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1.
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2.
The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average 13C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest 13C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.  相似文献   

3.
In this study kamacite was experimentally grown in taenite grains of Fe-Ni-P alloys containing between 5 and 10 wt% Ni and 0 and 1.0 wt% P. Both isothermal heat treatments and non-isothermal heat treatments at cooling rates of 2 to 5°C/day were carried out. Analytical electron microscopy was used to examine the orientation and chemical composition of the kamacite and the surrounding taenite matrix. The kamacite so produced is spindle or rod shaped and has a Widmanstätten pattern orientation. The presence of heterogeneous sites such as phosphides is necessary for the nucleation of the intergranular kamacite. During kamacite growth both Ni and P partition between kamacite and taenite with chemical equilibrium at the two phase interface. The growth kinetics are limited by the diffusion of Ni in taenite. Additional diffusion experiments showed that the volume diffusion coefficient of Ni in taenite is raised by a factor of 10 at 750°C in the presence of only 0.15 wt% P.A numerical model to simulate the growth of kamacite in Fe-Ni-P alloys, based on our experimental results, was developed and applied to estimate the cooling rates of the iron meteorites. The cooling rates predicted by the new model are two orders of magnitude greater than those of previous studies. For example the cooling rates of chemical groups I, IIIAB and IVA are 400–4000°C/106years, 150–1400°C/ 106 years and 750–6000°C/106years respectively. Previous models gave 1–4°C/106 years, 1–10°C/106 years and 3–200°C/106 years. Such fast cooling rates can be interpreted to indicate that meteorite parent bodies need only be a few kilometers in diameter or that iron meteorites can be formed near the surface of larger asteroidal bodies.  相似文献   

4.
本文对南丹IIICD铁陨石的矿物学特征进行了研究,并与同为铁陨石但化学分类不同的阿根廷IAB铁陨石和西伯利亚IIB铁陨石进行了对比,重点探讨了风化作用对铁陨石矿物学特征的影响.首先用偏光显微镜、静水称重、扫描电镜观察了样品的基本矿物学特征和微形貌特征,然后用振动式样品磁强计、X射线衍射与电子探针能谱半定量测试研究了样品的磁学性质、物相和化学组成.研究结果表明,南丹铁陨石在较强的自然风化作用下,光泽变弱为土状光泽,相对密度降低;风化产生的反铁磁性物质会使陨石的磁性下降;另外,样品表面物相组成也发生较大变化,以针铁矿(FeOOH)和磁铁矿(Fe3O4)等铁的次生矿物为主;但风化壳以下的矿物物相及化学成分均未发生明显变化,以Fe、Ni为主的铁纹石、镍纹石物相存在.  相似文献   

5.
In residual materials obtained on dissolution of iron meteorites in 2M H2SO4, the ratio of190Os/184Os has been measured by radiochemical neutron activation analysis. Most residues have a normal isotopic ratio (to within ±2%). However, in some residues both positive and negative deviations in the isotopic ratio are seen. The most spectacular deviations are in the insoluble fragments (nuggets) from Sikhote Alin iron meteorite where the190Os/184Os ratio is about 50% of the normal value. The new results confirm our earlier observations that iron meteorites contain pre-solar grains.  相似文献   

