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1.
The coarse-grained, Ca-rich inclusions in the Allende meteorite are the highest-temperature condensates from the cooling solar nebula and, as such, the oldest solid objects in the solar system. All refractory elements with condensation points above the accretion temperature of the inclusions whose concentrations in them have been measured are seen to be present in the inclusions in unfractionated proportion to one another relative to C1 chondrites when data are averaged for a large number of inclusions. Observational data for U and theoretical data for both U and Pu suggest that these elements exhibited refractory behavior in the solar nebula. An experiment is proposed in which fissiogenic Xe and U contents are measured in a suite of these inclusions to obtain the244Pu/238U ratio of the solar system at the time of initial condensation with an uncertainty of ±15%.  相似文献   

2.
The abundances of nine rare earth elements (REE) in phosphate separates from three ordinary chondrites, Saint Séverin (LL6), Bruderheim (L6) and Richardton (H5), were measured by instrumental neutron activation analysis. All REE except europium are enriched in the phosphate minerals (merrillite and chlorapatite) by factor of 200–300 relative to the chondritic average, whereas Eu is enriched by a factor of 40–50. Electron microprobe analysis showed no significant differences in phosphate mineral composition among the three chondrites studied, though the relative proportions of two minerals varied.According to our data, REE are enriched by almost the same factor in merrillite and chlorapatite in the Bruderheim and, with less certainty, in the other two chondrites. This behavior of REE contrast with that of the actinoid elements, Th, U and Pu, which are also enriched in phosphate but are fractionated between merrillite and chlorapatite. Since Pu and REE show different fractionation behavior in chondritic phosphates, it may be difficult to use REE as stand-ins for Pu in244Pu chronology.  相似文献   

3.
Recent models of Ca, Al-rich inclusion (CAI) petrogenesis suggest that refractory inclusions may be residues of interstellar dust aggregates that were incompletely evaporated and partially melted in the solar nebula. These models, and the recent availability of new thermodynamic data, have led us to re-examine the traditional interpretation that lithophile refractory trace elements (LRTE) condensed as oxides in solid solution in refractory major condensates, while refractory noble metals (RNM) condensed as micron-sized nuggets of Pt-metal alloys. Calculations of LRTE-RNM alloy stability fields under nebular oxygen fugacities and partitioning experiments lead us to conclude that: (1) Ti, Zr, Nb, Hf, U, and Ta form stable alloys with RNM under nebular conditions; (2) the observation that metallic Zr, Nb, and Ta occur in some Pt-metal nuggets and grains is explained by the stability of these LRTE-RNM alloys under normal nebular oxygen fugacities; (3) metallic Ti, Hf, and U may also occur in some nuggets; (4) the lanthanides, the other actinides (Th, Pu), and Y do not form stable alloys, and thus probably do not occur alloyed with RNM; and (5) the partitioning of U (but not Th, Pu, or the REE) into RNM is a novel actinide and REE/actinide fractionation mechanism that is based on metal/silicate fractionation (rather than on the relative volatility of their oxides).We propose that micron-sized Pt-metal nuggets formed from smaller grains of RNM alloys and compounds during the evaporation and melting of primitive dust aggregates. This process would have been enhanced by: (1) the possibility that the RNM were present as compounds (especially with As and S) as well as metallic alloys in interstellar dust and in some primitive meteoritical material, since they often exhibit non-siderophile behavior; and (2) the fluxing of volatiles through CAI's during distillation. Microscopic nuggets are common in melilite chondrules, indicating that residence in a slowly-cooled silicate melt may have favored their formation. Cation diffusivity and variations in localfO2 can explain why metallic LRTE-bearing nuggets are not common in CAI's (despite the relative stability of LRTE-RNM alloys). We propose that the lithophile component of Fremdlinge is enriched in super-refractory elements, and that Group II CAI's formed from Fremdlinge-poor dust. We interpret the Group II REE fractionation as a pre-solar event, and predict that Nd/Sm dating will yield an age greater than the canonical age of the solar system. If metal/silicate fractionation in a cold solar nebula can explain Group II REE patterns, the possibility that Group II CAI's are also distillation residues cannot be excluded.  相似文献   

