The nature,origin and modification of insoluble organic matter in chondrites,the major source of Earth’s C and N

All chondrites accreted ∼3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules.Large enrichments in D and ¹⁵N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion.Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.     

On the origins of GEMS grains

From their birth as condensates in the outflows of oxygen-rich evolved stars, processing in interstellar space, and incorporation into disks around new stars, amorphous silicates predominate in most astrophysical environments. Amorphous silicates were a major building block of our Solar System and are prominent in infrared spectra of comets. Anhydrous interplanetary dust particles (IDPs) thought to derive from comets contain abundant amorphous silicates known as GEMS (glass with embedded metal and sulfides) grains. GEMS grains have been proposed to be isotopically and chemically homogenized interstellar amorphous silicate dust. We evaluated this hypothesis through coordinated chemical and isotopic analyses of GEMS grains in a suite of IDPs to constrain their origins. GEMS grains show order of magnitude variations in Mg, Fe, Ca, and S abundances. GEMS grains do not match the average element abundances inferred for ISM dust containing on average, too little Mg, Fe, and Ca, and too much S. GEMS grains have complementary compositions to the crystalline components in IDPs suggesting that they formed from the same reservoir. We did not observe any unequivocal microstructural or chemical evidence that GEMS grains experienced prolonged exposure to radiation.We identified four GEMS grains having O isotopic compositions that point to origins in red giant branch or asymptotic giant branch stars and supernovae. Based on their O isotopic compositions, we estimate that 1-6% of GEMS grains are surviving circumstellar grains. The remaining 94-99% of GEMS grains have O isotopic compositions that are indistinguishable from terrestrial materials and carbonaceous chondrites. These isotopically solar GEMS grains either formed in the Solar System or were completely homogenized in the interstellar medium (ISM). However, the chemical compositions of GEMS grains are extremely heterogeneous and seem to rule out this possibility. Based on their solar isotopic compositions and their non-solar elemental compositions we propose that most GEMS grains formed in the nebula as late-stage non-equilibrium condensates.     

The nature of molecular cloud material in interplanetary dust

Eight interplanetary dust particles (IDPs) exhibiting a wide range of H and N isotopic anomalies have been studied by transmission electron microscopy, x-ray absorption near-edge structure spectroscopy, and Fourier-transform infrared spectroscopy. These anomalies are believed to have originated during chemical reactions in a cold molecular cloud that was the precursor to the Solar System. The chemical and mineralogical studies reported here thus constitute direct studies of preserved molecular cloud materials. The H and N isotopic anomalies are hosted by different hydrocarbons that reside in the abundant carbonaceous matrix of the IDPs. Infrared measurements constrain the major deuterium (D) host in the D-enriched IDPs to thermally labile aliphatic hydrocarbon groups attached to macromolecular material. Much of the large variation observed in D/H in this suite of IDPs reflects the variable loss of this labile component during atmospheric entry heating. IDPs with elevated ¹⁵N/¹⁴N ratios contain N in the form of amine (-NH₂) functional groups that are likely attached to other molecules such as aromatic hydrocarbons. The host of the N isotopic anomalies is not as readily lost during entry heating as the D-rich material. Infrared analysis shows that while the organic matter in primitive anhydrous IDPs is similar to that observed in acid residues of primitive chondritic meteorites, the measured aromatic:aliphatic ratio is markedly lower in the IDPs.     