6.
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.  相似文献   

7.
Morphologies, petrographic settings and carbon and nitrogen isotopic compositions of graphites in the Acapulco meteorite, the latter determined by secondary ionization mass spectrometry, are reported. Seven different graphite morphologies were recognized, the majority of which occur enclosed exclusively in kamacite. Individual graphite grains also rarely occur in the silicate matrix. Kamacite rims surrounding taenite cores of metal grains are separated from the Ni-rich metal cores by graphite veneers. These graphite veneers impeded or prevented Ni-Fe interdiffusion during cooling. In addition, matrix FeNi metal contains considerable amounts of phosphorous (≈ 700 ppm) and silicon (≈ 300 ppm) (Pack et al., 2005 in preparation) thus indicating that results of laboratory cooling experiments in the Fe-Ni binary system are inapplicable to Acapulco metals. Graphites of different morphologies display a range of carbon and nitrogen isotopic compositions, indicating a diversity of source regions before accretion in the Acapulco parent body. The isotopic compositions point to at least three isotopic reservoirs from which the graphites originated: (1) A reservoir with heavy carbon, represented by graphite in silicates (δ13C = 14.3 ± 2.4 ‰ and δ15N = −103.4 ± 10.9 ‰), (2) A reservoir with isotopically light carbon and nitrogen, characteristic for the metals. Its C- and N-isotopic compositions are probably preserved in the graphite exsolutions that are isotopically light in carbon and lightest in nitrogen (δ13C = −17 to −23 ‰ δ15N = −141 to −159 ‰). (3) A reservoir with an assumed isotopic composition (δ13C ∼ −5 ‰; δ15N ∼ −50 ‰). A detailed three-dimensional tomography in reflected light microscopy of the decorations of metal-troilite spherules in the cores of orthopyroxenes and olivines and metal-troilite veins was conducted to clarify their origin. Metal and troilite veins are present only near the fusion crust. Hence, these veins are not pristine to Acapulco parent body but resulted during passage of Acapulco in Earth’s atmosphere. A thorough search for symplectite-type silicate-troilite liquid quench textures was conducted to determine the extent of closed-system partial silicate melting in Acapulco.Metal-troilite spherules in orthopyroxenes and olivines are not randomly distributed but decorate ferromagnesian silicate restite cores, indicating that the metal-spherule decoration around restite silicates took place in a silicate partial melt. Graphite inclusions in these spherules have C- and N- isotopic compositions (δ13C = −2.9 ± 2.5 ‰ and δ15N = −101.2 ± 32 ‰) close to the average values of graphite in metals and in the silicate matrix, thus strongly suggesting that they originated from a mixture of graphite inclusions in metals and silicate matrix graphite during a closed system crystallization process subsequent to silicate-metal-sulfide partial melting. Troilite-orthopyroxene quench symplectite textures in orthopyroxene rims are clear evidence that silicate-sulfide partial melting took place in Acapulco. Due to petrographic heterogeneity on a centimeter scale, bulk REE abundances of individual samples or of individual minerals provide only limited information and the REE abundances alone are not entirely adequate to unravel the formational processes that prevailed in the acapulcoite-lodranite parent body. The present investigations demonstrate the complexity of the evolutionary stages of acapulcoites from accretion to parent body processes.  相似文献   

8.
Low molecular weight monocarboxylic acids are the most abundant water soluble organic compounds in the Murchison and many other CM type carbonaceous chondrites. In this study, we examined the monocarboxylic acids in Murchison and EET96029.20 carbonaceous meteorites using a new sample preparation and introduction technique for gas chromatograph recently developed for volatile, water-soluble organic compounds: solid phase micro-extraction (SPME). We identified more than 50 monocarboxylic acids from Murchison compared with the 18 compounds reported previously. Formic acid, a known interstellar molecule, has been fully analyzed in these carbonaceous meteorites, with its δD value suggesting an interstellar origin. We determined both carbon and hydrogen isotopic ratios of individual monocarboxylic acids in Murchison, to better define the origins and genetic relationships of these compounds. The compound-specific isotopic data reveal a large enrichment in 13C (δ13C up to + 32.5‰) and particularly D (δD up to + 2024‰). The branched acids are substantially enriched in both 13C and D relative to the straight chain acids, with those branched acids containing a quaternary carbon showing the greatest isotopic enrichment. The isotopic difference may be attributed to variations in the different synthetic regimes or terrestrial input of straight chain acids.  相似文献   

9.
The carbon isotopic composition of CO2 from fluid inclusions in granulite facies rocks has been determined. The “primary” carbonic fluid — most probably being of Upper Mantle origin — appears to have δ 13C-values around ?15%. or even lighter up to ?20%. During the late stages of retromorphosis an enrichment in the heavy carbon isotope seems to occur resulting in δ-values between ?5 and ?7%. which, on the basis of 13C/12C ratios of carbonatites, kimberlites and diamonds have been taken up till now as representative for juvenile carbon. The implications of these findings are discussed.  相似文献   

