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1.
Robert Keith Moniot 《Geochimica et cosmochimica acta》1980,44(2):253-271
Acid-resistant residues were prepared by HCl-HF demineralization of three H-type ordinary chondrites: Brownfield 1937 (H3), Dimmitt (H3,4), and Estacado (H6). These residues were found to contain a large proportion of the planetary-type trapped Ar, Kr, and Xe in the meteorites. The similarity of these acid residues to those from carbonaceous chondrites and LL-type ordinary chondrites suggests that the same phase carries the trapped noble gases in all these diverse meteorite types. Because the H group represents a large fraction of all meteorites, this result indicates that the gas-rich carrier phase is as universal as the trapped noble-gas component itself. When treated with an oxidizing etchant, the acid residues lost almost all their complement of noble gases. In addition, the Xe in at least one oxidized residue, from Dimmitt, displayed isotopic anomalies of the type known as CCFX or DME-Xe, which is characterized by simultaneous excesses of both the lightest and heaviest isotopes. The anomaly in the Dimmitt sample differs from that observed in carbonaceous-chondrite samples, however, in the relative proportions of the light- and heavy-isotope excesses.The results of this study do not show an inverse correlation between trapped and trapped 36Ar abundance, as has been reported for acid-resistant residues from LL-chondrites. The results of this work therefore fail to support the hypothesis that meteoritic trapped noble gas abundances were established at the time of condensation. 相似文献
2.
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
3.
Ramakant R. Mahajan 《地学前缘(英文版)》2017,8(1):205-210
The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components. 相似文献
4.
Nadia Vogel Heinrich Baur Addi Bischoff Rainer Wieler 《Chemie der Erde / Geochemistry》2011,(2):135-142
We analyzed noble gases and determined 3He, 21Ne, and 38Ar cosmic ray exposure ages (CREAs) of Rumuruti chondrites from North West Africa (NWA) to rule on potential pairings and/or source pairings of North Africa R chondrite samples. The 21Ne exposure ages range between 10 and 74 Ma, with NWA 2897 and 1668 having the highest known exposure ages among R chondrites. We also include other R chondrites from North Africa (Schultz et al., 2005) and, based on their noble gas characteristics and their 21Ne CREAs, propose pairings of the following samples: NWA 2198, 5069, 755, 4615, 845, 851, 978, 1471, and possibly DaG 013 belonging to one fall with a CREA of ∼10 Ma, and NWA 753, 4360, 4419, 5606, 1472, 1476, 1477, 1478, and 1566 representing one fall with a CREA of ∼14 Ma. NWA 2821, 2503, 2289, 3364, 3146, 4619, 4392, 3098, and 2446 seem to belong to one single fall with a CREA of ∼20 Ma, and NWA 2897 and 1668 seem to be paired and show a common CREA of ∼66 Ma. Overall, all R chondrite samples from North Africa analyzed for noble gases so far represent ∼16 individual falls. Comparing falls from North Africa to literature CREAs of R chondrites worldwide, it seems possible that a significant number of all R chondrite falls studied for noble gases were ejected from the R chondrite parent body during one large collisional event between 15 and 25 Ma ago. However, the database is still too small to draw definitive conclusions. The large portion of brecciated R chondrites in collections suggests severe impact brecciation of the R chondrite parent body. 相似文献
5.
Abundances of cosmic ray-produced noble gases and 26Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 106 yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic 3He, 21Ne, and 38Ar in chondrites to an adopted ‘average’ shielding: The measured unsaturated 26Al concentrations and the calculated equilibrium 26Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10?8 cm3/g 106 yr) are: 3He = 2.45,21Ne = 0.47, and 38Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived. 相似文献
6.
He, Ne and Ar isotopes have been measured in six, and 26Al in two diogenites. Cosmic-ray exposure ages corrected for shielding effects using 22Ne/21Ne ratios are generally concordant. Five diogenites have a group age of 14 Myr and three others may have a group age of 24 Myr, implying that two collisions may have produced 8 of the 9 diogenites. 相似文献
7.
