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1.
Abstract— The CR group of carbonaceous chondrites may represent some of the most primitive extraterrestrial materials available for analysis. However, in contrast to other chondrite groups, the CR organic fraction is poorly characterized. The carbonaceous chondrite literature shows that relatively anhydrous thermal processing results in a condensed, poorly alkylated, O‐poor macromolecular material, while for aqueous processing the converse is true. Such characteristics can be used to discern the alteration histories of the carbonaceous chondrites. We have performed bulk elemental and isotopic analysis and flash pyrolysis on four CR chondrites (Renazzo, Al Rais, Elephant Moraine [EET] 87770, and Yamato [Y‐] 790112) to determine the nature of their organic component. Renazzo, Al Rais, and Y‐790112 release qualitatively similar pyrolysis products, although there are some variations. Al Rais' macromolecular structure contains substantially higher relative abundances of alkylated and oxidized species and relatively lighter δ15N, suggesting that it has endured more extensive aqueous processing than the other CR chondrites. Renazzo appears relatively unprocessed, with a low degree of alkylation, a lack of detectable nitrogen‐bearing components, and low methylnaphthalene ratio. EET 87770's low abundance of alkylated species suggests its macromolecular structure may be relatively condensed, with condensation potentially assisted by a period of mild thermal alteration.  相似文献   

2.
The water‐soluble organic compounds in carbonaceous chondrite meteorites constitute a record of the synthetic reactions occurring at the birth of the solar system and those taking place during parent body alteration and may have been important for the later origins and development of life on Earth. In this present work, we have developed a novel methodology for the simultaneous analysis of the molecular distribution, compound‐specific δ13C, and enantiomeric compositions of aliphatic monocarboxylic acids (MCA) extracted from the hot‐water extracts of 16 carbonaceous chondrites from CM, CR, CO, CV, and CK groups. We observed high concentrations of meteoritic MCAs, with total carbon weight percentages which in some cases approached those of carbonates and insoluble organic matter. Moreover, we found that the concentration of MCAs in CR chondrites is higher than in the other meteorite groups, with acetic acid exhibiting the highest concentration in all samples. The abundance of MCAs decreased with increasing molecular weight and with increasing aqueous and/or thermal alteration experienced by the meteorite sample. The δ13C isotopic values of MCAs ranged from ?52 to +27‰, and aside from an inverse relationship between δ13C value and carbon straight‐chain length for C3–C6 MCAs in Murchison, the 13C‐isotopic values did not correlate with the number of carbon atoms per molecule. We also observed racemic compositions of 2‐methylbutanoic acid in CM and CR chondrites. We used this novel analytical protocol and collective data to shed new light on the prebiotic origins of chondritic MCAs.  相似文献   

3.
Abstract— Isotopic compositions and abundances of boron were measured in sixteen chondrules from seven chondrites by ion microprobe mass spectrometry. The chondrules are of the porphyritic, barred, and radial type and host meteorites include carbonaceous, ordinary, and enstatite chondrites. Boron abundances are generally low with average boron concentrations of between 80 and 500 ppb. These abundances are lower than those of bulk chondrites (0.35 to 1.2 ppm; Zhai et al., 1996), confirming earlier suggestions that boron is mostly contained in the matrix. No significant variation in the 11B/10B ratio is observed among these chondrules, outside our experimental error limits of several permil, and B‐isotopic compositions agree with those reported for bulk chondrites. The lack of a significant isotope fractionation between chondrules and matrix implies that the low boron abundances are not the result of a Rayleigh fractionation during chondrule formation. Isotopic heterogeneities within individual chondrules are constrained to be < ±20%0 at > 95% confidence level at a spatial scale of 20–30 μm, significantly lower than the value of about ±40%0 previously reported for chondrules from carbonaceous and ordinary chondrites (Chaussidon and Robert, 1995, 1998). The observed B‐isotopic homogeneity does not conflict with the presence of decay products from extinct 10Be, with (10Be/9Be)0 ? 10?3, as was inferred for calcium‐aluminum‐rich inclusions. Extinct 10Be in chondrules would shift the abundance ratio 11B/10B at best by several permil because of their commonly observed low Be/B ratios (<2). The results show that potential B‐isotopic heterogeneities in the solar nebula due to the presence of components with different B‐isotopic signatures, such as boron produced by high‐energy galactic cosmic rays (11B/10B ? 2.5), or by the hypothetical low‐energy particle irradiation (11B/10B ? 3.5–11) or boron from type II supernovae (11B/10B >> 1), did not survive the chondrule formation processes to a measurable extent.  相似文献   