The origin and evolution of chondrites recorded in the elemental and isotopic compositions of their macromolecular organic matter

Extraterrestrial organic matter in meteorites potentially retains a unique record of synthesis and chemical/thermal modification by parent body, nebular and even presolar processes. In a survey of the elemental and isotopic compositions of insoluble organic matter (IOM) from 75 carbonaceous, ordinary and enstatite chondrites, we find dramatic variations within and between chondrite classes. There is no evidence that these variations correlate with the time and/or location of chondrite formation, or with any primary petrologic or bulk compositional features that are associated with nebular processes (e.g., chondrule and volatile trace element abundances). Nor is there evidence for the formation of the IOM by Fischer-Tropsch-Type synthesis in the nebula or in the parent bodies. The elemental variations are consistent with thermal maturation and/or oxidation of a common precursor. For reasons that are unclear, there are large variations in isotopic composition within and between chondrite classes that do not correlate in a simple way with elemental composition or petrologic type. Nevertheless, because of the pattern of elemental variations with petrologic type and the lack of any correlation with the primary features of the chondrite classes, at present the most likely explanation is that all IOM compositional variations are the result of parent body processing of a common precursor. If correct, the range of isotopic compositions within and between chondrite classes implies that the IOM is composed of several isotopically distinct components whose relative stability varied with parent body conditions. The most primitive IOM is found in the CR chondrites and Bells (CM2). Isotopically, the IOM from these meteorites resembles the IOM in interplanetary dust particles. Chemically, their IOM resembles the CHON particles of comet Halley. Despite the large isotopic anomalies in the IOM from these meteorites, it is uncertain whether the IOM formed in the interstellar medium or the outer Solar System, although the former is preferred here.     

Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets

Oxygen isotopes were measured in four chondritic hydrated interplanetary dust particles (IDPs) and five chondritic anhydrous IDPs including two GEMS-rich particles (Glass embedded with metal and sulfides) by a combination of high precision and high lateral resolution ion microprobe techniques.All IDPs have isotopic compositions tightly clustered around that of solar system planetary materials. Hydrated IDPs have mass-fractionated oxygen isotopic compositions similar to those of CI and CM carbonaceous chondrites, consistent with hydration of initially anhydrous protosolar dust. Anhydrous IDPs have small ¹⁶O excesses and depletions similar to those of carbonaceous chondrites, the largest ¹⁶O variations being hosted by the two GEMS-rich IDPs. Coarse-grained forsteritic olivine and enstatite in anhydrous IDPs are isotopically similar to their counterparts in comet Wild 2 and in chondrules suggesting a high temperature inner solar system origin. The small variations in the ¹⁶O content of GEMS-rich IDPs suggest that most GEMS either do not preserve a record of interstellar processes or the initial interstellar dust is not ¹⁶O-rich as expected by self-shielding models, although a larger dataset is required to verify these conclusions.Together with other chemical and mineralogical indicators, O isotopes show that the parent-bodies of carbonaceous chondrites, of chondritic IDPs, of most Antarctic micrometeorites, and comet Wild 2 belong to a single family of objects of carbonaceous chondrite chemical affinity as distinct from ordinary, enstatite, K- and R-chondrites. Comparison with astronomical observations thus suggests a chemical continuum of objects including main belt and outer solar system asteroids such as C-type, P-type and D-type asteroids, Trojans and Centaurs as well as short-period comets and other Kuiper Belt Objects.     

Nitrogen isotopic composition of macromolecular organic matter in interplanetary dust particles

Nitrogen concentrations and isotopic compositions were measured by ion microprobe scanning imaging in two interplanetary dust particles L2021 K1 and L2036 E22, in which imaging of D/H and C/H ratios has previously evidenced the presence of D-rich macromolecular organic components. High nitrogen concentrations of 10-20 wt% and δ¹⁵N values up to +400‰ are observed in these D-rich macromolecular components. The previous study of D/H and C/H ratios has revealed three different D-rich macromolecular phases. The one previously ascribed to macromolecular organic matter akin the insoluble organic matter (IOM) from carbonaceous chondrites is enriched in nitrogen by one order of magnitude compared to the carbonaceous chondrite IOM, although its isotopic composition is still similar to what is known from Renazzo (δ¹⁵N = +208‰).The correlation observed in macromolecular organic material between the D- and ¹⁵N-excesses suggests that the latter originate probably from chemical reactions typical of the cold interstellar medium. These interstellar materials preserved to some extent in IDPs are therefore macromolecular organic components with various aliphaticity and aromaticity. They are heavily N-heterosubstituted as shown by their high nitrogen concentrations >10 wt%. They have high D/H ratios >10⁻³ and δ¹⁵N values ≥ +400‰. In L2021 K1 a mixture is observed at the micron scale between interstellar and chondritic-like organic phases. This indicates that some IDPs contain organic materials processed at various heliocentric distances in a turbulent nebula. Comparison with observation in comets suggests that these molecules may be cometary macromolecules. A correlation is observed between the D/H ratios and δ¹⁵N values of macromolecular organic matter from IDPs, meteorites, the Earth and of major nebular reservoirs. This suggests that most macromolecular organic matter in the inner solar system was probably issued from interstellar precursors and further processed in the protosolar nebula.     

