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1.
Xenon isotopic data were acquired by high resolution step pyrolysis and combined step pyrolysis/combustion of aliquots of size separated nanodiamonds. 129Xe excess (129Xe*) from in situ decay of 129I is preferentially associated with the larger grain size separates. This observation rules out trapping by recoil from surrounding material. The releases of Xe-P3 and 129Xe occur in the same low temperature pyrolysis steps and exhibit similar distributions among the size separates. These observations imply a common site for the components and, in consequence, suggest a common incorporation event.Whether one component or two, our observations require that 129Xe* and Xe-P3 were incorporated into a subpopulation of nanodiamonds before nanodiamonds were mixed and incorporated into parent bodies. Their susceptibilities to loss during heating in the laboratory are similar, but the ratio of 129Xe* to Xe-P3 varies among nanodiamond separates from different meteorites (literature data). We conclude that the 129Xe* we observe today was present as 129I during parent body processing. Furthermore, the range of 129Xe*/132XeP3 ratios across all the separates requires that even nanodiamonds from CI chondrites were at least 5-10× more rich in Xe-P3 during 129I decay than they are today.We present a simple model involving one degassing event per parent body between incorporation of nanodiamonds and final decay of 129I. The observed variations among parent bodies require degassing events separated by several 129I half lives (∼50Ma), consistent with low-temperature processing on parent bodies but longer than expected for nebular processing. In this model, nanodiamonds from ALHA77307 degassed at an unusually early stage, suggesting they alone may retain the signature of processing in the nebula in their P3 and 129Xe* abundances.The isotopic signature associated with Xe-P6 is also found only in the larger size separates. Concentration of Xe-HL increases with increasing grain size, but its relative abundance with respect to Xe-P3 and P6 is higher in smaller grain-size fractions. We argue that Xe-P6 is best seen as a variant of Xe-HL, and that they are both mixtures of a “normal” component akin to solar xenon and a slightly variable exotic component. We show that both current models of Xe-H formation can account for the observed variability, and propose a scenario according to which Xe-HL and P6 were implanted into separate diamond populations before incorporation of Xe-P3 and 129I.  相似文献   

2.
Twenty-two dark inclusions (DIs) from Allende (18), Leoville (2), Vigarano (1) and Efremovka (1) were studied by the I-Xe method. All except two of these DIs (Vigarano 2226 and Leoville LV2) produce well-defined isochrons, and precise I-Xe ages. The Allende DIs formed a tight group about 1.6 Ma older than Shallowater (4.566 ± 0.002 Ga), about 5 Ma older than four previously studied Allende CAIs. Most of the dark inclusions require trapped Xe with less 129Xe (or more 128Xe) than conventional planetary Xe (well restricted in composition by Q-Xe or OC-Xe). Studies of an irradiated/unirradiated DI pair from Allende demonstrate that the 128Xe/132Xe ratio in trapped is normal planetary, so that a 129Xe/132Xe ratio below planetary seems to be required. Yet, this is not possible given constraints on 129Xe evolution in the early solar system. Trends among all of the Allende DIs suggest that an intimate mixture of partially decayed iodine and Xe formed a pseudo trapped Xe component enriched in both 129Xe and 127I, and subsequently in 128Xe after n-capture during reactor irradiation. Enrichment in radiogenic 129Xe, but with a 129Xe/127I ratio less than that observed in the iodine host phase, places closure of this trapped mixture ≥13 Ma after precipitation of the major iodine-bearing phase. Because the I-Xe isochron is a mixing line between iodine-derived and trapped Xe (pseudo or not), I-Xe ages, given by the slope of this mixing line, are not compromised by the presence of pseudo trapped Xe, and the precision of the I-Xe ages is given by the statistics of the line fit.  相似文献   

