共查询到20条相似文献，搜索用时 31 毫秒
1.
2.
We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3ox), Axtell (CV3ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites.After accounting for parent-body metamorphism, abundances and characteristics of presolar components still show large variations across the classes of carbonaceous chondrites. These variations correlate with the bulk compositions of the host meteorites and imply that the same thermal processing that was responsible for generating the compositional differences between the various chondrite groups also modified the initial presolar-grain assemblages. The CI chondrites and CM2 matrix have the least fractionated bulk compositions relative to the sun and the highest abundances of most types of presolar material, particularly the most fragile types, and thus are probably most representative of the material inherited from the sun's parent molecular cloud. The other classes can be understood as the products of various degrees of heating of bulk molecular cloud material in the solar nebula, removing the volatile elements and destroying the most fragile presolar components, followed by chondrule formation, metal-silicate fractionation in some cases, further nebula processing in some cases, accretion, and parent body processing. If the bulk compositions and the characteristics of the presolar-grain assemblages in various chondrite classes reflect the same processes, as seems likely, then differential condensation from a nebula of solar composition is ruled out as the mechanism for producing the chondrite classes. Presolar grains would have been destroyed if the nebula had been completely vaporized. Our analysis shows that carbonaceous chondrites reflect all stages of nebular processing and thus are no more closely related to one another than they are to ordinary and enstatite chondrites. 相似文献
3.
《Chemie der Erde / Geochemistry》2014,74(4):519-544
Noble gases are not rare in the Universe, but they are rare in rocks. As a consequence, it has been possible to identify in detailed analyses a variety of components whose existence is barely visible in other elements: radiogenic and cosmogenic gases produced in situ, as well as a variety of “trapped” components – both of solar (solar wind) origin and the “planetary” noble gases. The latter are most abundant in the most primitive chondritic meteorites and are distinct in elemental and isotopic abundance patterns from planetary noble gases sensu strictu, e.g., those in the atmospheres of Earth and Mars, having in common only the strong relative depletion of light relative to heavy elements when compared to the solar abundance pattern. In themselves, the “planetary” noble gases in meteorites constitute again a complex mixture of components including such hosted by pre-solar stardust grains.The pre-solar components bear witness of the processes of nucleosynthesis in stars. In particular, krypton and xenon isotopes in pre-solar silicon carbide and graphite grains keep a record of physical conditions of the slow-neutron capture process (s-process) in asymptotic giant branch (AGB) stars. The more abundant Kr and Xe in the nanodiamonds, on the other hand, show a more enigmatic pattern, which, however, may be related to variants of the other two processes of heavy element nucleosynthesis, the rapid neutron capture process (r-process) and the p-process producing the proton-rich isotopes.“Q-type” noble gases of probably “local” origin dominate the inventory of the heavy noble gases (Ar, Kr, Xe). They are hosted by “phase Q”, a still ill-characterized carbonaceous phase that is concentrated in the acid-insoluble residue left after digestion of the main meteorite minerals in HF and HCl acids. While negligible in planetary-gas-rich primitive meteorites, the fraction carried by “solubles” becomes more important in chondrites of higher petrologic type. While apparently isotopically similar to Q gas, the elemental abundances are somewhat less fractionated relative to the solar pattern, and they deserve further study. Similar “planetary” gases occur in high abundance in the ureilite achondrites, while small amounts of Q-type noble gases may be present in some other achondrites. A “subsolar” component, possibly a mixture of Q and solar noble gases, is found in enstatite chondrites. While no definite mechanism has been identified for the introduction of the planetary noble gases into their meteoritic host phases, there are strong indications that ion implantation has played a major role.The planetary noble gases are concentrated in the meteorite matrix. Ca-Al-rich inclusions (CAIs) are largely planetary-gas-free, however, some trapped gases have been found in chondrules. Micrometeorites (MMs) and interplanetary dust particles (IDPs) often contain abundant solar wind He and Ne, but they are challenging objects for the analysis of the heavier noble gases that are characteristic for the planetary component. The few existing data for Xe point to a Q-like isotopic composition. Isotopically Q-Kr and Q-Xe show a mass dependent fractionation relative to solar wind, with small radiogenic/nuclear additions. They may be closer to “bulk solar” Kr and Xe than Kr and Xe in the solar wind, but for a firm conclusion it is necessary to gain a better understanding of mass fractionation during solar wind acceleration. 相似文献
4.