Circumstellar aluminum oxide and silicon carbide in interplanetary dust particles

A systematic NanoSIMS isotope imaging study of sub-micrometer phases in interplanetary dust particles (IDPs) has led to the discovery of two presolar grain types that previously were observed only in primitive meteorites. A 350 × 600 nm² Al₂O₃ grain has a large ¹⁷O enrichment and a slight ¹⁸O depletion, as well as a ²⁶Mg excess due to the decay of extinct ²⁶Al. Because of its relatively large size and prominent location within the IDP, this presolar Al₂O₃ grain is well characterized by SEM-EDX analyses. A second, much smaller presolar grain has a diameter of 150 nm and a ¹³C enrichment of more than 300%. Isotopic anomalies in C are rarely found in IDPs and the magnitude of this anomaly is unprecedented. This grain also has a ¹⁵N-rich composition and its isotopic makeup as well as its secondary ion yields identify it as a SiC grain. The discovery of presolar Al₂O₃ and SiC in IDPs seamlessly complements earlier notions of interplanetary dust particles as the most primitive extraterrestrial material currently available for laboratory analysis. Both Al₂O₃ and SiC are common presolar grain types in primitive meteorites, but they appeared conspicuously absent from the presolar grain inventory in interplanetary dust particles, which is dominated by silicate stardust. Not finding these presolar grain types in interplanetary dust would have been difficult to explain. Abundance estimates of the new presolar grain types in IDPs are hampered by limited statistics, but both Al₂O₃ and SiC are less common than presolar silicates which have been found at relatively high abundances in IDPs. The particle in which these presolar grains have been found belongs to the ‘isotopically primitive subgroup’ of IDPs, yet does not contain any presolar silicates.     

Carbonaceous components in the comet Halley dust

Cometary grains containing large amounts of carbon and/or organic matter (CHON) were discovered by in situ measurements of comet Halley dust composition during VEGA and GIOTTO flyby missions. In this paper, we report the classification of these cometary grains by means of cluster analysis, discuss the resulting compositional groups, and compare them with substances observed or hypothesized in meteorites, interplanetary dust particles, and the interstellar medium. Grains dominated by carbon and/or organic matter (CHON grains) represent approximately 22% of the total population of measured cometary dust particles. They usually contain a minor abundance of rock-forming elements as well. Grains having organic material are relatively more abundant in the vicinity of the nucleus than in the outer regions of the coma, which suggests decomposition of the organics in the coma environment. The majority of comet Halley organic particles are multicomponent mixtures of carbon phases and organic compounds. Possibly, the cometary CHON grains may be related to kerogen material of an interstellar origin in carbonaceous meteorites. Pure carbon grains, hydrocarbons and polymers of cyanopolyynes, and multi-carbon monoxides are present in cometary dust as compositionally simple and distinctive components among a variety of others. There is no clear evidence of significant presence of pure formaldehyde or HCN polymers in Halley dust particles. The diversity of types of cometary organic compounds is consistent with the interstellar dust model of comets and probably reflects differences in composition of precursor dust. Preservation of this heterogeneity among submicron particles suggests the gentle formation of cometary nucleus by aggregation of interstellar dust in the protosolar nebula without complete mixing or chemical homogenization at the submicron level.     