3.
We present I-Xe analyses of ten chondrules from Chainpur LL3.4 by IR laser-stepped heating. Five chondrules provided isochrons of varying quality, giving a range of ages from 0.5 Ma before Shallowater to 17.8 after Shallowater. This confirms the extended range of Chainpur chondrule ages determined by previous data. We discuss evidence for fluid alteration, shock, and thermal events in explaining the chondrule ages and suggest that chondrule remelting events, presumably from bombardment of the parent body surface, are responsible for resetting the I-Xe chronometer. Previous data show a negative correlation between 132Xe/129Xe of the trapped Xe component and 127I/129I of an initial iodine component. This behaviour that requires the presence of a component with trapped 129Xe/132Xe lower than the planetary value has been cited as evidence for closed system evolution of the I-Xe system. We find no evidence of an unambiguous trapped component lower than planetary and no evidence of a negative correlation in our data. Therefore we suggest that open system behaviour more suitably explains the I-Xe systematics of Chainpur chondrules.  相似文献   

4.
The IAB iron meteorites may be related to the chondrites: siderophile elements in the metal matrix have chondritic abundances, and the abundant silicate inclusions are chondritic both in mineralogy and in chemical composition. Silicate and troilite (FeS) from IAB irons were analyzed by the I-Xe technique. Four IAB silicate samples gave well-defined I-Xe ages [in millions of years relative to Bjurböle; the monitor error (± 2.5 Myr) is not included]: ?3.7 ± 0.3 for Woodbine, ?0.7 ± 0. 6 for Mundrabilla, +1.4 ± 0.7 for Copiapo, and +2.6 ± 0.6 for Landes. The (129Xe/132Xe)trapped ratios are consistent with previous values for chondrites, with the exception of Landes which has an extraordinary trapped ratio of 3.5 ± 0.2. Both analyses of silicate from Pitts gave anomalous I-Xe patterns.Troilite samples were also analyzed: Pitts troilite gave a complex I-Xe pattern, which suggests an age of +17 Myr; Mundrabilla troilite defined a good I-Xe correlation, which after correction for neutron capture on 128Te gave an age of ?10.8 ± 0.7 Myr. Thus, surprisingly, low-melting troilite substantially predates high-melting silicate in Mundrabilla.Abundances of Ga, Ge, and Ni in metal from these meteorites are correlated with I-Xe ages of the silicate; meteorites with older silicates have greater Ni contents. No model easily accounts for this result as well as other properties of IAB irons; nevertheless, these results, taken at face value, overall favor a nebular formation model (e.g. Wasson, 1970, Icarus 12, 407–423). The great age of troilite from Mundrabilla suggests that this troilite formed in a different nebular region from the silicate and metal, and was later mechanically mixed with these other phases.The correlation between the trace elements in the metal and the I-Xe ages of the silicate provides one of the first known instances in which another well-defined meteoritic property correlates with I-Xe ages. In addition, almost all the 129Xe in Mundrabilla silicate (etched in acid) was correlated with 128Xe. These two results further support the validity of the I-Xe dating method.  相似文献   

5.
129Xe, from the decay of the now-extinct 16.7 Ma129I, accumulates in iodine-bearing sites and since most iodine host phases are secondary, the I-Xe system is typically a chronometer for post-formational processes. The validity of the I-Xe chronometer is confirmed by comparison with Pb-Pb ages on phosphate and feldspar separates from twelve meteorites. Phosphate separates are found to be concordant with Pb-Pb for all six samples in which useful I-Xe data were obtained. Feldspar is a better iodine host than apatite in H chondrites, typically providing good I-Xe isochrons. These too are concordant with the Pb-Pb ages of the corresponding phosphates for five out of six feldspar separates. The exception is Allegan whose feldspar yields one of the oldest I-Xe ages observed, similar to those for CI and CM magnetites. We attribute this to a more primary mineralization, predating the secondary phosphate from which the comparison Pb-Pb age was obtained. Absolute I-Xe ages, found using the reported Pb-Pb age of Acapulco phosphate provide an absolute I-Xe age of 4.566 ± 0.002 Ga for both Shallowater and Bjurböle irradiation standards. This allows relative I-Xe ages to be interpreted in the context of absolute ages.  相似文献   

6.
Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and 40Ar-39Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic 40Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred PuU ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most 16O-rich meteorite known, gives a complex 40Ar-39Ar age pattern which suggests a recent (?0.85 Ga) severe thermal disturbance. The absence of excess 129Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between 40Ar-39Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.  相似文献   