4.
Abstract— We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high‐resolution stepped‐combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), Elephant Moraine (EET) 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (?1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole‐rock carbon isotopic composition (δ13C ? +24.4%o) of the chip was dominated by 13C‐enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; δ13C ? +67%o and δ18O ? +35%o, in the proportions ?4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (?2.6 wt%, δ13C ? ?9%o; 1033 ppm N, δ15N ? +77%o), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further ?18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (approximately 3650–4330 ppm) than other carbonaceous chondrites, along with ?8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.  相似文献   

5.
We identified 66 chromite grains from 42 of ~5000 micrometeorites collected from Indian Ocean deep‐sea sediments and the South Pole water well. To determine the chromite grains precursors and their contribution to the micrometeorite flux, we combined quantitative electron microprobe analyses and oxygen isotopic analyses by high‐resolution secondary ion mass spectrometry. Micrometeorite chromite grains show variable O isotopic compositions with δ18O values ranging from ?0.8 to 6.0‰, δ17O values from 0.3 to 3.6‰, and Δ17O values from ?0.9 to 1.6‰, most of them being similar to those of chromites from ordinary chondrites. The oxygen isotopic compositions of olivine, considered as a proxy of chromite in chromite‐bearing micrometeorites where chromite is too small to be measured in ion microprobe have Δ17O values suggesting a principal relationship to ordinary chondrites with some having carbonaceous chondrite precursors. Furthermore, the chemical compositions of chromites in micrometeorites are close to those reported for ordinary chondrite chromites, but some contribution from carbonaceous chondrites cannot be ruled out. Consequently, carbonaceous chondrites cannot be a major contributor of chromite‐bearing micrometeorites. Based on their oxygen isotopic and elemental compositions, we thus conclude with no ambiguity that chromite‐bearing micrometeorites are largely related to fragments of ordinary chondrites with a small fraction from carbonaceous chondrites, unlike other micrometeorites deriving largely from carbonaceous chondrites.  相似文献   

6.
Our detailed mineralogical, elemental, and isotopic study of the Miller Range (MIL) 07687 meteorite showed that, although this meteorite has affinities to CO chondrites, it also exhibits sufficient differences to warrant classification as an ungrouped carbonaceous chondrite. The most notable feature of MIL 07687 is the presence of two distinct matrix lithologies that result from highly localized aqueous alteration. One of these lithologies is Fe‐rich and exhibits evidence for interaction with water, including the presence of fibrous (dendritic) ferrihydrite. The other lithology, which is Fe‐poor, appears to represent relatively unaltered protolith material. MIL 07687 has presolar grain abundances consistent with those observed in other modestly altered carbonaceous chondrites: the overall abundance of O‐rich presolar grains is 137 ± 3 ppm and the overall abundance of SiC grains is 71 ± 11 ppm. However, there is a large difference in the observed O‐rich and SiC grain number densities between altered and unaltered areas, reflecting partial destruction of presolar grains (both O‐ and C‐rich grains) due to the aqueous alteration experienced by MIL 07687 under highly oxidizing conditions. Detailed coordinated NanoSIMS‐TEM analysis of a large hotspot composed of an isotopically normal core surrounded by a rim composed of 17O‐rich grains is consistent with either original condensation of the core and surrounding grains in the same parent AGB star, or with grain accretion in the ISM or solar nebula.  相似文献   

7.
Abstract— Ar‐rich noble gases, the so‐called “subsolar” noble gases, are a major component of heavy primordial noble gases in unequilibrated ordinary chondrites and some classes of anhydrous carbonaceous chondrites, whereas they are almost absent in hydrous carbonaceous chondrites that suffered extensive aqueous alteration. To understand the effects of aqueous alteration on the abundance of Ar‐rich noble gases, we performed an aqueous alteration experiments on the Ningqiang type 3 carbonaceous chondrite that consists entirely of anhydrous minerals and contains Ar‐rich noble gases. Powdered samples and deionized neutral water were kept at 200 °C for 10 and 20 days, respectively. Mineralogical analyses show that, during the 10‐day alteration, serpentine and hematite formed at the expense of olivine, low‐Ca pyroxene, and sulfide. Noble gas analyses show that the 10‐day alteration of natural Ningqiang removed 79% of the primordial 36Ar, 68% of the 84Kr, and 60% of the 132Xe, but only 45% of the 4He and 53% of the primordial 20Ne. Calculated elemental ratios of the noble gases removed during the 10‐day alteration are in the range of those of Ar‐rich noble gases. These results indicate that Ar‐rich noble gases are located in materials that are very susceptible to aqueous alteration. In contrast, heavy primordial noble gases remaining in the altered samples are close to Q gas in elemental and isotope compositions. This indicates that phase Q is much more resistant to aqueous alteration than the host phases of Ar‐rich noble gases. In the 20‐day sample, the mineralogical and noble gas signatures are basically similar to those of the 10‐day sample, indicating that the loss of Ar‐rich noble gases was completed within the 10‐day alteration. Our results suggest that almost all of the Ar‐rich noble gases were lost from primitive asteroids during early, low‐temperature aqueous alteration.  相似文献   