We have investigated the siting of noble gases in 6 E-chondrites, by analyzing fractions separated by density, grain size, and chemical resistance from Qingzhen (E3), Indarch (E4), Abee and Saint Sauveur (E4-5) and Yilmia and North West Forrest (E6).The new “subsolar” (i.e. Ar-rich) component in E6's is concentrated in the main, ensatite-rich fraction of the meteorites, with density 3.06–3.3 g/cm3. It is unaffected by HCl and HNO3 treatments of such fractions and remains in unchanged concentration when the samples are partially dissolved by HF. These properties suggest that the subsolar component is located in enstatite, or less likely, in a phase closely associated with it. E4-5's have at least half of their subsolar gases in HCl- and HNO3-resistant sites (enstatite?), but fail to show the increasing gas concentration with decreasing grain size that is characteristic of most other primordial gas carriers. This may mean that the subsolar gases originally were in some other phase, but were then transferred to enstatite by metamorphism.Most of the 129Xer of E6's is concentrated in the same fractions as the subsolar gases, again suggesting enstatite or an associated phase as the host. Only a few percent of the 129Xer is contained in fractions enriched in other major and minor minerals. In E4's, on the other hand, 129Xer is enhanced in finegrained, low density fractions and is also partly associated with chondrules. Perhaps 129I was originally contained in fine-grained matrix, but was transferred to enstatite during metamorphism.A carbon-rich fraction of Indarch (E4) is enhanced in Ne-A, CCF-Xe, and L-Xe. Interestingly, both the isotopic composition of Xe and the Ne/CCF-Xe ratios resemble those of C-chondrites, yet these two meteorite classes probably formed rather far apart. Thus, if these components were mixed at a late stage, it must have been in fairly constant ratio over a large scale. Alternatively, they may have been mixed at an earlier stage, into a common carrier that was spread through a significant portion of the solar nebula.The primordial gases of Qingzhen (E3) resemble those of Indarch: they are present in moderate amounts (, with little or no contribution from the subsolar component. Thus Qingzhen reinforces our earlier finding that E-chondrites show no regular increase in noble gas content with decreasing petrologic type. One notable feature of Qingzhen is its very low ratio of 1.07, which indicates that 3He has been lost by solar heating. Solar heating may also account for its low, discordant gas retention ages (U,Th-He age = 1.1 AE, KAr age = 3.2AE). 相似文献
5.
Eric Quirico Michèle Bourot-denise Gilles Montagnac 《Geochimica et cosmochimica acta》2011,75(11):3088-3102
The thermal history of a series of EH3 and EL3 chondrites has been investigated by studying the degree of structural order of the organic matter (OM) located and characterized in matrix areas by Raman micro-spectroscopy. By comparison with unequilibrated ordinary chondrites (UOCs) and CO and CV carbonaceous chondrites, the following petrologic types have been assigned to various E chondrites: Sahara 97096 and Allan Hills 84206: 3.1-3.4; Allan Hills 85170 and Parsa: 3.5; Allan Hills 85119: 3.7; Qingzhen, MacAlpine Hills 88136 and MacAlpine Hills 88184: 3.6-3.7. The petrologic type of Qingzhen is consistent with the abundance of the P3 noble gas component, a sensitive tracer of the grade of thermal metamorphism. The petrologic types are qualitatively consistent with the abundance of fine-grained matrix for the whole series. No significant effects of shock processes on the structure of OM were observed. However such processes certainly compete with thermal metamorphism and the possibility of an effect cannot be fully discarded, in particular in the less metamorphosed objects. The OM precursors accreted by the EH3 and EL3 parent bodies appear to be fairly similar to those of UOCs and CO and CV carbonaceous chondrites. Raman data however show some slight structural differences that could be partly accounted for by shock processes. The metamorphic history of EH3 and EL3 chondrites has often been described as complex, in particular regarding the combined action of shock and thermal metamorphism. Because OM maturity is mostly controlled by the temperature of peak metamorphism, it is possible to distinguish between the contributions of long duration thermal processes and that of shock processes. Comparison of the petrologic types with the closure temperatures previously derived from opaque mineral assemblages has revealed that the thermal history of EH3 and EL3 chondrites is consistent with a simple asteroidal onion shell model. Thermal metamorphism in enstatite chondrites appears to be fairly similar to that which takes place in other chondrite classes. The complex features recorded by mineralogy and petrology and widely reported in the literature appear to be mostly controlled by shock processes. 相似文献
6.