7.
The I-Xe chronometer is based upon decay of now-extinct 129I where the ratio of accumulated daughter 129Xe to stable 127I reflects the iodine isotopic ratio at closure of the host mineral. Since none of the parent remains, I-Xe is by nature a relative chronometer but, when referenced by a standard mineral of known age, the I-Xe system becomes an absolute chronometer reflecting true closure times. Most iodine hosts are secondary minerals so the I-Xe system is unique in providing details of post-formational chronometry not readily available with other chronometers. The short half-life of 129I gives it exceptional precision. However, the secondary nature of iodine host minerals, combined with the inherent precision of I-Xe, were responsible for a large database of “whole-rock” I-Xe ages that were not easily interpreted. As this problem evolved historically, doubts were cast upon the viability of the I-Xe system as a chronometer which persisted until it was tested against other chronometers in single-mineral systems. Properly calibrated, absolute I-Xe ages reflect the true closure time of the host minerals, and sequences of closure times in different hosts provide cooling rates for the parent object.  相似文献   

8.
Solid-state 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopic experiments have been performed on isolated meteoritic Insoluble Organic Matter (IOM) spanning four different carbonaceous chondrite meteorite groups; a CR2 (EET92042), a CI1 (Orgueil), a CM2 (Murchison), and the unique C2 meteorite, Tagish Lake. These solid state NMR experiments reveal considerable variation in bulk organic composition across the different meteorite group’s IOM. The fraction of aromatic carbon increases as CR2 < CI1 < CM2 < Tagish Lake. The increases in aromatic carbon are offset by reductions in aliphatic (sp3) carbon moieties, e.g., “CHx,” and “CHx(O,N).” Oxidized sp2 bonded carbon, e.g., carboxyls and ketones grouped as “CO,” are largely conservative across these meteorite groups. Single pulse (SP) 13C magic angle spinning (MAS) NMR experiments reveal the presence of nanodiamonds with an apparent concentration ranking in the IOM of CR2 < CI1 < CM2 < Tagish Lake. A pair of independent NMR experiments reveals that, on average, the aromatic moieties in the IOM of all four meteoritic IOM fractions are highly substituted. Fast spinning SP 1H MAS NMR spectral data combined with other NMR experimental data reveal that the average hydrogen content of sp3 bonded carbon functional groups is low, requiring a high degree of aliphatic chain branching in each IOM fraction. The variation in chemistry across the meteorite groups is consistent with alteration by low temperature chemical oxidation. It is concluded that such chemistry principally affected the aliphatic moieties whereas the aromatic moieties and nanodiamonds may have been largely unaffected.  相似文献   

9.
Analysis of noble gas proportions and their release kinetics during stepped pyrolysis and oxidation of meteoritic nanodiamonds, as well as their core-shell structure led to the following conclusions: (1) Noble gases of HL component with anomalous isotopic composition were presumably formed prior to implantation in the nanodiamonds owing to mixing of nucleosynthetic products of p- and r- process associated with explosion of type-II supernova with noble gases having “normal” isotopic composition; (2) isotopically normal P3 noble gases in the nanodiamonds grains are confined to the nondiamond (for instance, graphite-like) phase in the surface layer. The “layer” structure of nanodiamonds grains resulted from heating up to 800–900°C. Observed increase in contents of P3 noble gases with increasing grain sizes of meteoritic nanodiamonds is caused by the dependence of the degree of graphitization of the superfical layer at given temperature on the grain size and surface defect density; (3) bimodal release of noble gases during pyrolysis of the meteoritic nanodiamonds from weakly metamorphosed meteorites was caused by P3 and HL components, which are comparable in abundance but sharply differ in their release temperature.  相似文献   

