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1.
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   

2.
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   

3.
Here, we report the mineralogy, petrography, C‐N‐O‐stable isotope compositions, degree of disorder of organic matter, and abundances of presolar components of the chondrite Roberts Massif (RBT) 04133 using a coordinated, multitechnique approach. The results of this study are inconsistent with its initial classification as a Renazzo‐like carbonaceous chondrite, and strongly support RBT 04133 being a brecciated, reduced petrologic type >3.3 Vigarano‐like carbonaceous (CV) chondrite. RBT 04133 shows no evidence for aqueous alteration. However, it is mildly thermally altered (up to approximately 440 °C); which is apparent in its whole‐rock C and N isotopic compositions, the degree of disorder of C in insoluble organic matter, low presolar grain abundances, minor element compositions of Fe,Ni metal, chromite compositions and morphologies, and the presence of unequilibrated silicates. Sulfides within type I chondrules from RBT 04133 appear to be pre‐accretionary (i.e., did not form via aqueous alteration), providing further evidence that some sulfide minerals formed prior to accretion of the CV chondrite parent body. The thin section studied contains two reduced CV3 lithologies, one of which appears to be more thermally metamorphosed, indicating that RBT 04133, like several other CV chondrites, is a breccia and thus experienced impact processing. Linear foliation of chondrules was not observed implying that RBT 04133 did not experience high velocity impacts that could lead to extensive thermal metamorphism. Presolar silicates are still present in RBT 04133, although presolar SiC grain abundances are very low, indicating that the progressive destruction or modification of presolar SiC grains begins before presolar silicate grains are completely unidentifiable.  相似文献   

4.
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   

5.
Abstract— The Sahara Desert is a region of high diurnal temperature variation and sporadic rainfall that has recently yielded over 450 meteorites. Eighteen of these Saharan samples are carbonaceous chondrites, of which we have analysed 17 for C content and isotopic composition. Ten of the 18 are paired CR chondrites, of which four have also had N and H contents and compositions determined. A primitive ordinary chondrite (L/LL3.2) found in the region has also been analysed for C, N and H contents and isotopic composition. Saharan samples contain between 21% and 45% of the light elements of their non-Saharan counterparts. Paired Saharan samples show a greater heterogeneity in both C content and isotopic composition than multiple analyses of non-Saharan samples. The cause of the observed isotopic and abundance effects is due to the hot desert weathering processes experienced by these samples. Peak temperatures of meteorites on the desert floor may be in excess of 100 °C, leading to low-temperature hydrous pyrolysis and oxidation reactions, liberating volatile organics and CO2. This may also cause the remaining material to become partially solubilised and ultimately lost during rainfall. The low δD of the CR and ordinary chondrites can be attributed to the destruction and loss of organic material through dehydrogenation and exchange reactions on the desert surface. The increased 13C abundance suggests that the less tightly bound C from the macromolecular organic material is isotopically lighter than the remaining C. Carbon contents and isotopic compositions are also affected by the addition of terrestrial calcitic evaporite deposits, up to 10,000 ppm carbonate has been measured, with a δ13C of between 0 and ?10%0.  相似文献   

6.
Abstract— We performed in situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezö‐Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine‐grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ15N and δ13C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15N excesses are found, with δ15Nmax ～+955‰, confirming previous results. Excesses in 15N are also detected in fine‐grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ15N ranging up to +440‰. The 15N excesses are attributed to ion‐molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small‐ and large‐scale carbon isotopic variations in the solar nebula.  相似文献   

7.
Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for 26Al‐26Mg systematics, only one aluminum‐rich chondrule contains excess 26Mg from the in situ decay of 26Al; the inferred initial ratio (26Al/27Al)o = (6.8 ± 2.4) × 10?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16O‐rich compositions with a range in δ18O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26Mg from the in situ decay of 26Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of 26Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26Al was at least approximately homogeneously distributed across the chondrite‐forming region.  相似文献   

8.
Abstract— Oxygen‐isotopic compositions were determined for a suite of enstatite chondrites and aubrites. In agreement with previous work (Clayton et al., 1984), most samples have O‐isotopic compositions close to the terrestrial fractionation line (TFL), and there appear to be no significant differences in O‐isotopic compositions between individual EH and EL chondrites and aubrites. Five enstatite meteorites have O‐isotopic compositions that are significantly different from the other samples and >0.2% away from the TFL. Two of these have petrographic evidence of brecciation and interaction between other meteorite types; for the other three, similar scenarios are suggested. There appears to be a systematic increase in δ18O from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6. There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66. At the present time, detailed understanding of the origin of these O‐isotopic systematics remain elusive but clearly point to a complex accretion history, parent‐body evolution, or both.  相似文献   