Isotopic anomalies in organic nanoglobules from Comet 81P/Wild 2: Comparison to Murchison nanoglobules and isotopic anomalies induced in terrestrial organics by electron irradiation

Nanoglobules are a form of organic matter found in interplanetary dust particles and primitive meteorites and are commonly associated with ¹⁵N and D isotopic anomalies that are suggestive of interstellar processes. We report the discovery of two isotopically-anomalous organic globules from the Stardust collection of particles from Comet 81P/Wild 2 and compare them with nanoglobules from the Murchison CM2 meteorite. One globule from Stardust Cometary Track 80 contains highly aromatic organic matter and a large ¹⁵N anomaly (δ¹⁵N = 1120‰). Associated, non-globular, organic matter from this track is less enriched in ¹⁵N and contains a mixture of aromatic and oxidized carbon similar to bulk insoluble organic material (IOM) from primitive meteorites. The second globule, from Cometary Track 2, contains non-aromatic organic matter with abundant nitrile (CN) and carboxyl (COOH) functional groups. It is significantly enriched in D (δD = 1000‰) but has a terrestrial ¹⁵N/¹⁴N ratio. Experiments indicate that similar D enrichments, unaccompanied by ¹⁵N fractionation, can be reproduced in the laboratory by electron irradiation of epoxy or cyanoacrylate. Thus, a terrestrial origin for this globule cannot be ruled out, and, conversely, exposure to high-energy electron irradiation in space may be an important factor in producing D anomalies in organic materials. For comparison, we report two Murchison globules: one with a large ¹⁵N enrichment and highly aromatic chemistry analogous to the Track 80 globule and the other only moderately enriched in ¹⁵N with IOM-like chemistry. The observation of organic globules in Comet 81P/Wild 2 indicates that comets likely sampled the same reservoirs of organic matter as did the chondrite parent bodies. The observed isotopic anomalies in the globules are most likely preserved signatures of low temperature (<10 K) chemistry in the interstellar medium or perhaps the outer regions of the solar nebula. In other extraterrestrial samples, D isotopic anomalies, but not those of ¹⁵N, may be explained in part by exposure to ionizing electron radiation.     

Water and astrobiology

Water is formed from two of the three most abundant elements in the universe and so is abundant in interstellar space, in our Solar System, and on Earth, where it is an essential compound for the existence of life as we know it. Water ice acts as a substrate and reactant in interstellar clouds of gas and dust, enabling the formation of organic compounds that are important precursors to life and that eventually became incorporated into comets and asteroids in the early Solar System. Laboratory experiments have allowed us to infer the reaction pathways and mechanisms by which some of these compounds are formed. In these reactions, water can act as an energy transfer medium, increasing product yields, or it can lower yields by diluting reaction centers. Water can also destroy organic compounds when water ice decomposes under ionizing radiation and the decomposition products attack the compounds; whether this happens depends critically on temperature and structure of the ice, whether crystalline or amorphous. Ice structure and temperature also largely determine its gas content. As the solar nebula collapsed, icy mantles on interstellar grains probably sublimated and then recondensed onto other grains, thus influencing the transport of energy, mass, and angular momentum in the disk. Icy grains also influenced the temperature structure of the disk because they influence mean disk opacity. Outside the “snow line” at 3–5 AU icy grains accreted to become part of comets and planetesimals that occupy the region of the outer planets, the Kuiper belt, and the Oort cloud. Water was acquired by the growing Earth by several mechanisms. Evidence from noble gas isotopes indicates that Earth achieved sufficient mass fast enough to capture an early H-rich atmosphere from the Solar nebula itself. Although the remnant of this primary atmosphere is now found only in the mantle, it may also reside in the core, which could contain most of the H on Earth (or none at all). The bulk silicate Earth contains only 500–1100 ppm H₂O, an amount small enough to explain by “wet” accretion, although most of it probably accumulated with the latter half of Earth's mass from wetter planetary embryos originating beyond 1.5 AU. Degassing on impact delivered water to Earth's surface, where it dissolved into a magma ocean, a process that likely saved it from loss during subsequent catastrophic impacts such as the Moon-forming giant impact, which resulted in >99% loss of the noble gas inventory. Although most of Earth's water probably came from meteoritic material, the depletion on Earth of Xe relative to Kr strongly suggests a role for comets. The role of water in supporting life is an essential one on Earth and probably elsewhere, given the unusual properties of water compared with other potentially abundant compounds. Its dipolarity, high boiling point and heat of vaporization and, for ice, melting temperature; its expansion on freezing; and its solvent properties make it an ideal medium for life. Life originated early on Earth, indicating an abundance of water, nutrients, precursor molecules, substrates, and appropriate physical and chemical conditions. Life adapted quickly to (and may have originated in) extreme environments, of heat, cold, dryness, saltiness, and acidity. This adaptation to extreme conditions bodes well for the prospect of finding life elsewhere in our Solar System and in planetary systems around other stars.     