10.
We report data on neutron capture products of the secondary cosmic ray component, the inferred proton and neutron fluences, and the identification of double beta decay of 82Se in heavily shielded samples of the Cape York iron meteorite. One purpose of this study is to develop a new chronometer for cosmic ray exposure, based on the nuclides 129I (16 My half-life) and 129Xe from low energy cosmic ray reactions on Te. The abundance ratio of these two nuclides permits the determination of an (effective) exposure age of 93 ± 16 My, which represents the first exposure age datum of Cape York. The very small concentrations of spallogenic 38Ar = 6.5 × 10−10 cm3 STP/g in the metal and troilite (per g Fe) document the heavily shielded locations of our sample. An excess of 129Xe in the troilite is shown to be entirely due to the decay of cosmic-ray-produced 129I. On the other hand, an inclusion in the troilite reveals the presence of 129Xe from extinct 129I and documents its ~4.5 Gy formation age. Mono-isotopic excess of 82Kr is identified as due to ββ-decay of 82Se with an inferred half-life of 1.0 × 1010 y. This represents the first ββ-decay product observed in a meteorite.  相似文献   

11.
Analysis of the C, N, and Xe release kinetics of intermediate-sized nanodiamond fraction from the Orgueil CI meteorite during isothermal pyrolysis conducted for the first time and subsequent oxidation indicates that (a) the rate of C, N, and Xe release at pyrolysis at a constant temperature decreases with time; (b) the relative amount of released Xe, which mostly has a normal isotopic composition (Xe-P3) at various pyrolysis time up to 800°C, is controlled, first of all, by the heating temperature, whereas the amount of N is controlled by both the temperature and heating time; and (c) prolonged pyrolysis notably modifies the distribution of nitrogen of normal (δ15N = 0) and anomalous (δ15N= ?350‰) isotopic composition in diamond grains. The identified features of the C and N release kinetics are explained by differences in the binding energy of chemically adsorbed O with C atoms and the accommodation of the main amounts of N in extended defects of the crystal structure of nanodiamond. The major factors of the decrease in the Xe-P3 release rate during the isothermal pyrolysis of nanodiamond are either the differences between the Xe desorption parameters of the traps in graphite-like phases containing Xe-P3 or the differences between the radiation-induced defectiveness of grains of the population containing implanted Xe-P3. Our results led us to conclude that (1) meteoritic nanodiamond contains relatively low amounts of a phases carrying the P3 component of noble gases, regardless of the nature of this component, and (2) the population of nanodiamond grains containing most of isotopically anomalous nitrogen was produced at a high rate to preserve this nitrogen, first of all, at extended defects in the diamond crystal structure.  相似文献   

12.
Parameters for a number of neutron irradiations are examined and results intercompared for the Bjurböle meteorite; data for the 1967 Valecitos-1 irradiation are presented. Apparent I-Xe ‘formation’ ages are reproducible for three different samples of Bjurböle, suggesting isotopic homogeneity for initial iodine in the bulk material. The systematics of neutron capture in 135Xe (produced from 235U neutron fission) are examined and verified in irradiated BCR-1.  相似文献   

13.
A model for the composition of meteoritic nanodiamonds is suggested based on analysis of the concentrations and isotopic compositions of C, N, and Xe in the nanodiamond-rich grain-size fractions, which were separated for the first time from the Orgueil CI chondrite. According to the model, meteoritic nanodiamond consists of two populations of grains (denoted CHL and CN). The size distributions of grains in populations in the CHL and CN populations are different: the CHL population is finer grained than CN. The grains of the CHL population are characterized by a radial gradient in the carbon isotopic composition, and they contain implanted anomalous noble gases (HL component) and the heavy nitrogen isotope 15N. Following (Clayton et al., 1995), the probable astrophysical source of this population of nanodiamond grains is thought to be the mixing helium and hydrogen shells of a Type-II supernova, and the mechanism that produced these grains was the slow CVD process. The CN population grains have homogeneous isotopic compositions of carbon (δ13C ≡–100‰) and nitrogen (δ15N ≡–400‰) and contain almost all nitrogen of the nanodiamond-rich fractions. This population of nanodiamond grains was likely formed by a fast unequilibrated process, when shock waves affected organic compounds or gas rich in C- and N-bearing compounds during the early evolution of the protosolar nebula. Calculations within the framework of the model show that the nanodiamond-rich fractions separated from the Orgueil meteorite have the CN/CHL ratios varying from 1 in the finest grained fraction to 10 in the coarse-grained one. At these proportions of the populations, weighted mean δ13C values of CHL grains in the fractions lie within the range of 42 to 394‰, and the concentrations of 132Xe-HL and 15N are (49–563) × 10–8 cm3/gC and (1.1–6.2) × 10–5 cm3/gC, respectively.  相似文献   

