga and Ge, which makes it an excellent taxonomic parameter. BecauseK _Cu is near unity, Cu displays a small range of variation within most magmatic groups (less than a factor of 2.2) and, because of its high volatility, large variations can be noticed among groups.     

Formation of meteorite hydrocarbons from thermal decomposition of siderite (FeCO3)

Thermal decomposition of siderite has been proposed as a source of magnetite in martian meteorites. Laboratory experiments were conducted to evaluate the possibility that this process might also result in abiotic synthesis of organic compounds. Siderite decomposition in the presence of water vapor at 300°C generated a variety of organic products dominated by alkylated and hydroxylated aromatic compounds. The results suggest that formation of magnetite by thermal decomposition of siderite on the precursor rock of the martian meteorite ALH84001 would have been accompanied by formation of organic compounds and may represent a source of extraterrestrial organic matter in the meteorite and on Mars. The results also suggest that thermal decomposition of siderite during metamorphism could account for some of the reduced carbon observed in metasedimentary rocks from the early Earth.     

The concentration and isotopic composition of hydrogen,carbon and nitrogen in carbonaceous meteorites

Concentrations and isotopic compositions were determined for H₂, N₂ and C extracted by stepwise pyrolysis from powdered meteorites, from residues of meteorites partially dissolved with aqueous HF, and from residues of meteorites reacted with HF-HCl solutions. The meteorites treated were the carbonaceous chondrites, Orgueil, Murray, Murchison, Renazzo and Cold Bokkeveld. Data determined for whole rock samples are in approximate agreement with previously published data. Acidification of the meteorites removed the inorganic sources of H₂, so that H₂ in the HF-HCl acid residues came primarily from insoluble organic matter, which makes up 70–80% fraction of the total carbon in carbonaceous meteorites. The δD in the organic matter differs markedly from previously determined values in organic matter in meteorites. The δD values of organic matter from acid residues of C1 and C2 carbonaceous chondrites range from +650 to + 1150%. The acid residues of the Renazzo meteorite, whose total H₂ has a δD of +930‰, gave a δD value of +2500‰. Oxidation of the HF-HCl residue with H₂O₂ solution removes the high δD and the low δ¹⁵N components. The δ¹³C values range between ?10 and ?21 and δ¹⁵N values range between +40 and ?11. The δ¹⁵N of Renazzo is unusual; its values range between +150 and ?190.There is good correlation between δD and the concentration of H₂ in the acid residues, but no correlation exists between δD, δ¹³C and δ¹⁵N in them. A simple model is proposed to explain the high δD values, and the relationships between δD values and the concentration of H₂. This model depends on the irradiation of gaseous molecules facilitating reaction between ionic molecules, and indicates that an increase in the rate of polymerization and accumulation of organic matter on grains would produce an increase in the deuterium concentration in organic matter.     

Multiple parent bodies of polymict brecciated meteorites

In three brecciated meteorites, Bencubbin, Cumberland Falls and Plainview, the oxygen isotopic compositions of different rock types within each meteorite were determined to seek genetic relationships between them. In all cases the isotopic compositions are not consistent with derivation from a single parent body. There is no evidence that chondrites and achondrites could be derived from a common parent body. The chondritic inclusions in Bencubbin and Cumberland Falls cannot be identified with any of the ordinary chondritic meteorites. The carbonaceous chondritic fragments in Bencubbin are smilar to, but not identical with, C2 meteorites. The achondritic portion of Bencubbin has a very unusual isotopic composition, which, along with its close relative Weatherford, sets it in a class distinctly apart from other achondrites. Lithic fragments in brecciated meteorites provide a wider range of rock types than is represented by known macroscopic meteorites. Collisions between some meteorite parent bodies were of sufficiently low velocity that fragments of both are preserved in breccias.     

Amino acids of the Murchison meteorite: II. Five carbon acyclic primary β-, γ-, and δ-amino alkanoic acids

All ten of the possible five-carbon acyclic primary β-, γ-, and δ-amino alkanoic acids (amino position isomers of the valines) have been positively identified in hot-water extracts of the Murchison meteorite using combined gas chromatography-mass spectrometry (GC-MS) and ion exchange chromatography. With the exception of δ-aminovaleric acid, none of these amino acids has been previously reported to occur in meteorites or in any other natural material. The γ-amino acids (4-aminopentanoic acid, 4-aminc-2-meth-ylbutanoic acid, and 4-amino-3-methylbutanoic acid) are present at higher concentrations (about 5 nmol g^?1) than are the β-amino isomers (3-aminopentanoic acid, 3-amino-2-methylbutanoic acid, allo-3-amino-2-methylbutanoic acid, 3-amino-3-methylbutanoic acid, 3-amino-2-ethylpropanoic acid, and 3-amino-2,2-dimethylpropanoic acid) which are present at concentrations of 1–2 nmol g^?1. These amino acids are less abundant in the meteorite than either the corresponding α-amino acids or the four-carbon homologues. Thirty-six amino acids have now been positively identified in the Murchison meteorite, 17 of which are apparently unique to carbonaceous chondrites. The fact that the meteorite contains all possible five-carbon acyclic primary α-, β-, γ-, and δ-amino alkanoic acids is consistent with a synthetic process involving random combination of single-carbon precursors.     

云南西双版纳曼桂陨石雨调查

北京时间2018年6月1日21:43左右,在云南西双版纳景洪市的西南有火流星滑过夜空,通过目击者拍摄的视频初步判断又是一次陨石降落,这是继2017年10月4日(农历中秋节)后又一次目击到流星光顾云南省上空。6月2日有消息称在西双版纳勐海县勐遮镇的曼伦村发现了降落的陨石。随后的几天,陆续有很多媒体记者、陨石爱好者和陨石猎人赶往陨石降落地,随即,一场陨石雨呈现出来。截止目前,已经收集到的数据表明这次陨石雨的已知范围约3km×12km,从南东向北西方向以55.3°±2.5°的入射角度降落至地面,先后在曼南、曼光、曼央囡、曼潘、曼么代、曼楷龙、曼庄、曼垒、曼伦、曼迈回、曼朗、曼赛、蔓燕和曼桂村等10余个自然村发现陨石,据不完全统计,陨石数量超过1000个,质量由0.04 g至1280 g,总质量超过50 kg。该陨石雨陨石类型为L6,冲击程度为S4。     

Uranium in the silicate inclusions of stony-iron and iron meteorites

The microdistribution of U has been studied, using fission track techniques, in eleven mesosiderites, seven pallasites and four iron meteorites with silicate inclusions. When concentrated, U is usually found in phosphates: merrillite and/or chlorapatite. As in stony meteorites, the U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also exhibit variations in the same meteorite. Uranium is found to be concentrated in merrillite (0.25 to 1.43 ppm) in all the mesosiderites except Bondoc where none was observed. No U-rich phase was identified in six of the seven pallasites. In the seventh, Marjalahti, there are merrillite grains with concentrations ranging from 0.06 to 0.14 ppm. Where observed, the phosphates from silicate inclusions in the irons appear to have U concentrations similar to the mesosiderites.