Solubilities of noble gases in magnetite were determined by growing magnetite in a noble-gas atmosphere between 450 and 700°K. Henry's law is obeyed at pressures up to 10?2 atm for He, Ne, Ar and up to 10?5 atm for Kr, Xe, with the following distribution coefficients at 500° (cc STP g?1 atm?5): He 0.042, Ne 0.016, Ar 3.6, Kr 1.3, Xe 0.88, some 102–105 times higher than previous determinations on silicate and fluoride melts. Apparent heats of solution in kcal/mole are: He ?2.42 ±0.12, Ne ?2.20 ±0.10, Ar ?15.25 ±0.25, Kr ?13.0 ±0.3, Xe ?12-5 ± 0.5. These values, too, stand in sharp contrast with earlier determinations on melts which were small and positive, but are comparable to the values for clathrates. Presumably the gases are held in anion vacancies.Extrapolation of the magnetite data to the formation temperature of C1 chondrites, 360°K, shows that the Arp36 content of Orgueil magnetite could be acquired by equilibrium solubility at a total nebular pressure of 4 × 10?6 atm. In the absence of data for silicates (the principal host phase of planetary gas), an attempt is made to estimate the solubilities required to account for planetary gases in meteorites. These values do not appear grossly unreasonable in the light of the magnetite data, when structural differences between the two minerals are taken into account. It seems that equilibrium solubility may be able to account for four features of planetary gas: elemental ratios, amounts, correlations with other volatiles and retentive siting. It cannot account for the isotopic fractionation of planetary gas, however. 相似文献
8.
Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for 53Mn (t1.2 = 3.7Myr), 26Al(t1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for 26Al and 53Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for 26Al and 53Mn to deduce the respective saturation values 26Al0 and 53Mn0 and to obtain 21Ne production rates (21P).26Al and 53Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of 21p based on 53Mn and 26Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (?2.5Myr) increase in the cosmic ray intensity. 相似文献
9.
Noble gas data are reported for 12 E-chondrites. Combined with literature data, they show that K-Ar ages are >4 Æ for 14 out of 18 meteorites, yet U, Th-He ages are often shorter, perhaps due to late, mild reheating. Cosmic-ray exposure ages differ systematically between types 4 and 6, with E4's mostly below 16 Myr and E6's above 30 Myr. This may mean that the E-chondrite parent body contains predominantly a single petrologic type on the (~ 1 km) scale of individual impacts, in contrast to the more thoroughly mixed parent bodies of the ordinary chondrites.The heavy noble gases consist of at least two primordial components: the usual planetary component () and a less fractionated, ‘subsolar’ component (). The latter is found in highest concentration in the E4 chondrite South Oman (36Ar = 760 × 10?8cc/g, ). The isotopic compositions of both components are similar to typical planetary values, indicating that some factor other than mass controlled the noble gas elemental ratios. The heavy Xe isotopes occasionally show some of the lowest and ratios measured in bulk chondrites, suggestive of nearly fission-free Xe (e.g. ). Amounts of planetary gas in E4 E6 chondrites fall in the range for ordinary chondrites of types 4–6, but, in contrast to the ordinary chondrites. fail to correlate with petrologic type or volatile trace element contents. Another unusual feature of E-chondrites is that primordial Ne is present even in most 4's and 5's (20Nep ~ 1 to 7 × 10?8cc/g). with an isotopic composition consistent with planetary Ne.Analyses of mineral separates show that the planetary gases are concentrated in an HF- and HCl-insoluble mineral similar to phase Q, the poorly characterized, HNO3-soluble carrier of primordial gases in carbonaceous and ordinary chondrites. The subsolar gases, on the other hand, are located in an HCl- and HNO3-resistant phase, possibly enstatite or a minor phase included in enstatite. Much of the 129Xer (50% for E4's, > 70% for E6's) is in HCl-resistant but HF-soluble sites, suggestive of a silicate.A similar subsolar component may be responsible for the high ratios of some C3's, unequilibrated ordinary chondrites, and the unique aubrite Shallowater. The planet Venus also has a high ratio, well above the planetary range, and hence may have acquired its noble gases from an E-chondrite-like material, similar to South Oman. 相似文献
10.
《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
11.