10.
Fourteen ureilites were analyzed for stable C isotopic composition using stepped combustion. The δ13C values over the temperature range 500 to 1000°C are fairly constant for any particular meteorite although there are differences between samples. The similarity in combustion temperatures of pure diamond (600–1000δC) and pure graphite (600–800°C) makes it difficult to ascertain the relative proportions of either component within each sample. However, the constant δ13C values observed over the range 500 to 1000°C strongly suggests that ureilite diamond and graphite have the same isotopic composition. This would seem to confirm that the diamond in ureilites formed from the graphite during a process, presumably an impact event, which did not fractionate C isotopes.There is a variation in C isotopic composition of graphite/diamond intergrowths among ureilites, which is not continuous—the samples fall into two groups, with δ13C values clustered around ?10%. and ?2%. PDB. These groups are also distinguishable on the basis of the Fe content of their olivines, which may reflect the existence of more than one ureilite parent body. The brecciated ureilite North Haig has a δ13C value of ?6.5%. and it is thus possible that this sample contains components from mixed parent materials.Nitrogen abundance and stable isotope measurements were made on five samples using stepped combustion analysis. Nitrogen concentrations range from 25 to 150 ppm and CN ratios are substantially less than for carbonaceous chondrites. Variation in N isotopic composition is wide and there is evidence of different ratios in diamond/graphite, silicate and metal.  相似文献   

11.
Three hundred and thirty new 13C analyses of diamonds are presented, indicating, in conjunction with earlier published work, a range of about 30%. in the carbon isotopic composition of diamonds. The frequency distribution of diamond δ13C analyses shows a very pronounced mode at ?5 to ?6%.vs PDB, a large negative skewness, and a sharp boundary at about ?1%.. Analyses of diamonds from the Premier and Dan Carl mines, South Africa, demonstrate that: (1) differences in 13C content that can be related to diamond color and shape are smaller than 1%.; (2) the mean 13C content of kimberlite carbonates is 1–2%. lower than that of associated diamonds; (3) significant differences in 13C content exist between the mean isotopic compositions of diamonds from these two pipes; (4) the variability in δ13C differs from one mine to the other.Computations were carried out evaluating the effect on the 13C content of diamonds of: (i) various precipitation processes; (ii) the abundance of the species H2, H2O, CH4, CO, CO2 and O2 in the vapor; (iii) the initial isotopic composition variability of the source carbon; (iv) variations of the carbon isotope effects resulting from changes in pressure and temperature and (v) reservoir effects (Rayleigh fractionation). Fifty-eight genetic models were investigated for compatibility with the 13C distribution in diamonds and associated carbonate. The modeling does not permit an unambiguous answer to the question whether or not a vapor participated in diamond formation, although the presence of methane during diamond formation is compatible with the carbon isotopic composition data, possible oxygen fugacities in the mantle and with the composition of gases liberated from diamonds. In all probability carbon isotope effects in the diamond formation process were small, and the very large range in δ13C observed was inherited from the source carbon.  相似文献   

12.
Analyses were made of samples of the several classes of iron meteorites: (hexahedrites, octahedrites, ataxites, and troilite inclusions) in further study of the isotopic composition of primordial lead and toward establishing correlation between the distribution of lead among the mineral inclusions and the nickel-iron mass of the meteorite. Two groups of iron meteorites can be distinguished on the basis of isotopic composition lead suggesting two ages for the parent bodies of common iron meteorites. The distribution of lead in iron meteorites ranges markedly but no relation could be found between isotopic composition of lead and the several structures and compositions. The content of lead in troilites are one or two orders of magnitude higher than in the nickel-iron phase.-- M. Russell.  相似文献   