9.
Abstract— In situ io n microprobe analyses of spinel in refractory calcium‐aluminium‐rich inclusions (CAIs) from type 3 EH chondrites yield 16O‐rich compositions (δ 18O and δ 17O about‐40‰). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (δ 18O and δ 17O about ?5‰). A regression through the data results in a line with slope 1.0 on a three‐isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with 16O‐rich and 16O‐poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs.  相似文献   

10.
Abstract Diamonds isolated from primitive chondrites of the carbonaceous, ordinary and enstatite groups have been analysed by high-resolution stepped combustion, followed by measurement of their C and N isotopes using a newly adapted technique that allows quantitative measurements of C/N ratios. The δ13C of the diamond is shown to vary between meteorite groups from ?32 to ?38%0, and the measured C/N ratios suggest that the N concentration of diamond ranges over a factor of 7 from 1800 ppm (Tieschitz) to 13,000 ppm (Adrar 003). The δ15N of N released from diamond is constrained to ?348 ± 7%. The complexity of the C release pattern and C/N ratio during combustion implies the presence of more than one component, which suggests that either more than one type of diamond is present in the samples, or unidentified additional phases are located in the acid-resistant residue. The components are present in varying proportions between meteorite groups. The data are compatible with a model of a mix of different diamond populations (some probably presolar and some possibly solar) existing in the early solar nebula, where each population originally contributed a roughly equal amount to chondrites of every class. Subsequent metamorphism has resulted in overall variations in δ13C and C/N ratios in diamond isolated from meteorites of differing petrologic grade without significantly altering the N isotopic composition. Possible ways for this to be achieved are explored.  相似文献   

11.
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   

12.
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   

13.
John T. Wasson 《Icarus》2008,195(2):895-907
Studies of matrix in primitive chondrites provide our only detailed information about the fine fraction (diameter <2 μm) of solids in the solar nebula. A minor fraction of the fines, the presolar grains, offers information about the kinds of materials present in the molecular cloud that spawned the Solar System. Although some researchers have argued that chondritic matrix is relatively unaltered presolar matter, meteoritic chondrules bear witness to multiple high-temperature events each of which would have evaporated those fines that were inside the high-temperature fluid. Because heat is mainly transferred into the interior of chondrules by conduction, the surface temperatures of chondrules were probably at or above 2000 K. In contrast, the evaporation of mafic silicates in a canonical solar nebula occurs at around 1300 K and FeO-rich, amorphous, fine matrix evaporates at still lower temperatures, perhaps near 1200 K. Thus, during chondrule formation, the temperature of the placental bath was probably >700 K higher than the evaporation temperatures of nebular fines. The scale of chondrule forming events is not known. The currently popular shock models have typical scales of about 105 km. The scale of nebular lightning is less well defined, but is certainly much smaller, perhaps in the range 1 to 1000 m. In both cases the temperature pulses were long enough to evaporate submicrometer nebular fines. This interpretation disagrees with common views that meteoritic matrix is largely presolar in character and CI-chondrite-like in composition. It is inevitable that presolar grains (both those recognized by their anomalous isotopic compositions and those having solar-like compositions) that were within the hot fluid would also have evaporated. Chondrule formation appears to have continued down to the temperatures at which planetesimals formed, possibly around 250 K. At temperatures >600 K, the main form of C is gaseous CO. Although the conversion of CO to CH4 at lower temperatures is kinetically inhibited, radiation associated with chondrule formation would have accelerated the conversion. There is now evidence that an appreciable fraction of the nanodiamonds previously held to be presolar were actually formed in the solar nebula. Industrial condensation of diamonds from mixtures of CH4 and H2 implies that high nebular CH4/CO ratios favored nanodiamond formation. A large fraction of chondritic insoluble organic matter may have formed in related processes. At low nebular temperatures appreciable water should have been incorporated into the smoke that condensed following dust (and some chondrule) evaporation. If chondrule formation continued down to temperatures as low as 250 K this process could account for the water concentration observed in primitive chondrites such as LL3.0 and CO3.0 chondrites. Higher H2O contents in CM and CI chondrites may reflect asteroidal redistribution. In some chondrite groups (e.g., CR) the Mg/Si ratio of matrix material is appreciably (30%) lower than that of chondrules but the bulk Mg/Si ratio is roughly similar to the CI or solar ratio. This has been interpreted as a kind of closed-system behavior sometimes called “complementarity.” This leads to the conclusion that nebular fines were efficiently agglomerated. Its importance, however is obscured by the observation that bulk Mg/Si ratios in ordinary and enstatite chondrites are much lower than those in carbonaceous chondrites, and thus that complementarity did not hold throughout the solar nebula.  相似文献   