14.
Three troilite- and pentlandite-rich samples from the Allende C3 chondrite were analyzed for Xe (and in one case Ne and Ar) by mass spectrometry, in 13–22 temperature steps. All samples released a small ‘CCFXe’ component (enriched in the heavy isotopes Xe134, 136) at the relatively low temperature of 700–800°C, ahead of adsorbed atmospheric Xe (~900°C), radiogenic Xe129 (1000°C), and primordial Xe (1250°C). Though such a labile component suggests implanted fission recoils, the simultaneous release of Ne, Ar, and Xe124, 126 shows that it instead comes from carbon and perhaps chromite, two major host phases of CCFXe. Apparently small amounts of these phases are occluded in sulfides, and decompose by chemical reaction upon heating. Thus the experiment fails to resolve the nature of CCFXe.A marked enrichment of Xe124, without corresponding enrichments in Xe126 or Xe131–136, was observed in the 550–650° and 1400–1500° fractions. Though requiring confirmation, it supports earlier evidence for the complexity and variability of the light xenon component, contrary to claims that it is an integral part of CCFXe.  相似文献   

15.
Portales Valley, Sombrerete, and Northwest Africa (NWA) 176 are three unrelated meteorites, which consist of silicate mixed with substantial amounts of metal and which likely formed at elevated temperatures as a consequence of early impacts on their parent bodies. Measured 39Ar-40Ar ages of these meteorites are 4477 ± 11 Ma and 4458 ± 16 Ma (two samples of Portales Valley), 4541 ± 12 Ma, and 4524 ± 13 Ma, respectively (Ma = million years; all one-sigma errors). The Ar-Ar data for Portales Valley show no evidence of later open system behavior suggested by some other chronometers. Measured 129I-129Xe ages of these three meteorites are 4559.9 ± 0.5 Ma, 4561.9 ± 1.0 Ma, and ∼4544 Ma, respectively (relative to Shallowater = 4562.3 ± 0.4 Ma). From stepwise temperature release data, we determined the diffusion characteristics for Ar and Xe in our samples and calculated approximate closure temperatures for the K-Ar and I-Xe chronometers. Adopting results and interpretations about these meteorites from some previous workers, we evaluated all these data against various thermal cooling models. We conclude that Portales Valley formed 4560 Ma ago, cooled quickly to below the I-Xe closure temperature, then cooled deep within the parent body at a rate of ∼4 °C/Ma through K-Ar closure. We conclude that Sombrerete formed 4562 Ma ago and cooled relatively quickly. NWA 176 likely formed and cooled quickly ∼4544 Ma ago, or later than formation times of most meteorite parent bodies. For all three meteorites, the Ar-Ar ages are in better agreement with I-Xe ages and preferred thermal models if we increase these Ar-Ar ages by ∼20 Ma. Such age corrections would be consistent with probable errors in 40K decay parameters in current use, as suggested by others. The role of impact heating and possible disruption and partial reassembly of meteorite parent bodies to form some meteorites likely was an important process in the early solar system.  相似文献   

16.
The release kinetics of Xe of the isotopically normal component of noble gases (P3 component) from the coarse-grained fraction of nanodiamonds from the Orgueil (CI) meteorite and the kinetics of 4He release from lunar soil were studied by means of a numerical simulation. It is demonstrated that the release of these gases as a peak with a single pronounced maximum may not correspond to the diffusion model with a single activation energy and can in fact be controlled by a spectrum of activation energies with a number of peaks a number of peaks remaining unresolved at stepped pyrolysis. In particular, the amount of Xe-P3 preserved in nanodiamonds during thermal metamorphism of the Orgueil meteorite calculated using parameters of the diffusion process (activation energy and frequency factor) that were determined in the model with a single activation energy indicates that practically all Xe should be lost during a very short time. These losses are inconsistent with both the duration of thermal metamorphism of the meteorite parent bodies and the Xe-P3 concentrations measured in these meteorites. A much higher preservation of Xe-P3 during thermal metamorphism lasting for hundreds of years follows from calculations based on diffusion with a spectrum of activation energiesa for Xe release. The results of isothermal pyrolysis of a nanodiamonds fraction from Orgueil confirms a presence of several activation energies for Xe-P3 release from the nanodiamonds. The application of the diffusion model with a spectrum of activation energies to He release from lunar soil samples also shows that He can be retained in these samples at 20°C during a much longer time than it follows from the model with a single activation energy (Anufriev, 2010).  相似文献   