13.
Very precise silver (Ag) isotopic compositions have been determined for a number of terrestrial rocks, and high and low Pd/Ag meteorites by utilizing multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The meteorites include primitive chondrites, the Group IAB iron meteorites Canyon Diablo and Toluca, and the Group IIIAB iron meteorite Grant. Silver isotopic measurements are primarily of interest because 107Ag was produced by decay of the short-lived radionuclide 107Pd during the formation of the solar system and hence the Pd-Ag chronometer has set constraints on the timing of early planetesimal formation. A 2σ precision of ±0.05‰ can be obtained for analyses of standard solutions when Ag isotopic ratios are normalized to Pd, to correct for instrumental mass discrimination, and to bracketing standards. Caution must be exercised when making Ag isotopic measurements because isotopic artifacts can be generated in the laboratory and during mass spectrometry. The external reproducibility for geological samples based on replicate analyses of rocks is ±0.2‰ (2σ).All chondrites analyzed have similar Ag isotopic compositions that do not differ significantly (>0.3‰) from the ‘terrestrial’ value of the NIST SRM 978a Ag isotope standard. Hence, they show no evidence of excess 107Ag derived from 107Pd decay or, of stable Ag isotope fractionation associated with volatile element depletion within the accretion disk or from parent body metamorphism. The Group IAB iron meteorite samples analyzed show evidence of complex behavior and disturbance of Ag isotope systematics. Therefore, care must be taken when using this group of iron meteorites to obtain chronological information based on the Pd-Ag decay scheme.  相似文献   

14.
Stability of the isotopic composition of carbon in endogenic terrestrial substances, as well as in meteorites, consistently lower in C13 than the biogenic marine carbonates, suggests both presence and stability of a certain zone under the crust of the earth in which the C systems are maintained at certain equilibria, at different levels, typical of certain geological processes operative therein. Isotopic exchanges and recurrent fractionations of the C isotopes, in the course of the migrations of carbon, are indicated by the available evidence, the net result of which is an impoverishment of C13 in living substance and in its derivatives (oils, coals, etc.), and its enrichment in the biogenic residual carbonates. – IGR Staff.  相似文献   

15.
Stromatolites forming today on a small scale in hydrothermal environments are chemical and biological analogues of much larger Precambrian formations. Carbon isotopic composition varied as a function of CO2 concentration, pH, and species composition. Stratiform, layered stromatolites grew in silica-depositing springs at 55° to 70°C; they consisted mainly of a unicellular alga, Synechococcus, and a filamentous, photosynthetic bacterium, Chloroflexus. These thermophiles become enriched in 12C as the concentration of carbon dioxide in the effluent waters increases. At a concentration of 40 ppm total inorganic C, and δ13C of organic carbon was ～ ?12%., whereas at 900 ppm total inorganic C, the δ13C of similar species was ～ ?25%.. Conical stromatolites or conophytons (principally a filamentous, blue-green alga Phormidium and Chloroflexus) grew at 40°-55°C. In older, broader conophytons, Chloroflexus was the dominant organism. Their δ13C values were ～ ?18%. in a variety of hot springs. In carbonate-depositing springs, i.e., carbon dioxide saturated, conophytons and stromatolites consisting of a variety of blue-green algae and photosynthetic bacteria had the most negative δ13C values (to ?30%.). These carbon isotope ratios are directly comparable to carbon isotope ratios of kerogen from Precambrian stromatolites. The presence and activity of methanogenic bacteria or heterotrophic, aerobic and anaerobic bacteria did not alter significantly the δ13C of the original organic matter.The hydrogen isotopic fractionation between thermophilic organisms and water is 0 to ?74 for temperatures of 85° to 46°C, respectively. Acidophilic algae fractionated hydrogen isotopes to a lesser extent than did the photosynthetic organisms inhabiting neutral pH springs. Because organic matter retains some of its original isotopic signature, relationships of CO2 levels, pH, temperature, and species composition between modern stromatolites and their environment and those of the Precambrian can be inferred.  相似文献   