Identification of isotopically primitive interplanetary dust particles: A NanoSIMS isotopic imaging study

We have carried out a comprehensive survey of the isotopic compositions (H, B, C, N, O, and S) of a suite of interplanetary dust particles (IDPs), including both cluster and individual particles. Isotopic imaging with the NanoSIMS shows the presence of numerous discrete hotspots that are strongly enriched in ¹⁵N, up to ∼1300‰. A number of the IDPs also contain larger regions with more modest enrichments in ¹⁵N, leading to average bulk N isotopic compositions that are ¹⁵N-enriched in these IDPs. Although C isotopic compositions are normal in most of the IDPs, two ¹⁵N-rich hotspots have correlated ¹³C anomalies. CN⁻/C⁻ ratios suggest that most of the ¹⁵N-rich hotspots are associated with relatively N-poor carbonaceous matter, although specific carriers have not been determined. H isotopic distributions are similar to those of N: D anomalies are present both as distinct D-rich hotspots and as larger regions with more modest enrichments. Nevertheless, H and N isotopic anomalies are not directly correlated, consistent with results from previous studies. Oxygen isotopic imaging shows the presence of abundant presolar silicate grains in some of the IDPs. The O isotopic compositions of the grains are similar to those of presolar oxide and silicate grains from primitive meteorites. Most of the silicate grains in the IDPs have isotopic ratios consistent with meteoritic Group 1 oxide grains, indicating origins in oxygen-rich red giant and asymptotic giant branch stars, but several presolar silicates exhibit the ¹⁷O and ¹⁸O enrichments of Group 4 oxide grains, whose origin is less well understood. Based on their N isotopic compositions, the IDPs studied here can be divided into two groups. One group is characterized as being “isotopically primitive” and consists of those IDPs that have anomalous bulk N isotopic compositions. These particles typically also contain numerous ¹⁵N-rich hotspots, occasional C isotopic anomalies, and abundant presolar silicate grains. In contrast, the other “isotopically normal” IDPs have normal bulk N isotopic compositions and, although some contain ¹⁵N-rich hotspots, none exhibit C isotopic anomalies and none contain presolar silicate or oxide grains. Thus, isotopically interesting IDPs can be identified and selected on the basis of their bulk N isotopic compositions for further study. However, this distinction does not appear to extend to H isotopic compositions. Although both H and N anomalies are frequently attributed to the survival of molecular cloud material in IDPs and, thus, should be more common in IDPs with anomalous bulk N compositions, D anomalies are as common in normal IDPs as they are in those characterized as isotopically primitive, based on their N isotopes.     