4.
Isotopic dilution technique and mass spectrometry are used in order to measure isotopic compositions and concentrations of some particular elements on the Oklo sample KN50-3548, which are interesting for describing the nuclear reaction. These analyses give coherent results leading to the determination of the following parameters: neutron fluence, spectrum index, total number of fissions relative to uranium, and percentage of238U and239Pu fissions. From the percentage of plutonium fissions, the duration of the nuclear reaction is estimated to be 800,000 years. The age of the nuclear reaction is measured from the lead analysis and from the number of uranium fissions; these two determinations give respectivelyd = 1730 ± 10m.y. andd = 2000 ± 100m.y. The second method, based on the assumption that uranium and rare earths have not migrated, could mean that the nuclear reaction might be older than the Francevillian. It is also shown that no contamination by natural elements has occurred for lead, rare earths, palladium and rare gases, after the reaction.  相似文献   

5.
Fossil fission tracks have been found in a coarse-grained white inclusion of the Allende chondrite. Tracks are present in excess of those produced by238U spontaneous fission and cosmic rays. The ratio of excess tracks to238U tracks is ～20, intermediate to ratios previously observed in meteorites but much lower than might be expected in light of the high initial244Pu/U ratio measured in these inclusions from Xe isotope ratios.  相似文献   

6.
Solid source mass spectrometry has been used to determine the relative cumulative fission yields of five elements in three samples of uranium ore from reactor zones in the Oklo mine site. Eighteen fission chains covering the mass range from105 ≤ A ≤ 130 have been measured for Pd, Ag, Cd, Sn and Te. These measurements have enabled a number of nuclear parameters to be calculated including the relative proportions of235U,238U and239Pu involved in the fission process.The concentration of the five elements in the Oklo samples have also been measured using the stable isotope dilution technique. These values have then been compared to the estimates of the amount of these elements produced by fission under the conditions that are appropriate to the three samples. This procedure enables the retentivity of the elements in the reactor zones to be evaluated. Our work confirms the fact that Pd and Te are retained almost in their entirety in the samples, whereas the other three elements have been partially lost from the reactor site. Almost all the Cd fission products have been lost, and more than 50% of the Ag and Sn fission-produced material has been removed.  相似文献   

7.
The actinides exhibit a number of unique chemical and nuclear properties. Of particular interest are the man-made actinides (Np, Pu, Cm and Am) that are produced in significant enough quantities that they are a source of energy in fission reactions, a source of fissile material for nuclear weapons and of environmental concern because of their long half-lives and radiotoxicity. During the past 50 yr, over 1400 mT of Pu and substantial quantities of the “minor” actinides, such as Np, Am and Cm, have been generated in nuclear reactors. There are two basic strategies for the disposition of these elements: (1) to “burn” or transmute the actinides using nuclear reactors or accelerators; (2) to “sequester” the actinides in chemically durable, radiation-resistant materials that are suitable for geologic disposal. There has been substantial interest in the use of isometric pyrochlore, A2B2O7 (A=rare earths; B=Ti, Zr, Sn and Hf), for the immobilization of actinides, particularly plutonium. Systematic studies of rare-earth pyrochlores have led to the discovery that certain compositions (B=Zr, Hf) are stable to very high doses of α-decay event damage. The radiation stability of these compositions is closely related to the structural distortions that occur for specific pyrochlore compositions and the electronic structure of the B-site cation. This understanding provides the basis for designing materials for the safe, long-term immobilization and sequestration of actinides.  相似文献   

8.
We have developed a technique for revealing nuclear tracks in the mineral hibonite (CaAl12O19), found in the refractory inclusions from carbonaceous chondrites. The tracks in hibonitesfrom Murchison carbonaous chondrite are dominated by fission tracks from244Pu (constituting more than 90% of the total). The measured uranium contents in these crystals range from 1.2 to 62 ppb. We deduce that the average value for the244Pu/238U ratio in most of the Murchison hibonites at the time of track retention is0.022 ± 0.011.  相似文献   