Jun-Ichi Matsuda Roy S. Lewis Hiroshi Takahashi Edward Anders 《Geochimica et cosmochimica acta》1980,44(11):1861-1874
Noble gases were measured in bulk samples of the C3V chondrites Grosnaja, Vigarano, and Leoville, and in HF,HCl-insoluble residues before and after etching with HNO3. The residues were characterized by INAA and SEM. Gas components were determined, directly or by subtraction, for the following fractions: HF,HCl-solubles (?98% of the meteorite), ‘sphase Q’, a poorly characterized trace mineral that is insoluble in HCl-HF but soluble in HNO3, and an insoluble residue, consisting of ferrichromite, carbonaceous matter, and spinel.Bulk meteorites show some correlation of the noble-gas pattern with McSween's subclasses: two ‘oxidized’ C3V's—Allende (LEWIS et al, 1975) and Grosnaja— have lower Ar/Xe but higher Ne/Xe ratios than the ‘reduced’ C3V's—Vigarano and Leoville—which are transitional to LL3's and C3O chondrites in both respects. An HCl-soluble mineral of high Ar/Xr ratio seems to be responsible. In other respects, the 3 C3V's of this study resemble Allende, with only moderate differences. Phase Q contains most of the Ar, Kr, Xe, but only small amounts of Ne; the etched residues contain planetary Ne () and the controversial CCFXe component, enriched in the heavy Xe isotopes (). The CCFXe is accompanied by an ‘L-Xe’ component that is enriched in the light Xe isotopes. The proportion of the two is virtually constant in C3V's. as in all other C-chondrites. in contrast to the ~ 2-fold variation in ordinary chondrites.C3V's have systematically higher ratios, and hence higher ratios of CCFXe to planetary Xe, than do other chondrite classes. This may reflect some peculiarity in their formation conditions, favoring uptake of CCFXe. 相似文献
12.
Noble gases in three meteoritic samples were examined by stepwise heating, in an attempt to relate peaks in the outgassing curves to specific minerals: NeKrXe in Allende (C3V) and an Allende residue insoluble in HF-HCl, and Xe in Abee (E4). In Allende, chromite and carbon contain most of the trapped Ne (20Ne/22Ne ≈ 8.7) and anomalous Xe enriched in light and heavy isotopes, and release it at ~850°C (bulk meteorite) or 1000°C (residue). Mineral Q, containing most of the trapped Ar, Kr, Xe as well as some Ne (20Ne/22Ne ≈ 10.4), releases its gases mainly between 1200 and 1600°C, well above the release temperatures of organic polymers (300–500°) or amorphous carbon (800–1000°). The high noble-gas release temperature, ready solubility in oxidizing acids, and correlation with acid-soluble Fe and Cr all point to an inorganic rather than carbonaceous nature of Q.All the radiogenic 129Xe is contained in HCl, HF-soluble minerals, and is distributed as follows over the peaks in the release curve: Attend 1000° (75%), 1300° (25%); Abee (data of Hohenberg and Reynolds, 1969) ~850° (15%), 1100° (60%), 1300° (25%). No conclusive identifications of host phases can yet be given; possible candidates are troilite and silicates for Allende, and djerfisherite, troilite and silicates for Abee.Mineral Q strongly absorbs air xenon, and releases some of it only at 800–1000°C. Dilution by air Xe from Q and other minerals may explain why temperature fractions from bulk meteorites often contain less 124–130Xe for a given enrichment in heavy isotopes than does xenon from etched chromitecarbon samples, although chromite-carbon is the source of the anomalous xenon in either case. Air xenon contamination thus is an important source of error in the derivation of fission xenon spectra. 相似文献
13.
O Eugster N Grögler M.D Mendia P Eberhardt J Geiss 《Geochimica et cosmochimica acta》1973,37(9):1991-2003
He, Ne, Ar, Kr and Xe concentrations and isotopic abundances were measured in three bulk grain size fractions prepared from sample L-16-19, No. 120 (C level, 20–22 cm depth) returned by the Luna 16 mission. The expected anticorrelation between the concentrations of trapped solar wind noble gases and grain size is observed. Elemental abundances of solar wind trapped noble gases are similar to those previously found in corresponding grain size fractions of the Apollo 11 and 12 fines. The trapped ratio varies in the soils from different lunar maria due to diffusion losses. A rough correlation of with the proportion of ilmenite in these samples is apparent. The elemental and isotopic ratios of the surface correlated noble gases in Luna 16 resemble those previously found in Apollo fines. Based on 21Ne, 78Kr and 126Xe a cosmic ray exposure age of 360 my was determined. This age is similar to those obtained for the soils from other lunar maria. 相似文献
14.