16.
The first data are reported on the carbon isotopic composition of diamond crystals from the Grib pipe kimberlite deposit of the Archangelsk diamond province (ADP). The δ13C value of the crystals ranges from ?2.79 to ?9.61‰. The isotopic composition of carbon was determined in three zoned crystals (δ13C of ?5.8 ?6.96 ‰, ?5.64/ ?5.85 ‰, and ?5.94/ ?5.69 ‰), two “diamond in diamond” samples (diamond inclusion with δ13C of ?4.05 and ?6.34 ‰ in host diamond crystals with δ13C of ?8.05 and ?7.54 ‰, respectively), and two samples of coated diamonds (cores with δ13C of ?6.98 and ?6.78‰ and coats with δ13C of ?7.51 and ?8.01 ‰, respectively). δ13C values were obtained for individual diamond crystals from bort-type aggregates (δ13C of ?4.24/ ?4.05 ‰, ?6.58/ ?7.48 ‰, and ?5.48/ ?6.08 ‰). Correlations were examined between the carbon isotopic composition of diamonds and their crystal morphology; the color; the concentration of nitrogen, hydrogen, and platelet defects; and mineral inclusions content. It was supposed that the observed δ13C variations in the crystals are most likely related to the fractionation of carbon isotopes rather than to the heterogeneity of carbon sources involved in diamond formation. The isotopic characteristics of diamonds from the Grib pipe were compared with those of previously investigated diamonds from the Lomonosov deposit. It was found that diamonds from these relatively closely spaced kimberlite fields are different; this also indicates the existence of spatially localized peculiarities of isotope fractionation in processes accompanying diamond formation.  相似文献   

17.
Bulk carbonate samples of hemipelagic limestone–marl alternations from the Middle and Upper Triassic of Italy are analysed for their isotopic compositions. Middle Triassic samples are representative of the Livinallongo Formation of the Dolomites, while Upper Triassic hemipelagites were sampled in the Pignola 2 section, within the Calcari con Selce Formation of the Southern Apennines in Southern Italy. Triassic hemipelagites occur either as nodular limestones with chert nodules or as plane‐bedded limestone–marl alternations which are locally silicified. In the Middle Triassic Livinallongo Formation, diagenetic alteration primarily affected the stable isotopic composition of sediment surrounding carbonate nodules, whereas the latter show almost pristine compositions. Diagenesis lowered the carbon and oxygen isotope values of bulk carbonate and introduced a strong correlation between δ13C and δ18O values. In the Middle Triassic successions of the Dolomites, bulk carbonate of nodular limestone facies is most commonly unaltered, whereas carbonate of the plane‐bedded facies is uniformly affected by diagenetic alteration. In contrast to carbonate nodules, plane‐bedded facies often show compaction features. Although both types of pelagic carbonate rocks show very similar petrographic characteristics, scanning electron microscopy studies reveal that nodular limestone consists of micrite (< 5 μm in diameter), whereas samples of the plane‐bedded facies are composed of calcite crystals ca 10 μm in size showing pitted, polished surfaces. These observations suggest that nodular and plane‐bedded facies underwent different diagenetic pathways determined by the prevailing mineralogy of the precursor sediment, i.e. probably high‐Mg calcite in the nodular facies and aragonite in the case of the plane‐bedded facies. Similar to Middle Triassic nodular facies, Upper Triassic nodular limestones of the Lagonegro Basin are also characterized by uncorrelated δ13C and δ18O values and exhibit small, less than 5 μm size, crystals. The alternation of calcitic and aragonitic precursors in the Middle Triassic of the Dolomites is thought to mirror rapid changes in the type of carbonate production of adjacent platforms. Bioturbation and dissolution of metastable carbonate grains played a key role during early lithification of nodular limestone beds, whereby early stabilization recorded the carbon isotopic composition of sea water. The bulk carbonate δ13C values of Middle and Upper Triassic hemipelagites from Italy agree with those of Tethyan low‐Mg calcite shells of articulate brachiopods, confirming that Triassic hemipelagites retained the primary carbon isotopic composition of the bottom sea water. A trend of increasing δ13C from the Late Anisian to the Early Carnian, partly seen in the data set presented here, is also recognized in successions from tropical palaeolatitudes elsewhere. The carbon isotopic composition of Middle and Upper Triassic nodular hemipelagic limestones can thus be used for chemostratigraphic correlation and palaeoenvironmental studies.  相似文献   