A. B. Verchovsky 《Geochemistry International》2017,55(11):957-970
Bulk meteorite samples of various chemical classes and petrologic types (mainly carbonaceous chondrites) were systematically investigated by the stepped combustion method with the simultaneous isotopic analysis of carbon, nitrogen, and noble gases. A correlation was revealed between planetary noble gases associating with the Q phase and isotopically light nitrogen (δ15N up to –150‰). The analysis of this correlation showed that the isotopically light nitrogen (ILN) is carried by Q. In most meteorites, isotopically heavy nitrogen (IHN) of organic compounds (macromolecular material) is dominant. The ILN of presolar grains (diamond and SiC) and Q can be detected after separation from dominant IHN. Such a separation of nitrogen from Q and macromolecular material occurs under natural conditions and during laboratory stepped combustion owing to Q shielding from direct contact with oxygen, which results in Q oxidation at temperatures higher than the temperatures of the release of most IHN. There are arguments that ILN released at high temperature cannot be related to nanodiamond and SiC. The separation effect allowed us to constrain the contents of noble gases in Q, assuming that this phase is carbon-dominated. The directly measured 36Ar/C and 132Xe/C ratios in ILN-rich temperature fractions are up to 0.1 and 1 × 10–4 cm3/g, respectively. These are only lower constraints on the contents. The analysis of the obtained data on the three-isotope diagram δ15N–36Ar/14N showed that Q noble gases were lost to a large extent from most meteorites during the metamorphism of their parent bodies. Hence, the initial contents of noble gases in Q could be more than an order of magnitude higher than those directly measured. Compared with other carbon phases, Q was predominantly transformed to diamond in ureilites affected by shock metamorphism. The analysis of their Ar–N systematics showed that, similar to carbonaceous chondrites, noble gases were lost from Q probably before its transformation to diamond. 相似文献
7.
Noble gas components in planetary atmospheres and interiors in relation to solar wind and meteorites
We discuss observed xenon isotopic signatures in solar system reservoirs and possible relationships. The predominant trapped
xenon component in ordinary chondrites (OC) is OC-Xe and its isotopic signature differs from Xe in ureilites, in carbonaceous
chondrites, in the atmospheres of Earth and Mars, and in the solar wind. Additional minor Xe components were identified in
type 3 chondrites and in the metal phase of chondrites. The OC-Xe and ureilite signatures are both consistent with varying
mixtures of HL-Xe and slightly mass fractionated solar-type Xe. Xenon in the Martian atmosphere is found to be strongly mass
fractionated by 37.7‰ per amu, relative to solar Xe, favoring the heavy isotopes. Xenon in SNC’s from the Martian mantle show
admixture of solar-type Xe, which belongs to an elementally strongly fractionated component. The origin of the isotopic signatures
of Ne and Xe in the terrestrial atmosphere are discussed in the light of evidence that the Xe isotopic fractionations in the
Martian and terrestrial atmospheres are consistent. However, in the terrestrial atmospheric Xe component excesses are observed
for132Xe and also for129,131Xe, relative to fractionated solar Xe. The suggested chemically fractionated fission Xe component (CFF-Xe) seems to closely
match the above excesses. We discuss models of origin for planetary volatiles and possible processes driving their evolution
to present day compositions. 相似文献
8.