8.
Abstract– The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine‐grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar δ15N and δ13C; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium‐heterogeneity and having mean H‐ and N‐isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer‐sized areas highly enriched in 15N (δ15N up to 1600‰). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.  相似文献   

9.
Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CBb and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15N (δ15N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CBb chondrites, ～20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CBb and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic 38Ar is ～34 Ma, similar to that of the Bencubbin (CBa) meteorite.  相似文献   

10.
Abstract– An IDP nicknamed Andric, from a stratospheric dust collector targeted to collect dust from comet 55P/Tempel‐Tuttle, contains five distinct presolar silicate and/or oxide grains in 14 ultramicrotome slices analyzed, for an estimated abundance of approximately 700 ppm in this IDP. Three of the grains are 17O‐enriched and probably formed in low‐mass red giant or asymptotic giant branch (AGB) stars; the other two grains exhibit 18O enrichments and may have a supernova origin. Carbon and N isotopic analyses show that Andric also exhibits significant variations in its N isotopic composition, with numerous discrete 15N‐rich hotspots and more diffuse regions that are also isotopically anomalous. Three 15N‐rich hotspots also have statistically significant 13C enrichments. Auger elemental analysis shows that these isotopically anomalous areas consist largely of carbonaceous matter and that the anomalies may be hosted by a variety of components. In addition, there is evidence for dilution of the isotopically heavy components with an isotopically normal endmember; this may have occurred either as a result of extraterrestrial alteration or during atmospheric entry. Isotopically primitive IDPs such as Andric share many characteristics with primitive meteorites such as the CR chondrites, which also contain isotopically anomalous carbonaceous matter and abundant presolar silicate and oxide grains. Although comets are one likely source for the origin of primitive IDPs, the presence of similar characteristics in meteorites thought to come from the asteroid belt suggests that other origins are also possible. Indeed the distinction between cometary and asteroidal sources is somewhat blurred by recent observations of icy comet‐like planetesimals in the outer asteroid belt.  相似文献   

11.
To examine the iron (Fe) isotopic heterogeneities of CI and ordinary chondrites, we have analyzed several large chips (approximately 1 g) from three CI chondrites and three ordinary chondrites (LL5, L5, and H5). The Fe isotope compositions of five different samples of Orgueil, one from Ivuna and one from Alais (CI chondrites), are highly homogeneous. This new dataset provides a δ56Fe average of 0.02 ± 0.04‰ (2SE, n = 7), which represents the best available value for the Fe isotopic composition of CI chondrites and probably the best estimate of the bulk solar system. We conclude that the homogeneity of CI chondrites reflects the initial Fe isotopic homogeneity of the well‐mixed solar nebula. In contrast, larger (up to 0.26‰ in δ56Fe) isotopic variations have been found between separate approximately 1 g pieces of the same ordinary chondrite sample. The Fe isotope heterogeneities in ordinary chondrites appear to be controlled by the abundances of chondritic components, specifically chondrules, whose Fe isotope compositions have been fractionated by evaporation and recondensation during multiple heating events.  相似文献   

12.
Abstract— High-purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases. P3 , released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios. HL , consisting of isotopically anomalous Xe-HL and Kr-H, Ar with high 38Ar/36Ar, and most of the gas making up Ne-A2 and He-A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase. P6 has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble-gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds. We find no evidence of separability of Xe-H and Xe-L or other isotopic variations in the HL component. However, because ～1010 diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble-gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble-gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was probably inherited from the Sun's parent molecular cloud.  相似文献   