地外有机化合物

球粒陨石中的有机化合物起源于星际介质,是构成太阳星云的初始组分,并与其他物质一起吸积形成小行星和行星。在小行星内,有机质经历了不同程度的水蚀变和热变质作用。球粒陨石中的有机化合物尽管是非生命成因,但组成极为复杂,主要是类似于干酪根的大分子物质,以及少量可溶性有机物。大部分可溶有机分子也发现于地球生物圈,但前者可具有完全不同的H、C、N等同位素组成,这也是它们来源于地球之外的重要证据。星云中宇宙线和紫外线（UV）的辐射、小行星的热变质和水蚀变,是地外有机质演化的主要过程。球粒陨石中的有机质是地球生命起源的物质基础,是生命起源不可或缺的重要环节。同样重要的是,大量的火星探测表明,火星历史上有过满足生命存在的基本条件,而在火星陨石中还发现了一些生物活动相关的线索。未来很可能首先在火星上发现地外生命存在的证据。     

Presolar diamond, silicon carbide, and graphite in carbonaceous chondrites: implications for thermal processing in the solar nebula

We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3_ox), Axtell (CV3_ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites.After accounting for parent-body metamorphism, abundances and characteristics of presolar components still show large variations across the classes of carbonaceous chondrites. These variations correlate with the bulk compositions of the host meteorites and imply that the same thermal processing that was responsible for generating the compositional differences between the various chondrite groups also modified the initial presolar-grain assemblages. The CI chondrites and CM2 matrix have the least fractionated bulk compositions relative to the sun and the highest abundances of most types of presolar material, particularly the most fragile types, and thus are probably most representative of the material inherited from the sun's parent molecular cloud. The other classes can be understood as the products of various degrees of heating of bulk molecular cloud material in the solar nebula, removing the volatile elements and destroying the most fragile presolar components, followed by chondrule formation, metal-silicate fractionation in some cases, further nebula processing in some cases, accretion, and parent body processing. If the bulk compositions and the characteristics of the presolar-grain assemblages in various chondrite classes reflect the same processes, as seems likely, then differential condensation from a nebula of solar composition is ruled out as the mechanism for producing the chondrite classes. Presolar grains would have been destroyed if the nebula had been completely vaporized. Our analysis shows that carbonaceous chondrites reflect all stages of nebular processing and thus are no more closely related to one another than they are to ordinary and enstatite chondrites.     

Petrography and classification of NWA 7402: A new sulfide-rich unequilibrated ordinary chondrite

We classify a new chondritic find Northwest Africa (NWA) 7402. This meteorite is highly unequilibrated, and is therefore potentially significant for the study of primitive Solar System materials. Mineralogy, mineral chemistry, and modal abundances of minerals indicate that NWA 7402 is most likely an L chondrite. However, the specimen contains a higher abundance of sulfide than commonly seen in ordinary chondrites. The structural order of organic matter in the matrix and the chromium content of Fe-rich olivine grains indicate a petrologic type of 3.1. NWA 7402 largely escaped thermal metamorphism, and secondary phases formed by aqueous alteration are rare to absent. Minor planar fractures and undulatory extinction of olivine grains suggest that NWA 7402 experienced shock up to stage 2 or 3. Terrestrial weathering is heterogeneous in the specimen; much of the stone's exterior shows substantial Fe oxidation (weathering grade 2), while some parts of the interior remain relatively fresh (weathering grade 1). NWA 7402 has some unusual features that should be investigated further. The sulfide abundance is higher than reported sulfide contents for other L chondrites, and the chromium content of the olivines does not fall on the trend established for unequilibrated ordinary chondrites by Grossman and Brearley (2005).     