Chronology and shock history of the Bencubbin meteorite: A nitrogen, noble gas, and Ar-Ar investigation of silicates, metal and fluid inclusions 总被引：1，自引：0，他引：1
We have investigated the distribution and isotopic composition of nitrogen and noble gases, and the Ar-Ar chronology of the Bencubbin meteorite. Gases were extracted from different lithologies by both stepwise heating and vacuum crushing. Significant amounts of gases were found to be trapped within vesicles present in silicate clasts. Results indicate a global redistribution of volatile elements during a shock event caused by an impactor that collided with a planetary regolith. A transient atmosphere was created that interacted with partially or totally melted silicates and metal clasts. This atmosphere contained 15N-rich nitrogen with a pressure ?3 × 105 hPa, noble gases, and probably, although not analyzed here, other volatile species. Nitrogen and noble gases were re-distributed among bubbles, metal, and partly or totally melted silicates, according to their partition coefficients among these different phases. The occurrence of N2 trapped in vesicles and dissolved in silicates indicates that the oxygen fugacity (fO2) was greater than the iron-wüstite buffer during the shock event. Ar-Ar dating of Bencubbin glass gives an age of 4.20 ± 0.05 Ga, which probably dates this impact event. The cosmic-ray exposure age is estimated at ∼40 Ma with two different methods. Noble gases present isotopic signatures similar to those of “phase Q” (the major host of noble gases trapped in chondrites) but elemental patterns enriched in light noble gases (He, Ne and Ar) relative to Kr and Xe, normalized to the phase Q composition. Nitrogen isotopic data together with 40Ar/36Ar ratios indicate mixing between a 15N-rich component (δ15N = +1000‰), terrestrial N, and an isotopically normal, chondritic N.Bencubbin and related 15N-rich meteorites of the CR clan do not show stable isotope (H and C) anomalies, precluding contribution of a nucleosynthetic component as the source of 15N enrichments. This leaves two possibilities, trapping of an ancient, highly fractionated atmosphere, or degassing of a primitive, isotopically unequilibrated, nitrogen component. Although the first possibility cannot be excluded, we favor the contribution of primitive material in the light of the recent finding of extremely 15N-rich anhydrous clasts in the CB/CH Isheyevo meteorite. This unequilibrated material, probably carried by the impactor, could have been insoluble organic matter extremely rich in 15N and hosting isotopically Q-like noble gases, possibly from the outer solar system. 相似文献
9.
This paper deals with the effects of thermal and collision events which had been experienced by the Jilin(H5) and Qingzhen(EH3) chondrites before they fell to the earth .The HRTEM and opti-cal microscopic investigations show that the Jilin chondrite has undergone more extensive thermal heating and two stages of collision,while the Qingzhen chondrite has experienced weak thermal events after the accretion of its parent body and one stage of moderate collision.The schematic dia-grams of the process of formation and evolution of these two meteorites are given in the present pa-per. 相似文献
10.
This paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine CV3 chondrites: Allende, Axtell, Bali, Mokoia, Grosnaja, Efremovka, Vigarano, Leoville, and Kaba. The maturity of the organic matter in matrix, determined by Raman spectroscopy, has been used as a powerful metamorphic tracer, independent of the mineralogical context and extent of aqueous alteration. This tracer has been used along with other metamorphic tracers such as Fe zoning in type-I chondrules of olivine phenocrysts, presolar grain abundance and noble gas abundance (bulk and P3 component). The study shows that the petrologic types determined earlier by Induced ThermoLuminescence were underestimated and suggests the following values: PT (Allende-Axtell) >3.6; PT (Bali-Mokoia-Grosnaja) ∼3.6; PT (Efremovka-Leoville-Vigarano) = 3.1-3.4; PT (Kaba) ∼3.1. The most commonly studied CV3, Allende, is also the most metamorphosed. Bali is a breccia containing clasts of different petrologic types. The attribution suggested by this study is that of clasts of the highest petrologic types, as pointed out by IOM maturity and noble gas bulk abundance. CV3 chondrites have complex asteroidal backgrounds, with various degrees of aqueous alteration and/or thermal metamorphism leading to complex mineralogical and petrologic patterns. (Fe,Mg) chemical zoning in olivine phenocrysts, on the borders of type I chondrules of porphyritic olivine- and pyroxene-rich textural types, has been found to correlate with the metamorphism grade. This suggests that chemical zoning in some chondrules, often interpreted as exchanges between chondrules and nebular gas, may well have an asteroidal origin. Furthermore, the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration. This does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through (Fe,Mg) equilibration. On the other hand, it may provide clues on the degree of aqueous alteration vs. thermal metamorphism and on the timing of both processes. In particular, Mokoia experienced significant aqueous alteration after the metamorphism peak, whereas Grosnaja, which has similar metamorphism grade, did not. 相似文献
11.