14.
Abstract— The thermal metamorphism grade of organic matter (OM) trapped in 6 unequilibrated ordinary chondrites (UOCs) (Semarkona [LL 3.0], Bishunpur [L/LL 3.1], Krymka [LL 3.1], Chainpur [LL 3.4], Inman [L/LL 3.4], and Tieschitz [H/L 3.6]) has been investigated with Raman spectroscopy in the region of the first‐order carbon bands. The carbonaceous chondrite Renazzo (CR2) was also investigated and used as a reference object for comparison, owing to the fact that previous studies pointed to the OM in this meteorite as being the most pristine among all chondrites. The results show that the OM thermal metamorphic grade: 1) follows the hierarchy Renazzo << Semarkona << other UOCs; 2) is well correlated to the petrographic type of the studied objects; and 3) is also well correlated with the isotopic enrichment δ15N. These results are strikingly consistent with earlier cosmochemical studies, in particular, the scenario proposed by Alexander et al. (1998). Thermal metamorphism in the parent body appears as the main evolution process of OM in UOCs, demonstrating that nebular heating was extremely weak and that OM burial results in the destabilization of an initial isotopic composition with high δD and δ15N. Furthermore, the clear discrimination between Renazzo, Semarkona, and other UOCs shows: 1) Semarkona is a very peculiar UOC—by far the most pristine; and 2) Raman spectroscopy is a valid and valuable tool for deriving petrographic sub‐types (especially the low ones) that should be used in the future to complement current techniques. We compare our results with other current techniques, namely, induced thermo‐luminescence and opaques petrography. Other results have been obtained. First, humic coals are not strictly valid standard materials for meteoritic OM but are helpful in the study of evolutionary trends due to thermal metamorphism. Second, terrestrial weathering has a huge effect on OM structure, particularly in Inman, which is a find. Finally, the earlier statement that fine‐grained chondrule rims and matrix in Semarkona could be the source of smectite‐rich IDPs is not valid, given the different degree of structural order of their OM.  相似文献   

15.
The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as CN and CF), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (CN) contain volume-bound nitrogen with δ15N = ?350‰; (c) the grains of both populations contain surface-bound nitrogen (δ15N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the CN and CF populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ12C isotope relative to the remainder of the grain. While the value of δ13C is ?37.3 ± 1.1‰ for carbon in the central part, it is ?32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as CHL), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the CHL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.  相似文献   

16.
Abstract— Nitrogen abundances and isotopic compositions of four CK chondrites (ALH85002, EET92002, Yamato6903 and Karoonda) were measured by a stepped-combustion method. Neon and Ar were also measured for the same samples. Two types of isotopically light N were observed. One of them is labile N released at low temperatures (～300 °C). This N is observed only in ALH85002. The other N is extracted at high temperatures (900?1200 °C) from all CK chondrites; although, the isotopic compositions are somewhat variable. There is a fair correlation between the excess 15N values and the abundance of trapped 36Ar for the high-temperature component, suggesting presolar origin of these species. The light N (δ15N = ?106.8‰) observed in Karoonda is one of the lightest N components ever reported for bulk chondrites.  相似文献   

17.
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   

18.
Abstract— Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide‐rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial 20Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial 36Ar. Our preferred interpretation is that‐in contrast to CAIs‐the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial 20Ne and 36Ar show low presolar‐diamond‐like 36Ar/20Ne ratios. In contrast, the metal‐sulfide‐rich coatings generally show higher gas concentrations and Q‐like 36Ar/20Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar‐carrying phase Q became enriched in the metal‐sulfide‐rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne‐carrying presolar diamonds survived the melting event leading to the low observed 36Ar/20Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar‐type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.  相似文献   

19.
Abstract— Nitrogen and Ar in more than 20 primitive ordinary chondrites were studied by a stepped combustion method. Several N carriers that are characterized by N isotopic composition, N release pattern and trapped Ar release pattern are recognized in the primitive ordinary chondrites. Large fractions of anomalous N and associated Ar are removed by acid treatment in most cases. The N isotopic anomalies cannot be explained by known presolar grains (with a possible exception of graphite), and some of the N isotopic anomalies may be due to unknown presolar grains. There is no specific relationship between the type of N carriers contained in an ordinary chondrite and the chemical type (H, L, or LL) of the chondrite. It is likely that as a result of impacts, the carriers of isotopically anomalous N were mixed in various parent bodies as rock fragments rather than as individual fine particles. The presence of distinctive N isotopic anomalies in primitive meteorites indicates that the primitive solar nebula may have been heterogeneous either spatially or temporally.  相似文献   

20.
Abstract— A new grouplet of primitive, metal‐rich chondrites, here called the CB (C, carbonaceous; B, bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of the more inclusive CR clan. CB chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB chondrites, as well as other CR clan chondrites, have much heavier nitrogen (higher 15N/14N) than that in other chondrite groups. The primitive characteristics of the CB chondrites suggest that they contain one of the best records of early nebular processes. Another chondrite, Grosvenor Mountains 95551, is petrographically similar to the CB chondrites, but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir.  相似文献   

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