The C3O chondrites Kainsaz, Lancé and Ornans were studied by an acid dissolution technique, to characterize the noble-gas components in 3 mineral fractions: HF, HCl-solubles (99% of the meteorite), chromite and carbon (0.3–0.9%), and ‘phase Q’, a poorly characterized trace mineral (0.05–0.4%) containing most of the Ar, Kr, Xe. For all fractions, gas contents decline in the order Kainsaz > Lancé > Ornans; this trend parallels volatile contents but not heterogeneity of olivine composition or degree of metamorphism and seems to reflect progressively higher condensation temperatures from the solar nebula.Solubles contain nearly unfractionated Xe, and show ratios up to 850. Hence the high ratios (200–400) of bulk C3O chondrites must be due to an HF-soluble mineral (possibly magnetite). Phase Q contains ordinary planetary gases and a Ne component of Chromite and carbon contain Ne of and ‘CCF’ xenon (a peculiar component of possibly fissiogenic origin, enriched in the heavy isotopes but accompanied by a component enriched in the light isotopes).In all primitive chondrites, both the amount and the chemical separability of CCFXe parallel the abundance of promordial noble gases and other volatiles, such as C, N, Tl, Bi and In. The close correlation of CCFXe with various properties of undoubtedly local origin (volatile content, petrologic type, presence of ferrichromite and carbon, etc.) is more consistent with a local than with an extrasolar origin of this component. A volatile superheavy element seems to be the most plausible source, but the evidence is not conclusive. 相似文献
15.
《Comptes Rendus Geoscience》2007,339(14-15):937-945
The origin of the Earth's atmosphere can be constrained by the study of noble gases in oceanic basalts. If it is clear that the mantle is degassed and formed part of the present atmosphere, it has been proposed that an important subduction of atmospheric noble gases in the mantle occurred during Earth's history, altering the primordial signature of the solid Earth. This subduction process has been suggested on the basis of the measurements of light xenon isotopes in CO2 well gases. Moreover, the fact that the 38Ar/36Ar ratio is atmospheric in all oceanic basalts, even for uncontaminated samples (e.g. with high 20Ne/22Ne), may also suggest that a massive subduction of atmospheric argon occurred, if the primitive Earth had a solar-like 38Ar/36Ar. This also implies that the atmosphere suffered a massive gas loss accompanied by mass fractionation (e.g. hydrodynamic escape) after mantle degassing or that a late veneer with an atmospheric composition occurred. Such a hypothesis is explored for rare gases, by developing a model in which degassing and subduction of atmospheric noble gases started ∼4.4 Ga ago. In the model, both radiogenic and non-radiogenic isotopic ratios are used (e.g. 38Ar/36Ar and 40Ar/36Ar; 124Xe/130Xe and 129Xe/130Xe) to constrain the subduction flux and the degassing parameters. It is shown that subduction and massive contamination of the entire mantle is possible, but implies that the 40Ar/36Ar and the 129Xe/130Xe ratios were higher in the past than today, which is not observed in Archean samples. It also implies that the sediments and the altered oceanic crust did not loose their noble gases during subduction or that the contaminated mantle wedge is mixed by the convective mantle. Moreover, such a model has to apply to the oceanic island source, since this later shows the same signature of argon and xenon non-radiogenic isotopic ratios. A scenario where the isotopic compositions of the argon and xenon were settled before or during accretion is therefore preferred to the subduction. 相似文献
16.
Thomas J. Bernatowicz 《Contributions to Mineralogy and Petrology》1981,76(1):84-91
This work reports the results of noble gas (Ne, Ar, Kr, Xe) analyses of accidental mantle xenoliths from San Carlos, Arizona. Except for the addition of radiogenic 40Ar and mass fractionation effects, the isotopic structures of these gases are indistinguishable from atmospheric composition. The absence of 129Xe excesses in these rocks may reflect indirect mixing of atmospheric gases with the source region of the xenoliths. The dominant influence on the noble gas abundances in the San Carlos xenoliths appears to have been diffusive gas loss, which may have occurred in a mantle metamorphic event or during contact with the host basanite magma. Evidence is presented for the partitioning of significant amounts of the heavy noble gases into fluid inclusions in the xenolith minerals; the proportion of each gas in the inclusions increases with increasing atomic weight of the gas, possibly reflecting solubility effects. The noble gases are present in greater concentration in pyroxenes than in olivine, similar to the behavior of other incompatible elements. 相似文献
17.