17.
Sixteen amorphous carbon (lampblack) samples that had been exposed to Xe127 and pumped for >9 hrs to remove the most labile gas were examined by etching with HNO3, for comparison with the release pattern of meteoritic xenon. Samples originally exposed at 100–200°C lost 90% of their Xe very readily, when the surface had been etched to a mean depth of only ~0.2 Å. This suggests that the Xe is adsorbed mainly at rare sites that are unusually reactive to HNO3. The adsorbed Xe survived several months' storage in vacuum, but on exposure to air, part of it was lost within a few hours, while the remainder persisted without measurable exchange. Samples exposed at 800–1000°C had a similar adsorbed component, as well as a second, tightly bound component extending to a mean depth of up to 30 Å; this component had apparently diffused into the carbon during exposure. The (microscopic) diffusion coefficient for graphitic crystallites is 5 × 10?20 cm2/sec at 1000°C.PVDC carbon lost its adsorbed Xe at about the same rate as lampblack on exposure to air or HNO3, though it differs from lampblack in being non-graphitizable and more porous. It had only a small diffused component, however.The most tightly bound part of the Xe adsorbed on lampblack resembles planetary Xe in most characteristics: surface siting, etchability, persistence in vacuum, and lack of exchange with atmospheric Xe. The Xe concentrations—if interpreted as equilibrium distribution coefficients—are some 106× too small to account for meteoritic Xe, but it appears that equilibrium had not been reached by any of the samples, even after 1 day's exposure to Xe. If the uptake of Xe is controlled by rate rather than equilibrium, then the high noble gas concentrations in meteorites may simply reflect the much longer uptake times in the solar nebula. It seems likely that the trapping mechanisms discussed here can also explain two other features: elemental fractionations of noble gases, and the close correlation between planetary Xe and CCFXe.  相似文献   

18.
Noble gases trapped in primitive meteorites are quantitatively hosted by a poorly defined organic phase, labeled phase Q. Xenon is enriched in heavy isotopes by +1.30 ± 0.06% per atomic mass unit (amu, 1σ) in phase Q relative to solar. To understand the origin of this fractionation, we have performed adsorption experiments of xenon atoms and ions, ionized in a radiofrequency plasma. Within the reaction vessel, anthracite was heated and the resulting smoke deposited onto the walls of the vessel, resulting in carbon-rich films. Xenon was trapped in the carbon films either as ions in the ionization zone of the vessel, or as neutral atoms outside this zone. Xenon trapped as ionic Xe is tightly bound and is enriched by +1.36 ± 0.05%/amu (1σ) in heavy isotopes, reproducing the isotopic fractionation of xenon trapped in phase Q relative to solar. Neutral xenon is more loosely trapped, is in much lower concentration, and is not isotopically fractionated. Ionized conditions allow the constant xenon isotopic composition observed in meteorite during stepwise heating release to be reproduced. Furthermore, the trapping efficiency of Xe+ estimated from these experiments is consistent with the high xenon concentration measured in phase Q of primitives meteorites.Xenon was not trapped in the film by implantation because the energies of the incident Xe atoms and ions were far too low (<1 eV). From the difference of behavior between ionic and neutral forms, we propose that xenon ions were trapped via chemical bonding at the surface of the newly created C-rich film. The observed mass-dependent fractionation of xenon is unlikely to have occurred in the gas phase. It is more probably related to variations in chemical bonding strengths of Xe isotopes as chemical bonds involving heavy Xe isotopes are more stable than those involving light ones. For young stars, including the young Sun, photons emitted in the far UV energy range able to ionize noble gases (<100 nm) were orders of magnitude more abundant than for the Present-day Sun, allowing efficient ionization of gaseous species. A way to achieve Q-noble gas fractionation and trapping was UV irradiation by nearby young stars from O/B association of the surface of growing organic grains in the outer part of the solar system or by the young Sun at the edge of the disk.  相似文献   

