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21.
Kevin D. Mckeegan Robert M. Walker Ernst Zinner 《Geochimica et cosmochimica acta》1985,49(9):1971-1987
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. 相似文献
22.
Sachiko Amari Peter Hoppe Ernst Zinner Roy S. Lewis 《Meteoritics & planetary science》1995,30(6):679-693
Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < 12C/13C < 120; 200 < 14N/15N; Si isotopes on slope 1.34 line), grains A (12C/13C < 3.5), grains B (3.5 < 12C/13C < 10), grains X (15N excesses, large 28Si excesses) and grains Y (150 < 12C/13C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements. 相似文献
23.
Ernst Zinner Sachiko Amari Brigitte Wopenka Roy S. Lewis 《Meteoritics & planetary science》1995,30(2):209-226
Abstract— One hundred forty-three carbon grains, ranging in size from 2 to 8 μm, from two chemical and physical separates from the Murchison CM2 chondrite, were analyzed by ion microprobe mass spectrometry for their C- and N-isotopic compositions. Both separates are enriched in the exotic noble gas component Ne-E(L). Ninety grains were also analyzed for their H and O contents and 118, for Si. Thirteen grains were analyzed by micro-sampling laser Raman spectroscopy. Round grains have large C-isotopic anomalies with 12C/13C ratios ranging from 7 to 4500 (terrestrial ratio = 89). Nitrogen in these grains is also anomalous but shows much smaller deviations from the terrestrial composition, 14N/15N ratios ranging from 193 to 680 (terrestrial ratio = 272). Spherulitic aggregates and non-round compact grains have normal C-isotopic ratios but 15N excesses (up to 35%). Raman spectra of the analyzed grains indicate varying degrees of crystalline disorder of graphite with estimated in-plane crystallite dimensions varying from 18 Å (highly disordered, similar to terrestrial kerogen) to ～750 Å (well-crystallized graphite). Element contents of H, O, and Si are correlated with one another, and H and O are probably present in the form of organic molecules. On the basis of morphology, the round grains fall into two groups: grains with smooth, shell-like surfaces (“onions”) and grains that appear to be dense aggregates of small scales (“cauliflowers”). “Onions” tend to have lower trace element contents, isotopically light C (12C/13C > 89) and a high degree of crystalline order, whereas “cauliflowers” have a larger spread in trace element contents and C-isotopic ratios (they range from isotopically light to heavy) but tend to have a low degree of crystalline order. However, these differences exist only on average, and no clear distinction can be made for individual grains. A few limited conclusions can be drawn about the astrophysical origin of the carbon grains of this study. The 15N excesses in spherulitic aggregates and non-round grains can be explained as the result of ion-molecule reactions in molecular clouds. The round grains, on the other hand, must have formed in stellar atmospheres (circumstellar grains). Grains with isotopically light C must have formed in stellar environments characterized by He-burning, either in the atmosphere of Wolf-Rayet stars during the WC phase or in the He-burning, 12C-rich zone of a massive star, ejected by a supernova explosion. Isotopically heavy C is produced by H-burning in the CNO cycle. Possible sources for grains with heavy C are carbon stars (AGB stars during the thermally pulsing phase) or novae, but the detailed distribution of 12C/13C ratios agree neither with the distribution observed in carbon stars nor with theoretical predictions for these two types of stellar sources. 相似文献
24.
Abstract— Primitive meteorites contain a few parts per million (ppm) of pristine interstellar grains that provide information on nuclear and chemical processes in stars. Their interstellar origin is proven by highly anomalous isotopic ratios, varying more than 1000-fold for elements such as C and N. Most grains isolated thus far are stable only under highly reducing conditions (C/O > 1), and apparently are “stardust” formed in stellar atmospheres. Microdiamonds, of median size ～ 10 Å, are most abundant (～ 400–1800 ppm) but least understood. They contain anomalous noble gases including Xe-HL, which shows the signature of the r- and p-processes and thus apparently is derived from supernovae. Silicon carbide, of grain size 0.2–10 μm and abundance ～ 6 ppm, shows the signature of the s-process and apparently comes mainly from red giant carbon (AGB) stars of 1–3 solar masses. Some grains appear to be ≥109 a older than the Solar System. Graphite spherules, of grain size 0.8–7 μm and abundance <2 ppm, contain highly anomalous C and noble gases, as well as large amounts of fossil 26Mg from the decay of extinct 26Al. They seem to come from at least three sources, probably AGB stars, novae, and Wolf-Rayet stars. 相似文献
25.
Ahmed El Goresy Ernst Zinner Catherine Caillet 《Geochimica et cosmochimica acta》2005,69(18):4535-4556
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
26.
F.J. Stadermann T.K. Croat S. Amari R.M. Walker E. Zinner 《Geochimica et cosmochimica acta》2005,69(1):177-188
Presolar graphite spherules from the Murchison low-density separate KE3 contain a large number of internal TiC crystals that range in size from 15 to 500 nm. We have studied one such graphite grain in great detail by successive analyses with SEM, ims3f SIMS, TEM and NanoSIMS. Isotopic measurements of the ‘bulk’ particle in the ims3f indicate a supernova origin for this graphite spherule. The NanoSIMS measurements of C, N, O and Ti isotopes were performed directly on TEM ultramicrotome sections of the spherule, allowing correlated studies of the isotopic and mineralogical properties of the graphite grain and its internal crystals. We found isotopic gradients in 12C/13C and 16O/18O from the core of the graphite spherule to its perimeter, with the most anomalous compositions being present in the center. These gradients may be the result of isotopic exchange with isotopically normal material, either in the laboratory or during the particle’s history. No similar isotopic gradients were found in the 16O/17O and 14N/15N ratios, which are normal within analytical uncertainty throughout the graphite spherule. Due to an unusually high O signal, internal TiC crystals were easily located during NanoSIMS imaging measurements. It was thus possible to determine isotopic compositions of several internal TiC grains independent of the surrounding graphite matrix. These TiC crystals are significantly more anomalous in their O isotopes than the graphite, with 16O/18O ratios ranging from 14 to 250 (compared to a terrestrial value of 499). Even the most centrally located TiC grains show significant variations in their O isotopic compositions from crystal to crystal. Measurement of the Ti isotopes in three TiC grains found no variations among them and no large differences between the compositions of the different crystals and the ‘bulk’ graphite spherule. However, the same three TiC crystals vary by a factor of 3 in their 16O/18O ratios. It is not clear in what form the O is associated with the TiC grains and whether it is cogenetic or the result of surface reactions on the TiC grains before they accreted onto the growing graphite spherule. The presence of 44Ca from short-lived 44Ti (t1/2 = 60y) in one of the TiC subgrains confirms the identification of this graphite spherule as a supernova condensate. 相似文献