18.
We determined the He, Ne, and Ar isotopic abundances in the four acapulcoites Dhofar (DHO) 125, DHO 290, DHO 312, and Graves Nunataks 98028, the metal-rich diogenite Northwest Africa (NWA) 1982, and a unique achondrite, NWA 1058, that resembles the acapulcoites in its chemical composition. The noble gases in these meteorites consist of three components: trapped gases, cosmic-ray produced nuclides, and nuclides produced by K, Th, and U decay. The four acapulcoites yield cosmic-ray exposure (CRE) ages in the range of 5.0-5.7 Ma and confirm earlier conclusions concerning break-up of all acapulcoites from a common S-type parent asteroid, possibly in three events 4.9, 5.9, and 14.8 Ma ago. We also discuss the other characteristics (mineralogy, chemistry, formation ages, and oxygen and trapped noble gas isotopes) of all other acapulcoites and their relatives, the lodranites. We propose that the acapulcoite/lodranite parent asteroid had a shell structure similar to that of the H chondrites: The less metamorphosed acapulcoites correspond to the H3 and H4 chondrites and originate from the exterior layers, whereas the more severely metamorphosed lodranites, similar to the H5 and H6 chondrites, represent the inner regions of their parent body. Ungrouped achondrite NWA 1982, probably a diogenite, shows a CRE age of 18.9 ± 2.0 Ma that falls on the major exposure age cluster of the diogenites. The unique achondrite NWA 1058 differs in cosmic-ray exposure age (38.9 ± 4.0 Ma) and in oxygen-isotopic composition from the acapulcoites and lodranites and is probably a winonaite. 相似文献
19.
Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio 38Ar/36Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of 40Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported 36Ar excesses due to possible decay of extinct 36Cl in the Efremovka meteorite.Trapped 22Ne in the range of (1.4-2.5) × 10−8 cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. 21Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309)All ureilites may have started with nearly the same initial elemental ratio (132Xe/36Ar)0, established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios 132Xe/36Ar and 84Kr/36Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (132Xe/36Ar)0 is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ17O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magmaThe elemental ratios 132Xe/36Ar and 84Kr/36Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ionThe similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio 129Xe/132Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of 129I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature. 相似文献
20.
The noble gases (He, Ne, Ar, Kr and Xe) are powerful geochemical tracers because they have distinctive isotopic compositions in the atmosphere, crust and mantle. This study illustrates how noble gases can be used to trace fluid origins in high-temperature metamorphic and mineralising environments; and at the same time provides new information on the composition of noble gases in deeper parts of the crust than have been sampled previously.We report data for H2O and CO2 fluid inclusions trapped at greenschist to amphibolite facies metamorphic conditions associated with three different styles of mineralisation and alteration in the Proterozoic Mt Isa Inlier of Australia. Sulphide fluid inclusions are dominated by crustal 4He. However, co-variations in fluid inclusion 20Ne/22Ne, 21Ne/22Ne, 40Ar/36Ar and 136Xe/130Xe indicate noble gases were derived from three or more reservoirs. In most cases, the fluid inclusions elemental noble gas ratios (e.g. Ne/Xe) are close to the ranges expected in sedimentary and crystalline rocks. However, the elemental ratios have been modified in some of the samples providing evidence for independent pulses of CO2, and interaction of CO2 with high-salinity aqueous fluids.Compositional variation is attributed to mixing of: (i) magmatic fluids (or deeply sourced metamorphic fluids) characterised by basement-derived noble gases with 20Ne/22Ne ∼ 8.4, 21Ne/22Ne ∼ 0.4, 40Ar/36Ar ∼ 40,000 and 136Xe/130Xe ∼ 8; (ii) basinal-metamorphic fluids with a narrow range of compositions including near-atmospheric values and (iii) noble gases derived from the meta-sedimentary host-rocks with 20Ne/22Ne ∼ 8-9.8, 21Ne/22Ne < 0.1, 40Ar/36Ar < 2500 and 136Xe/130Xe ∼ 2.2.These data provide the strongest geochemical evidence available for the involvement of fluids from two distinct geochemical reservoirs in Mt Isa’s largest ore deposits. In addition the data show how noble gases in fluid inclusions can provide information on fluid origins, the composition of the crust’s major lithologies, fluid-rock interactions and fluid-fluid mixing or immiscibility processes. 相似文献