陨石学及天体化学研究某些新进展

陨石是来自含气体-尘粒的太阳早期星云盘凝聚和吸积的原始物质,大多数原始物质因吸积后的作用过程而改变（如月球、地球及火星样品）,但有一些却完整的保存下来（如球粒陨石或球粒陨石中的难熔包体）。这些原始的物质通常依据同位素丰度特征来识别,依据其矿物-岩石学特征和成因可将已知的陨石划分许多更小的类型。陨石学及天体化学的新近进展包括：新近识别的陨石群;发现新类型球粒陨石及行星际尘粒中发现前太阳和星云组分;利用短寿命放射性核素完善了早期太阳系年代学;洞察宇宙化学丰度、分馏作用及星云源区及通过次生母体的作用过程阐释星云和前星云的记录。本文概述了早期太阳系内从星云到陨石的演化过程。依据这些资料,对早期太阳系所经历的多种核合成的输入、瞬时加热事件与星云动力学有一些新的认识,以及认识到小星子和行星体系的演化比以前预期的更快速。     

Ion microprobe isotopic measurements of individual interplanetary dust particles

Ion microprobe measurements of D/H ratios in individual fragments of eight stratospheric dust particles give δD values ranging from ?386 to +2534‰ relative to SMOW. The δD values in five particles far exceed those in terrestrial samples and prove that the samples are interplanetary dust particles (IDPs). The hydrogen isotopic composition is heterogeneous on a scale of a few microns demonstrating that the dust is unequilibrated. Measurements of D/H ratios in conjunction with elemental and molecular ion signals in different fragments of individual IDPs show that a carbonaceous phase, not water, is the carrier of the D enrichments. Previous infrared transmission measurements have shown that IDPs fall into three main spectral classes. Particles from two of those three IR classes show large D/H ratios. Two particles studied from the third class do not. However, one of these contains solar flare tracks and is extraterrestrial. Thus, most, but not all, IDPs contain hydrogen with a non-terrestrial isotopic composition.Carbon isotopic measurements on fragments of three IDPs give ratios similar to terrestrial values and show a largely uniform isotopic composition for a given particle. Small, but significant, differences in δ¹³C of ~40‰ between particles are seen. No correlations between the hydrogen and carbon isotopic compositions are observed.The magnesium and silicon isotopic compositions of fragments of three IDPs are found to be normal within measurement errors.     

Extreme Enrichment of Tellurium in Deep-Sea Sediments

Tellurium is a sort of scattered rare element on the earth. Its concentration is very low in earth＇s crust, only 1.0 ng/g. However, it has extremely high abundance in Co-rich crusts, marine polymetallic nodules, deep-sea sediments and aerolites. To find out the origin of tellurium enrichment in deep-sea sediments, we analyzed and compared tellurium concentrations and helium isotope compositions in the magnetic parts and those in the bulk parts of deep-sea sediments. The result indicates that the helium content, 3He/4He ratio and tellurium concentration are obviously higher in the magnetic parts than those in the bulk parts. The 3He abundance varies synchronously with the tellurium concentration. 3He and Te have a distinct positive correlation with each other. It is the first time that the paper brings forward that the extreme enrichment of tellurium in deep-sea sediments, like helium isotope anomalies, probably results from the input of interplanetary dust particles （IDPs）. Similarly, the extreme enrichment of tellurium in marine polymetallic nodules and Co-rich crusts is possibly related to IDPs.     

Sicilian serpentinite xenoliths containing abiotic organics with nanodiamond clusters as key model for prebiotic processes

Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic Tethys Ocean entered as xenoliths in a Miocenic diatreme,hence brought to the surface,in the Hyblean Plateau(Sicily).Some xenoliths consist of strongly serpentinized ultramafic rocks bearing blebs of abiotic organic matter,where clusters of amorphous carbon nanoparticles,including nanodiamonds,are immersed.Such an occurrence conjures up established hypotheses that diamond surfaces are suitable catalytic platforms stimulating the assemblage of complex bio-organic molecules relevant to the emergence of life on Earth.The appearance of bio-organic molecules under primitive Earth conditions is one of the major unsolved questions on the origin of life.Here we report new micro-Raman spectra on blebs of abiotic organic matter from a selected xenolith.Diamond bands were related to hydrogenated nanocrystalline diamonds,with size of nearly 1-1.6 nm,formed from organics at low pressures and temperatures.In particular,diamond surfaces can give rise to crystalline interfacial water layers that may have played a fundamental role in the early biosphere evolution as a good medium for rapidly transporting positive charges in the form of hydrated protons.Nowadays,proton gradients in alkaline hydrothermal vents along oceanic ridges are generally viewed as key pre-biotic factors.In general,serpentinites span the entire geological record,including prebiotic times.These hydrous ultramafic rocks often display evidence of abiotic carbon species,both organic and inorganic,including nanodiamonds,being also capable to give rise to chemiosmotic processes and proton gradients necessary to the organisms,such as the"Last Universal Common Ancestor"(LUCA),in the prebiotic Earth.     