J.D. Gilmour 《Geochimica et cosmochimica acta》2010,74(1):380-6266
Models capable of explaining the differences between the isotopic compositions of the planetary noble gas components “Q” and “P3” (widespread in primitive meteorites) and average solar system material as sampled by the solar wind are presented, and their implications discussed.Small, variable amounts of known presolar components and 129Xe from 129I decay are present in Q gases alongside a solar composition that has been mass fractionated. These most likely arise either from mixing during parent body processing or co-release from poorly retentive phases during analysis. Thus the heavy noble gas budget of primitive meteorites is dominated by a component derived from material with the average composition of the solar system.In contrast, P3 seems best explained as a presolar component, consistent with isolation from bulk material that subsequently evolved to the solar composition as newly synthesised material was added. Examination of Kr-P3 identifies the addition as having had the signature of the weak s-process, and demonstrates that a second process that contributes the isotopes of krypton not produced in the s-process (residual isotopes) must also have added material to the reservoir as it evolved to the solar composition. Total s-process contributions required of 132Xe and 84Kr are at least ∼1% of the present budget, as is that of the residual krypton “component”.While concentrations of P3 vary with extents of parent body processing, concentrations of 129Xe excess from 129I decay associated with P3 are roughly constant in those least processed meteorites that retain a P3 signature (apart from ALH77307). This has a natural explanation in the different chemical behaviours of iodine and xenon if 129I was alive in P3 in the early solar system. The presence of live 129I in P3 in the early solar system imposes a loose constraint that the P3 component was isolated from a parent reservoir less than ∼100 Myr before the formation of the solar system.The model of evolution from P3 gases to the solar composition requires that residual krypton isotopes are not products of a conventional r-process that also synthesises 129I. Separate sites for synthesis of residual isotopes of krypton and the heavy element r-process are consistent with observations of metal poor stars, but do not correspond to the two r-processes invoked to account for variations between the 182Hf and 129I systems. Since the weak s-process is implicated as the source of the s-process material contributed as the P3 reservoir evolved to a solar composition, the massive stars that host it may also host the process that synthesises residual krypton isotopes. The recent presence of nearby massive stars is consistent with an emerging picture of the environment of solar system formation. 相似文献
12.
We have investigated the presolar grain inventories of two CR chondrites, QUE 99177 and MET 00426, which are less altered than most members of this meteorite group. Both meteorites contain high abundances of O-anomalous presolar grains, with concentrations of 220 ± 40 and 160 ± 30 ppm for QUE 99177 and MET 00426, respectively. The presolar grain inventories are dominated by ferromagnesian silicates with group 1 oxygen isotopic compositions, indicative of origins in low mass red giant or asymptotic giant branch stars. Grains with pyroxene-like compositions are somewhat more common than those with olivine-like compositions, but most grains are non-stoichiometric with compositions intermediate between these two phases, consistent with recent work suggesting that amorphous interstellar silicates have stoichiometries between olivine and pyroxene type silicates. Although structural data are not available, one grain contains only Si and O, and has a stoichiometry consistent with SiO2.Our presolar grains are much more Fe-rich than predicted by astronomical observations. Although secondary alteration may play a role in enhancing the Fe contents of presolar grains, it seems unlikely that the large and ubiquitous Fe enrichments observed in the grains from this study can be due only to secondary processing, particularly given the highly primitive nature of these two meteorites. Grain condensation in the stellar outflows where these grains formed likely proceeded under rapidly changing kinetic conditions that may have enhanced the incorporation of Fe into the grains over that expected based on equilibrium condensation theory.Both QUE 99177 and MET 00426 appear to contain unusually low abundances of oxide grains and have higher silicate/oxide ratios than other primitive meteorites analyzed to date. We explore various possibilities for this discrepancy, but note that most scenarios are not likely to result in the preferential destruction of oxides relative to silicates. Thus, the highest silicate/oxide ratios, such as those observed in the CR chondrites, should reflect the true initial proportions of presolar silicate and oxide grains in the parent molecular cloud from which the solar nebula evolved. 相似文献
13.
Two examined fragments of the Kaidun meteorite principally differ in the concentrations of isotopes of noble gases and are very heterogeneous in terms of the isotopic composition of the gases. Because these fragments belong to two basically different types of meteoritic material (EL and CR chondrites), these characteristics of noble gases could be caused by differences in the cosmochemical histories of the fragments before their incorporation into the parent asteroid. As follows from the escape kinetics of all gases, atoms of trapped and cosmogenic noble gases are contained mostly in the structures of two carrier minerals in the samples. The concentrations and proportions of the concentrations of various primary noble gases in the examined fragments of Kaidun are obviously unusual compared to data on most currently known EL and CR meteorites. In contrast to EL and CR meteorites, which contain the primary component of mostly solar provenance, the elemental ratios and isotopic composition of Ne and He in the fragments of Kaidun correspond to those typical of the primary components of A and Q planetary gases. This testifies to the unique conditions under which the bulk of the noble gases were trapped from the early protoplanetary nebula. The apparent cosmic-ray age of both of the Kaidun fragments calculated based on cosmogenic isotopes from 3He to 126Xe varies from 0.027 to 246 Ma as a result of the escape of much cosmogenic isotopes at relatively low temperatures. The extrapolated cosmic-ray age of the Kaidun meteorite, calculated from the concentrations of cosmogenic isotopes of noble gases, is as old as a few billion years, which suggests that the material of the Kaidun meteorite could be irradiated for billions of years when residing in an unusual parent body. 相似文献
14.