18.
The abundance of C in carbonaceous and ordinary chondrites decreases exponentially with increasing shock pressure as inferred from the petrologic shock classification of Scott et al. [Scott, E.R.D., Keil, K., Stoffler, D., 1992. Shock metamorphism of carbonaceous chondrites. Geochim. Cosmochim. Acta 56, 4281-4293] and Stoffler et al. [Stoffler, D., Keil, K., Scott, E.R.D., 1991. Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845-3867]. This confirms the experimental results of Tyburczy et al. [Tyburczy, J.A., Frisch, B., Ahrens, T.J., 1986. Shock-induced volatile loss from a carbonaceous chondrite: implications for planetary accretion. Earth Planet. Sci. Lett. 80, 201-207] on shock-induced devolatization of the Murchison meteorite showing that carbonaceous chondrites appear to be completely devolatilized at impact velocities greater than 2 km s-1. Both of these results suggest that C incorporation would have been most efficient in the early stages of accretion, and that the primordial C content of the Earth was between 10(24) and 10(25) g C (1-10% efficiency of incorporation). This estimate agrees well with the value of 3-7 x 10(24) g C based on the atmospheric abundance of 36Ar and the chondritic C/36Ar (Marty and Jambon, 1987). Several observations suggest that C likely was incorporated into the Earth's core during accretion. (1) Graphite and carbides are commonly present in iron meteorites, and those iron meteorites with Widmanstatten patterns reflecting the slowest cooling rates (mostly Group I and IIIb) contain the highest C abundances. The C abundance-cooling rate correlation is consistent with dissolution of C into Fe-Ni liquids that segregated to form the cores of the iron meteorite parent bodies. (2) The carbon isotopic composition of graphite in iron meteorites exhibits a uniform value of -5% [Deines, P., Wickman, F.E. 1973. The isotopic composition of 'graphitic' carbon from iron meteorites and some remarks on the troilitic sulfur of iron meteorites. Geochim. Cosmochim. Acta 37, 1295-1319; Deines, P., Wickman, F.E., 1975. A contribution to the stable carbon isotope geochemistry of iron meteorites. Geochim. Cosmochim. Acta 39, 547-557] identical to the mode in the distribution found in diamonds, carbonatites and oceanic basalts [Mattey, D.P., 1987. Carbon isotopes in the mantle. Terra Cognita 7, 31-37]. (3) The room pressure solubility of C in molten iron is 4.3 wt% C. Phase equilibria confirm that the Fe-C eutectic persists to 12 GPa, and thermochemical calculations for the Fe-C-S system by Wood [Wood, B.J., 1993. Carbon in the core. Earth Planet. Sci. Lett. 117, 593-607] predict that C is soluble in Fe liquids at core pressures. The abundance of 36Ar in chondrites decreases exponentially with increasing shock pressure as observed for C. It is well known that noble gases are positively correlated and physically associated with C in meteorites [e.g. Otting, W., Zahringer J., 1967. Total carbon content and primordial rare gases in chondrites. Geochim. Cosmochim. Acta 31, 1949-1960; Reynolds, J.H., Frick, U., Niel, J.M., Phinney, D.L., 1978. Rare-gas-rich separates from carbonaceous chondrites. Geochim. Cosmochim. Acta, 42, 1775-1797]. This suggests a mechanism by which primordial He and other noble gases may have incorporated into the Earth during accretion. The abundance of He in the primordial Earth required to sustain the modern He flux for 4 Ga (assuming a planetary 3 He/4 He; Reynolds et al. [Reynolds, J.H., Frick, U., Niel, J.M., Phinney, D.L., 1978. Rare-gas-rich separates from carbonaceous chondrites. Geochim. Cosmochim. Acta 42, 1775-1797] is calculated to be > or = 10(-8) cm3 g-1. This minimum estimate is consistent with a 1-10% efficiency of noble gas retention during accretion and the observed abundance of He in carbonaceous chondrites (10(-5) to 10(-4) cm3 g-1 excluding spallogenic contributions).  相似文献   

19.
The oxygen (δ18O, δ17O) and carbon (δ13C, FMOD14C-the fraction of modern 14C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ13C and FMOD14C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO2 that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.  相似文献   

20.
New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (?5.1%., s = ±l.4%.vsPDB) is indistinguishable from that of 13 kimberlite pipes (?4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (?5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O18 by several permil with respect to those of carbonates from the subvolcanic type of carbonatite.The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC13 values from ?5.8 to ?7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (?5.8 to ?7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C13 content (?2.4 to ?4.4%.).If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as ?5.2%. and the range is ?2 to ?8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.  相似文献   

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