13.
Carbonaceous chondrites contain numerous indigenous organic compounds and could have been an important source of prebiotic compounds required for the origin of life on Earth or elsewhere. Extraterrestrial amino acids have been reported in five of the eight groups of carbonaceous chondrites and are most abundant in CI, CM, and CR chondrites but are also present in the more thermally altered CV and CO chondrites. We report the abundance, distribution, and enantiomeric and isotopic compositions of simple primary amino acids in six metal‐rich CH and CB carbonaceous chondrites that have not previously been investigated for amino acids: Allan Hills (ALH) 85085 (CH3), Pecora Escarpment (PCA) 91467 (CH3), Patuxent Range (PAT) 91546 (CH3), MacAlpine Hills (MAC) 02675 (CBb), Miller Range (MIL) 05082 (CB), and Miller Range (MIL) 07411 (CB). Amino acid abundances and carbon isotopic values were obtained by using both liquid chromatography time‐of‐flight mass spectrometry and fluorescence, and gas chromatography isotope ratio mass spectrometry. The δ13C/12C ratios of multiple amino acids fall outside of the terrestrial range and support their extraterrestrial origin. Extracts of CH chondrites were found to be particularly rich in amino acids (13–16 parts per million, ppm) while CB chondrite extracts had much lower abundances (0.2–2 ppm). The amino acid distributions of the CH and CB chondrites were distinct from the distributions observed in type 2 and 3 CM and CR chondrites and contained elevated levels of β‐, γ‐, and δ‐amino acids compared to the corresponding α‐amino acids, providing evidence that multiple amino acid formation mechanisms were important in CH and CB chondrites.  相似文献   

14.
Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped 20Ne and 36Ar, and the ratio (36Ar/20Ne)t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (36Ar/20Ne)t and trapped 36Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (36Ar/20Ne)t ratio, Ne‐isotopic composition, and excess of 4He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (21Ne/22Ne)c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.  相似文献   

15.
Abstract— Concentration and isotopic composition of the light noble gases as well as of 84Kr, 129Xe, and 132Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high 129Xe/132Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic‐ray exposure ages are calculated from cosmogenic 21Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ～9 and ～29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.  相似文献   

16.
Abstract– Insight into the chemical history of an ungrouped type 2 carbonaceous chondrite meteorite, Wisconsin Range (WIS) 91600, is gained through molecular analyses of insoluble organic matter (IOM) using solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy, X‐ray absorption near edge structure spectroscopy (XANES), and pyrolysis‐gas chromatography coupled with mass spectrometry (pyr‐GC/MS), and our previous bulk elemental and isotopic data. The IOM from WIS 91600 exhibits similarities in its abundance and bulk δ15N value with IOM from another ungrouped carbonaceous chondrite Tagish Lake, while it exhibits H/C, δ13C, and δD values that are more similar to IOM from the heated CM, Pecora Escarpment (PCA) 91008. The 13C NMR spectra of IOM of WIS 91600 and Tagish Lake are similar, except for a greater abundance of CHxO species in the latter and sharper carbonyl absorption in the former. Unusual cross‐polarization (CP) dynamics is observed for WIS 91600 that indicate the presence of two physically distinct organic domains, in which the degrees of aromatic condensation are distinctly different. The presence of two different organic domains in WIS 91600 is consistent with its brecciated nature. The formation of more condensed aromatics is the likely result of short duration thermal excursions during impacts. The fact that both WIS 91600 and PCA 91008 were subjected to short duration heating that is distinct from the thermal history of type 3 chondrites is confirmed by Carbon‐XANES. Finally, after being briefly heated (400 °C for 10 s), the pyrolysis behavior of Tagish Lake IOM is similar to that of WIS 91600 and PCA 91008. We conclude that WIS 91600 experienced very moderate, short duration heating at low temperatures (<500 °C) after an episode of aqueous alteration under conditions that were similar to those experienced by Tagish Lake.  相似文献   