Literature data show that refractory-lithophile elements in most chondrite groups are unfractionated relative to CI chondrites; the principal exception is the EL-chondrite group whose observed falls (all of which are type 6) are depleted in Ca and light REE. In contrast, literature data and our new INAA data on EL3 PCA 91020, EL3 MAC 88136 and EL4 Grein 002 show that some replicates of these samples have nearly flat REE patterns (unlike those of EL6 chondrites); other replicates exhibit fractionated REE patterns similar to those of EL6 chondrites. Petrographic examination shows that many EL6 (and some EL3 and EL4) chondrites are impact-melt breccias or contain impact-melted portions. We suggest that the same impact processes that formed these breccias and produced melt are responsible for the observed bulk compositional fractionations in refractory-lithophile elements, i.e., EL6 chondrites were produced from initially unequilibrated EL3 material. When large amounts of impact heat were deposited, plagioclase and/or oldhamite (CaS) (the major REE carriers in enstatite chondrites) may have been melted and then transported appreciable (>10 cm) distances. EL6 chondrites represent the residuum that is depleted in REE (particularly in LREE) and Ca. Unlike the case for EL chondrites, our new INAA data on ALH 84170, EET 87746 and SAH 97096 (all EH3) show some scatter but are consistent with the EH group having uniform refractory-lithophile abundances. 相似文献
15.
Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peña Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the 81Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the 81Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface” on their parent body(ies): (i) neutron capture 36Ar, 80Kr, 82Kr and/or 128Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peña Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peña Blanca Spring and ALHA 78113). The concentrations of 128Xe from neutron capture on 127I vary among four measured specimens of Cumberland Falls (0.5-76 × 10−14 cm3STP/g), but are correlated with those of radiogenic 129Xe, implying that the concentrations of (128Xe)n and (129Xe)rad reflect variable abundances of iodine among specimens. The ratios of (128Xe)n/(129Xe)rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic 129Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10−7, 9.4 × 10−10 and 2.8 × 10−10 cm3STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peña Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely 244Pu-derived, and about 10-20% seems to be 238U-derived at 136Xe. The observed (136Xe)Pu corresponds to 0.019-0.16 ppb of 244Pu, from which the 244Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after ∼4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas signatures, oxygen with chondritic isotopic compositions was found only in a specimen of Cumberland Falls (Δ17O of ∼0.3‰). The other four aubrites and the other two measured specimens of Cumberland Falls are concurrent with the typical range for aubrites. 相似文献
16.
Cadmium stable isotope cosmochemistry 总被引：2，自引：0，他引：2
Frank Wombacher Mark Rehkämper Klaus Mezger Carsten Münker 《Geochimica et cosmochimica acta》2008,72(2):646-667
Cadmium stable isotope compositions are reported for a comprehensive suite of carbonaceous, ordinary, enstatite, and Rumuruti chondrites as well as achondrites and lunar samples (soils, breccias, pristine anorthosite). The Cd isotope analyses were performed by multiple collector ICP-MS with an external reproducibility of ±0.43‰ (2 sd) for δ114/110Cd. None of the samples shows evidence of nucleosynthetic anomalies and cosmogenic isotope effects from neutron-capture by 113Cd were only observed for two lunar samples.The Cd stable isotope compositions of type 1, 2, and some type 3 carbonaceous chondrites, EH4 enstatite chondrites, eucrites, and the Earth are essentially identical at δ114/110Cd ≈ 0.0 ± 0.4. This suggests that the portion of the solar nebula from which the inner solar system bodies accreted was homogeneous with respect to its Cd isotope composition. It also indicates that the primary volatile element depletion of the inner solar system did not involve partial kinetic Rayleigh evaporation or condensation. Furthermore no resolvable Cd isotope effects were generated during the accretion and initial differentiation of the planetary bodies.In contrast, the analyses reveal large Cd isotope effects for ordinary and some enstatite chondrites, which display δ114/110Cd values between about −8 and +16. Smaller fractionations are observed for the Rumuruti and some type 3 to 5 carbonaceous chondrites. These Cd isotope variations are thought to reflect secondary depletion or redistribution of Cd, due to open system thermal metamorphism on the meteorite parent bodies.One CAI and chondrule separates from the Allende meteorite have unexpectedly high Cd concentrations and fractionated light Cd isotope compositions with δ114/110Cd ≈ −1 to −4. These characteristics may have been established by the interaction of originally Cd-poor materials with a volatile-rich gas prior to the final accretion of the Allende parent body. The general Cd enrichment of the lunar soil and regolith mainly reflects early volcanic activity. However, decreasing Cd abundances in lunar soils correlate well with an enrichment of the heavy Cd isotopes. This relationship is best explained by suppressed Rayleigh fractionation in response to space weathering. 相似文献
17.