9.
We report on extensive isotopic studies of Pb, Sr and Xe and on chemical abundance measurements of K, Rb, Sr, Ba, Nd, Sm, U and Th for total meteorite and mineral separates of the Angra dos Reis achondrite. U-Pb, Th-Pb and Pb-Pb ages are concordant at 4.54 AE for the total meteorite and for high-purity whitlockite in Angra dos Reis. This establishes Angra dos Reis as an early planetary differentiate which has not been disturbed for these systems since 4.54 AE ago. Measured87Sr/86Sr in pyroxene and whitlockite for Angra dos Reis (ADOR) are distinctly below BABI by two parts in 104 and only one part in 104 above the lowest87Sr/86Sr (ALL) measured in an Allende inclusion. The difference in ADOR-ALL corresponds to an interval of condensation in the solar nebula of ～3 m.y. If26Al was the heat source for the magmatism on the parent planets of Angra dos Reis and the basaltic achondrites (BABI) then the relatively large difference in87Sr/86Sr, BABI - ALL, must be the result of planetary evolution rather than condensation over ～10 m.y. Xe isotopic measurements confirm the presence of large amounts of244Pu-produced fission Xe and show that244Pu was enriched in the whitlockite relative to the pyroxene by a factor of ～18. We present chemical element enrichment factors between the whitlockite and the fassaitic pyroxene in Angra dos Reis. The enrichment factors demonstrate close analogy between the rare earth elements and their actinide analogs. The enrichment factor for Pu is intermediate to the enrichment factors of Nd and Sm.  相似文献   

10.
Distribution of trace elements in spinel and garnet peridotites   总被引：6，自引：1，他引：5  
The distribution of trace elements in the upper mantle has been discussed on the basis of the trace element abundances in bulk rocks and constituent minerals of two spinel and garnet facies peridotite xenoliths in alkali basalts from eastern China. The data presented are consistent with the suggestion that highly incompatible elements (Rb, Ba, Th, U, Sr, Nb, Ta) mainly reside in intergranular components, and to a lesser extent in fluid inclusions in minerals. The LILE composition in olivine and orthopyroxene can be seriously affected by the presence of fluid inclusions. Consequently the subsolidus partitioning of the LILE cannot be used to infer the olivine-melt and orthopyroxene-melt partition coefficients for these elements. There is a significant difference in (Opx/Cpx)HREE ratios for spinel and garnet peridotites, suggesting a P-T control on equilibrium partition coefficients.  相似文献   

11.
The times at which phyllosilicate matrix and euhedral olivines became associated have been determined for five C2 meteorites. The ages, calculated from fission track densities on crystal surfaces, are based on an initial244Pu/238U ratio in the matrix material of 0.0154 at 4.6 b.y., and range from 4.22 b.y. for Nogoya to 4.42 b.y. for Murray. Unless the initial244Pu/238U ratio was less than 0.004, the meteorites cannot have existed in their present form for 4.6 × 109 yr. The measured ages place limits on the time when pre-compaction effects such as micrometeorite craters and solar flare tracks were produced, and they may approximately date the formation of the olivine crystals themselves.  相似文献   

12.
The analyses of plutonium isotopes in dated strata of polar ice sheets indicate that the241Pu/239+240Pu activity ratio produced in the U.S.-dominated nuclear tests during the 1950s was about 26, while that in the U.S.S.R.-dominated weapons tests in the early 1960s was between 12 and 14. This difference provides time horizons for sedimentary deposits. Further, the239Pu/240Pu ratio may show a similar difference in fallout values from these two periods of weapons testing and may provide an additional, and perhaps more sensitive, chronology for sediments.  相似文献   

13.
(1) The observed anomalies in meteoritic oxygen isotope compositions are not due to an incomplete mixing of several dust or gas-plus-dust components in the solar nebula. If they were, other elements would display similar anomalies. (The FUN inclusions in Allende appear to be exceptions to this premise.) (2) The anomalies must therefore stem from differing degrees of incomplete exchange of oxygen isotopes between the primordial gas and dust components of the nebula. The dust is more likely to have been the16O-enriched component. (3) Since the isotopic difference between dust and gas probably could not have been preserved if the dust was ever completely vaporized in the nebula, the Ca,Al-rich inclusions (CAI's) in carbonaceous chondrites are unlikely to be condensates, but instead are distillation residues. (4) If so, the observed depletion of super-refractory elements in the Group II CAI's cannot have been accomplished by fractional condensation in the solar nebula. (5) Then this depletion, and a number of other properties of the components of primitive meteoritic material, must be relics of pre-solar system fractionations among different populations of interstellar dust grains.  相似文献   