17.
Abstract– Stable hydrogen, carbon, and nitrogen isotopic ratios (δD, δ13C, and δ15N) of organic compounds can reveal information about their origin and formation pathways. Several formation mechanisms and environments have been postulated for the amino acids detected in carbonaceous chondrites. As each proposed mechanism utilizes different precursor molecules, the isotopic signatures of the resulting amino acids may indicate the most likely of these pathways. We have applied gas chromatography with mass spectrometry and combustion isotope ratio mass spectrometry to measure the compound‐specific C, N, and H stable isotopic ratios of amino acids from seven CM and CR carbonaceous chondrites: CM1/2 Allan Hills (ALH) 83100, CM2 Murchison, CM2 Lewis Cliff (LEW) 90500, CM2 Lonewolf Nunataks (LON) 94101, CR2 Graves Nunataks (GRA) 95229, CR2 Elephant Moraine (EET) 92042, and CR3 Queen Alexandra Range (QUE) 99177. We compare the isotopic compositions of amino acids in these meteorites with predictions of expected isotopic enrichments from potential formation pathways. We observe trends of decreasing δ13C and increasing δD with increasing carbon number in the α‐H, α‐NH2 amino acids that correspond to predictions made for formation via Strecker‐cyanohydrin synthesis. We also observe light δ13C signatures for β‐alanine, which may indicate either formation via Michael addition or via a pathway that forms primarily small, straight‐chain, amine‐terminal amino acids (n‐ω‐amino acids). Higher deuterium enrichments are observed in α‐methyl amino acids, indicating formation of these amino acids or their precursors in cold interstellar or nebular environments. Finally, individual amino acids are more enriched in deuterium in CR chondrites than in CM chondrites, reflecting different parent‐body chemistry.  相似文献   

18.
Abstract— I have determined the composition via instrumental neutron activation analysis of a bulk pristine sample of the Tagish Lake carbonaceous chondrite fall, along with bulk samples of the CI chondrite Orgueil and of several CM chondrites. Tagish Lake has a mean of refractory lithophile element/Cr ratios like those of CM chondrites, and distinctly higher than the CI chondrite mean. Tagish Lake exhibits abundances of the moderately volatile lithophile elements Na and K that are slightly higher than those of mean CM chondrites. Refractory through moderately volatile siderophile element abundances in Tagish Lake are like those of CM chondrites. Tagish Lake is distinct from CM chondrites in abundances of the most volatile elements. Mean CI‐normalized Se/Co, Zn/Co and Cs/Co for Tagish Lake are 0.68 ± 0.01, 0.71 ± 0.07 and 0.76 ± 0.02, while for all available CM chondrite determinations, these ratios lie between 0.31 and 0.61, between 0.32 and 0.58, and between 0.39 and 0.74, respectively. Considering petrography, and oxygen isotopic and elemental compositions, Tagish Lake is an ungrouped member of the carbonaceous chondrite clan. The overall abundance pattern is similar to those of CM chondrites, indicating that Tagish Lake and CMs experienced very similar nebular fractionations. Bells is a CM chondrite with unusual petrologic characteristics. Bells has a mean CI‐normalized refractory lithophile element/Cr ratio of 0.96, lower than for any other CM chondrite, but shows CI‐normalized moderately volatile lithophile element/Cr ratios within the ranges of other CM chondrites, except for Na which is low. Iridium, Co, Ni and Fe abundances are like those of CM chondrites, but the moderately volatile siderophile elements, Au, As and Sb, have abundances below the ranges for CM chondrites. Abundances of the moderately volatile elements Se and Zn of Bells are within the CM ranges. Bells is best classified as an anomalous CM chondrite.  相似文献   

19.
The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH2/pH2O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show that unequilibrated enstatite chondrites have light bulk N (δ 15N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution in enstatite chondrites of nitride condensates with δ15 N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within 15N-rich organic matter. The major carrier of N in ordinary chondrites is unknown.  相似文献   

20.
Abstract— The fall and recovery of the Tagish Lake meteorite in British Columbia in January 2000 provided a unique opportunity to study relatively pristine samples of carbonaceous chondrite material. Measurements of the oxygen isotopic composition of water extracted under stepped pyrolysis from a bulk sample of this meteorite have allowed us to make comparisons with similar data obtained from CI and CM chondrites and so further investigate any relationships that may exist between these meteorites. The much lower yield of water bearing a terrestrial signature in Tagish Lake is indicative of the pristine nature of the meteorite. The relationship between the isotopic composition of this water and reported isotopic values for carbonates, bulk matrix and whole rock have been used to infer the extent and conditions under which parent‐body aqueous alteration occurred. In Tagish Lake the difference in Δ17O isotopic composition between the water and other phases is greater than that found in either CM or CI chondrites suggesting that reaction and isotopic exchange between components was more limited. This in turn suggests that in the case of Tagish Lake conditions during the processes of aqueous alteration on the parent body, which ultimately controlled the formation of new minerals, were distinct from those on both CI and CM parent bodies.  相似文献   

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