19.
Silicate inclusions from two IIE iron meteorites were dated by the I-Xe and 40Ar-39Ar techniques. Weekeroo Station, a ‘normal’ IIE iron, shows no loss of radiogenic 40Ar at low temperature, and the well-defined 40Ar-39Ar plateau yields an age of 4.54 ± 0.03 Byr. The xenon data define a good I-Xe correlation with an age of +10.9 ± 0.5 Myr relative to Bjurböle [the monitor error (±2.5 Myr) is not included].^Despite its relatively young age, Weekeroo Station's (129Xe132Xe)trappad ratio (= 0.84 ± 0.05) lies significantly below the solar value. Netschaëvo silicate has a chondritic composition, unlike ‘normal’ IIE silicate which is more differentiated. Nevertheless Netschaëvo gives a 40Ar-39Ar plateau-age of only 3.79 ± 0.03 Byr, with the xenon data failing to define an I-Xe isochron.Only irons from the IAB and IIE groups contain silicate inclusions, but these two groups differ in many other respects, mostly suggesting that IAB meteorites are more primitive. The I-Xe chronology supports this suggestion inasmuch as Weekeroo Station formed well after (8–15 Myr) IAB silicates. In terms of Scott and Wasson's (1976) model, ages for Weekeroo Station date the shock event which formed ‘normal’ IIE irons by mixing the low-melting fraction of the parent silicate with shock-liquefied metal. Scott and Wasson's suggestion that Netschaëvo represents IIE parent material, however, is contradicted by Netschaëvo's 3.8 Byr age.The four silicate-bearing IIE irons which have now been dated can be subdivided into distinct pairs: Weekeroo Station and Colomera formed near the beginning of the solar system, while Netschaëvo and Kodaikanal both formed only 3.8 Byr ago. A review of other properties of these meteorites generally support this subdivision.This work underscores the complexity of the history of IIE meteorites; in particular, an adequate model must account for the formation of two IIE irons at 3.8 Byr without disturbing rare gases in Weekeroo Station. All formation models are quite speculative, but the one which seems best to fit the available evidence postulates two parent bodies: the 3.8 Byr old silicate formed on one parent body, all other IIE material resided in a separate body, and subsequent collision(s) mixed the young silicate with IIE metal.  相似文献   

20.
We have determined the isotopic composition of the xenon component trapped in EETA 79001 lithologies B and C, which we refer to as SPB-xenon. SPB-xenon is isotopically distinct from known xenon reservoirs, but differs in regular fashion. Normalized to 132Xe, the light isotope ratios are indistinguishable from air, the 129Xe/132Xe ratio is about 2.4, and 134Xe and 136Xe are enhanced relative to the terrestrial atmosphere or AVCC. The apparent heavy-isotope enrichments are not generated by in situ fission and there is no spectral evidence for the presence of 244Pu fission xenon. However, the xenon composition does match that of fractionated AVCC except at 129Xe, and consequently may be derived from or related to that component. ALHA 77005, Shergotty and EETA 79001 lithology A also have enhanced 129Xe/132Xe ratios in most temperature steps, and are seemingly consistent with varying mixtures of SPB-xenon and terrestrial xenon.Our results for neon and argon in EETA 79001 confirm earlier results on the exposure ages. We have also verified that the trapped 38Ar/36Ar ratio in lithology C is apparently substantially different from the terrestrial or meteoritic value. Krypton in EETA 79001,C is more fractionated with respect to AVCC than is terrestrial krypton and in the opposite direction as xenon. EETA 79001,C contains excess 80Kr (and perhaps 82Kr and 128Xe), presumably from neutron capture on bromine and iodine, but these neutron captures do not appear to have occurred by in situ processes.  相似文献   

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