14.
Relative yields of Kr and Xe isotopes from the spontaneous fission of 248Cm and 250Cf have been determined mass spectrometrically. The yields are as follows: 83Kr/84Kr/85Kr/86Kr = 0.223/0.458/0.596/ ≡ 1.00 and 0.306/0.582/0.793/ ≡ 1.00;131Xe/132Xe/134Xe/136Xe = 0.486/0.819/1.075/ ≡ 1.00 and 0.343/0.506/0.851/ ≡ 1.00 from 248Cm and 250Cf, respectively. The Xe yields from 248Cm agree with an earlier determination by Leich et al. [24]. Neither of these yield patterns matches that of “fissiogenic” Kr and Xe in carbonaceous chondrites and hence 248Cm and 250Cf are ruled out as progenitors of the meteoritic Kr and Xe. In general, none of the spontaneously fissioning nuclides of actinide elements can be identified as a possible progenitor. Even the mixtures of actinides, including a combination of 248Cm and 250Cm, are unsuitable. The origin of “anomalous” Kr and Xe in carbonaceous chondrites must then be traced either to the spontaneous fission of a superheavy element or to peculiarities in specific nucleosynthetic reactions.  相似文献   

15.
Alkali basalt, trachybasalt and basanite magmas, containing abundant xenoliths of upper mantle origin, were erupted during the Plio-Pleistocene (2.4-0.14 Ma) in northern Sardinia. The magmas are enriched in K, Rb, Th and Ba relative to mid-ocean ridge basalts (MORB) and most ocean island basalts (OIB), resulting in high K/Nb, Th/Nb, Ba/Nb and Rb/Nb ratios. The large number of spinel peridotite inclusions in these lavas suggests that these chemical features cannot be explained by combined assimilation and fractional crystallization within the continental crust. However, volcanic rock chemistry can be explained by the assimilation of sialic rocks by turbulently convecting, mafic magmas during their ascent to the surface. Fractionation of Ba and K from the light rare earth elements (LREE) is required to explain the positive correlation of K/La and Ba/La with 87Sr/86Sr(i). Consequently, bulk assimilation of crystalline basement rocks by rising, hot basaltic magmas cannot explain the observed chemical trends, and preferential melting of a low melting quartzo-feldspathic crustal component probably occurred, leaving the REE in residual phases such as apatite, zircon, sphene and amphibole. Alternatively, large ion lithophile element (LILE) enrichment may have been related to interaction of rising mafic lavas with metasomatized lithospheric mantle or enriched asthenosphere.  相似文献   

16.
The concentrations of 25 major, minor and trace elements have been determined in four clasts, a metal-rich inclusion and two dark metal-poor inclusions from the Abee enstatite chondrite. The clasts are heterogeneous, displaying 2-fold enrichments or depletions in some elements. The data suggest that there are two generations of metal, one with low, the other with high concentrations of refractory siderophiles. The other elemental patterns can be understood in terms of variations in the abundance of major minerals. We infer that Sc and Mn are located largely in the niningerite ((Fe,Mg)S), V in the troilite (FeS) and rare earth elements in the oldhamite (CaS).Heterogeneities among the clasts are probably primary, resulting from the accretion-agglomeration process, although shock processes in a regolithic setting remain a possibility provided that they were followed by a period of metamorphism sufficient to erase petrologic evidence.In the dark inclusions the concentrations of the rare earths, Eu excepted, are 4 × higher than mean EH levels; this infers enhanced amounts of CaS. The dark inclusions are low in siderophiles, Sc, Mn, K, Na and Al, implying low amounts of metal, niningerite and feldspar. The origin of the dark inclusions is unclear; they do not appear to be the result of a simple, single-stage process.  相似文献   