The insoluble organic matter in carbonaceous chondrites: Chemical structure,isotopic composition and origin

Carbonaceous chondrites are characterized by their enrichment in organic matter, mainly represented by insoluble organic matter (IOM), which consists of small aromatic units linked by short-branched aliphatic chains. Furthermore, IOM contains organic radicals heterogeneously distributed along with diradicaloids. These chemical features discriminate IOM from terrestrial counterparts. Isotopic compositions, especially the D/H isotopic ratio, are also distinct. IOM is highly enriched in D (D/H > 350 × 10⁻⁶), and the D/H isotopic ratio is heterogeneous. The isotopic composition is the result of interstellar-like processes that could have taken place during the first ages of the protosolar nebula. Chemical structure and isotopic composition clearly show that IOM is synthesized by an abiotic process and is subsequently affected by aqueous alteration or high-temperature metamorphism on the parent body.     

The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite

The two textural varieties of olivine-rich Allende inclusions (rimmed and unrimmed olivine aggregates) consist primarily of a porous, fine-grained mafic constituent (inclusion matrix) that differs from the opaque meteorite matrix of CV3 chondrites by being relatively depleted in sulfides, metal grains, and (perhaps) carbonaceous material. Olivine is the most abundant mineral in Allende inclusion matrix; clinopyroxene, nepheline, sodalite, and Ti-Al-pyroxene occur in lesser amounts. Olivine in unrimmed olivine aggregates (Type 1A inclusions) is ferrous and has a narrow compositional range (Fo_50–65). Olivine in rimmed olivine aggregates (Type 1B inclusions) is, on average, more magnesian, with a wider compositional range (Fo_53–96). Olivine grains in the granular rims of Type 1B inclusions are zoned, with magnesian cores (Fo_>80) and ferrous rinds (Fo_<70). Ferrous olivines (Fo_<65) in both varieties of inclusions commonly contain significant amounts of Al₂O₃ (as much as ~0.7 wt%), CaO (as much as ~0.4 wt%), and TiO₂ (as much as ~0.2 wt%), refractory elements that probably occur in submicroscopic inclusions of Ca,Al,Ti-rich glass (rather than in the olivine crystal structure). Defocussed beam analyses of Allende matrix materials demonstrate that: (1) inclusion matrix in Type 1A inclusions is more enriched in olivine and FeO than inclusion matrix in the cores of Type 1B inclusions; (2) opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix; (3) the bulk compositions of Type 1A and Type 1B inclusions overlap; and (4) excluding sulfides and metal, the bulk compositions of Allende matrix materials cluster in a complementary pattern around the bulk composition of C1 chondrites.Inclusion matrix and meteorite matrix in Allende and other CV3 chondrites are probably relatively primitive nebular material, but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition. Allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing, non-equilibrium conditions from supercooled, supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula; CV3 meteorite matrix contains, in addition, a proportion of interstellar material that was heated (but not vaporized) in the nebula. Granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula. The mineral chemistry of matrix olivine in Allende seems to have been established by three different processes: non-equilibrium vapor → solid condensation; recrystallization and partial melting in the nebula; and $\text{Fe}\text{Mg}$ equilibration (without textural homogenization) in the meteorite parent body.