Ratan K. Mohapatra Susanne P. Schwenzer Siegfried Herrmann Ulrich Ott Jamie D. Gilmour 《Geochimica et cosmochimica acta》2009,73(5):1505-1522
Meteorite “finds” from the terrestrial hot deserts have become a major contributor to the inventory of Martian meteorites. In order to understand their nitrogen and noble gas components, we have carried out stepped heating experiments on samples from two Martian meteorites collected from hot deserts. We measured interior and surface bulk samples, glassy and non-glassy portions of Dar al Gani 476 and Sayh al Uhaymir 005. We have also analyzed noble gases released from the Antarctic shergottite Lewis Cliff 88516 by crushing and stepped heating. For the hot desert meteorites significant terrestrial Ar, Kr, Xe contamination is observed, with an elementally fractionated air (EFA) component dominating the low temperature releases. The extremely low Ar/Kr/Xe ratios of EFA may be the result of multiple episodes of trapping/loss during terrestrial alteration involving aqueous fluids. We suggest fractionation processes similar to those in hot deserts to have acted on Mars, with acidic weathering on the latter possibly even more effective in producing elementally fractionated components. Addition from fission xenon is apparent in DaG 476 and SaU 005. The Ar-Kr-Xe patterns for LEW 88516 show trends as typically observed in shergottites - including evidence for a crush-released component similar to that observed in EETA 79001. A trapped Ne component most prominent in the surface sample of DaG 476 may represent air contamination. It is accompanied by little trapped Ar (20Ne/36Ar > 50) and literature data suggest its presence also in some Antarctic finds. Data for LEW 88516 and literature data, on the other hand, suggest the presence of two trapped Ne components of Martian origin characterized by different 20Ne/22Ne, possibly related to the atmosphere and the interior. Caution is recommended in interpreting nitrogen and noble gas isotopic signatures of Martian meteorites from hot deserts in terms of extraterrestrial sources and processes. Nevertheless our results provide hope that vice-versa, via noble gases and nitrogen in meteorites and other relevant samples from terrestrial deserts, Martian secondary processes can be studied. 相似文献
18.
We present results of a study of the 53Mn-53Cr isotope systematics in the enstatite chondrites and achondrites (aubrites). The goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short-lived radionuclide 53Mn. Our earlier work (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2000a) has shown that the asteroid belt bodies are characterized by essentially the same initial 53Mn abundance. However, we have found the presence of a gradient in the abundance of the radiogenic 53Cr between the earth-moon system, Mars, and the asteroid Vesta. If this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated. This can be explained either by an early, volatility controlled Mn/Cr fractionation in the nebula or by an original radially heterogeneous distribution of 53Mn. The enstatite chondrites are suggested to form in the inner zones of the solar nebula, much closer to the Sun than the ordinary chondrites. Therefore, their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53Mn.We have studied the bulk samples of the EH4-chondrites Indarch and Abee and the EL6-chondrite Khairpur. Although these meteorites have essentially the same Mn/Cr ratio as the ordinary chondrites, the relative abundance of the radiogenic 53Cr is three times smaller than in the ordinary chondrites. Because these meteorites are primitive (undifferentiated) and no Mn/Cr fractionation had occurred within their parent bodies, this difference is a strong argument in favor of an initially heterogeneous distribution of 53Mn in the early inner solar system. This finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula. Using the characteristic 53Cr excess of the enstatite chondrites and the observed gradient, their place of origin falls at about 1.4 AU or somewhat closer to the Sun (i.e. >1.0-1.4 AU).We also present chronological results for the enstatite chondrites and achondrites. The ‘absolute’ 53Mn-53Cr ages of the EH4-chondrites are old: ∼4565 Ma. The EL6-chondrite Khairpur is ∼4.5 Ma younger, which is in good agreement with the 129I-129Xe data from the literature. The age of the aubrite Peña Blanca Spring appears to be similar to those of the enstatite chondrites while that of the aubrite Bishopville is at least ∼10 Ma younger, which is also in agreement with the 129I-129Xe data. The results from bulk samples of aubrites indicate that the last Mn/Cr fractionation in their parent body occurred ∼ 4563 Ma ago and imply an evolution of the Mn-Cr isotope system in an environment with an higher than chondritic Mn/Cr ratio for several millions of years. 相似文献
19.
Yves Marrocchi Angelina Razafitianamaharavo Bernard Marty 《Geochimica et cosmochimica acta》2005,69(9):2419-2430
Noble gases trapped in meteorites are tightly bound in a carbonaceous carrier labeled “phase Q.” Mechanisms having led to their retention in this phase or in its precursors are poorly understood. To test physical adsorption as a way of retaining noble gases into precursors of meteoritic materials, we have performed adsorption experiments for Ar, Kr, and Xe at low pressures (10−4 mbar to 500 mbar) encompassing pressures proposed for the evolving solar nebula. Low-pressure adsorption isotherms were obtained for ferrihydrite and montmorillonite, both phases being present in Orgueil (CI), for terrestrial type III kerogen, the best chemical analog of phase Q studied so far, and for carbon blacks, which are present in phase Q and can be considered as possible precursors.Based on adsorption data obtained at low pressures relevant to the protosolar nebula, we propose that the amount of noble gases that can be adsorbed onto primitive materials is much higher than previously inferred from experiments carried out at higher pressures. The adsorption capacity increases from kerogen, carbon blacks, montmorillonite to ferrihydrite. Because of its low specific surface area, kerogen can hardly account for the noble gas inventory of Q. Carbon blacks in the temperature range 75 K-100 K can adsorb up to two orders of magnitude more noble gases than those found in Q. Irreversible trapping of a few percent of noble gases adsorbed on such materials could represent a viable process for incorporating noble gases in phase Q precursors. This temperature range cannot be ruled out for the zone of accretion of the meteorite precursors according to recent astrophysical models and observations, although it is near the lower end of the temperatures proposed for the evolving solar nebula. 相似文献
20.
Henner Busemann Silvio Lorenzetti Otto Eugster 《Geochimica et cosmochimica acta》2006,70(21):5403-5425
We analyzed the spallogenic, trapped, fissiogenic and radiogenic noble gas components in various bulk samples of the angrites D’Orbigny and Sahara 99555 as well as in glass separates of D’Orbigny. The D’Orbigny glass samples show hints of solar-like noble gases, as deduced from the trapped elemental and Ne isotopic compositions; the bulk samples do not contain detectable amounts of trapped gases. These observations indicate that D’Orbigny experienced a complex history shortly after its formation 4.56 Ga ago. The glass of D’Orbigny most likely represents magma that rose from the interior of the angrite parent body (APB) and was quenched near the surface. Hence, the APB may contain—similar to the interior of Earth and Mars—solar noble gases. This would call into question the suggested trapping mechanism for solar noble gases in the Earth and Mars, which involves the solution of early atmospheres into magma oceans, due to the APB’s inability to retain a primordial atmosphere. The first detection of—possibly parentless—radiogenic excess 129Xe and solar noble gases in the glass of D’Orbigny indicates that the interior of APB degassed to a lesser degree than the outer regions. Therefore primordially trapped, fossil 129I was kept. The APB was not completely devolatilized. Sahara 99555 yields a cosmic-ray exposure age of 6.8 ± 0.3 Ma, while D’Orbigny was exposed to cosmic rays for 11.9 ± 1.2 Ma. Both ages are different than those found in the other angrites. Hence, the angrites analyzed so far sampled surface material from the APB that was ejected in at least five events. In contrast to the bulk sample, the D’Orbigny glass separates yield concordant ages of only 3.0 ± 1.1 Ma, apparently suggesting a pre-exposure of the host material. However, such a scenario is unlikely, due to very similar Mn-Cr ages found in the bulk and glass of D’Orbigny. Most likely, this discrepancy is the result of additional, secondary gas-free glass. Such glass might have been formed during the meteorite’s entry into the Earth’s atmosphere. Isotopically anomalous Xe due to the decay of 247Cm has not been found. The presence of 247Cm in glass of D’Orbigny has been suggested based on Pb isotope constraints. 相似文献