17.
广东南山花岗岩体位于陂头复式岩体西端,锆石的SHRIMP U-Pb年龄为158.1±1.8Ma,是燕山早期岩浆活动的产物。岩石化学特征显示岩体以高硅、富碱、贫Ca和Mg以及高TFeO/MgO、低CaO/Na2O为特征。其K2O/Na2O〉1,A/NK=7.8~11.92,A/CNK=1.33~1.68,属过铝质碱性岩石。在稀土和微量元素组成上,岩石富含稀土元素（除明显的负Eu异常,δEu=0.09~0.16）以及Zr、Y、Th、U、Nb等高场强元素,贫Ba、Sr、Ti等,高10000x Ga/Al（比值大于2.6）。在Zr、Nb、Ce、Y对10000×Ga/Al以及TFeO/MgO-SiO2等A型花岗岩多种判别图上,投影点主要落在A型花岗岩区,而与高分异的I、S型花岗岩明显不同。这些特征均指示,南山岩体具有铝质A型花岗岩的特点。通过Y-Nb-3Ga和Y-Nb-Ce构造环境判别图解将其进一步划分为A2型花岗岩,代表其形成于拉张的构造背景之下。本文在此研究基础上,认为南山花岗质岩浆可能形成于相对挤压的中侏罗世。而在晚侏罗世早期相对拉张的作用下,岩石圈减薄,软流圈地幔上涌,地壳的泥质岩和少量砂质岩受到幔源流体富集后发生部分熔融后上侵形成铝质A型花岗岩,且有较强的结晶分异作用。  相似文献   

18.
Measurements are reported of K, Rb and Sr levels and Sr isotopic compositions in silicate inclusions in diamonds, and of U, Th and Pb levels and Pb isotopic compositions in sulphide inclusions in diamonds. Samples are from the Premier Mine (Transvaal), the Finsch Mine (northern Cape Province) and the Kimberley mines (northern Cape Province). The isotopic composition of Pb in sulphide inclusions indicates that the diamonds containing these inclusions are xenocrysts in the host kimberlite. Model Pb ages for the sulphide inclusions in the diamonds from Finsch and Kimberley are in excess of 2 b.y., although the host kimberlites are Cretaceous. The sulphide inclusions from Premier, however, have a model age similar to the emplacement age of the Premier kimberlites (approximately 1.2 b.y.).In addition, K, Rb, Sr, U and Pb concentrations and Sr and Pb isotopic compositions in clinopyroxenes from 14 mantle-derived xenoliths were measured. Samples were eclogites from the Roberts Victor Mine (Orange Free State), peridotites from the Matsoku Pipe (Lesotho) and diopside megacrysts from the four mines in Kimberley. The samples from the Roberts Victor Mine define a large spread in Sr and Pb isotopic composition. The slope of a regression line through the Pb data in the 207/204 vs. 206/204 plot defines an age of roughly 2.5 b.y. The Matsoku samples have87Sr/86Sr ratios around 0.704. One group of samples (“fertile peridotites”) shows uniform, anomalously radiogenic Pb isotopic compositions, and bears evidence of a young event involving isotopic homogenisation. Lead isotopic heterogeneity, indicating an old age, has persisted in another group (“common peridotites”). The megacrysts from the four Kimberley mines have87Sr/86Sr ratios around 0.704 and uniform, anomalously radiogenic Pb isotopic compositions, indicating a young age. Their Pb isotopic uniformity contrasts with the spread shown by kimberlites in Kimberley, and shows that the megacrysts did not crystallise in equilibrium with their host kimberlites.The similarity between the Pb isotopic data obtained on clinopyroxenes from xenoliths and the data obtained on sulphide inclusions in diamonds from Finsch and Kimberley suggests that these diamonds might be cogenetic with old rock provinces in the subcontinental mantle.  相似文献   

19.
The Earth's mantle contains a mixture of primordial noble gases, in particular solar-type helium and neon, and radiogenic rare gases from long-lived U, 232Th, 40K and short-lived 129I, 244Pu. Rocks derived from deep mantle plume magmatism like on Hawaii or Iceland contain a higher proportion of primordial nuclides than rocks from the shallow upper mantle, e.g. mid ocean ridge basalts (MORBs). This is widely regarded as the key evidence for survival of a less degassed and more “primitive” reservoir within the lower mantle. We present an evaluation of noble gas composition showing the shallow mantle to have about five times more radiogenic (relative to primordial) isotopes than Hawaii/Iceland-type plume reservoirs, no matter if short- or long-lived decay systems are considered. This fundamental property suggests that both MORB and plume-type noble gases are mixtures of: (1) a homogeneous radiogenic component present throughout most of the mantle and (2) a uniform primordial noble gas component with very minor radiogenic ingrowth. This conclusion depends crucially on the observed excess of radiogenic Xe in plume-derived rocks, and is only valid if this Xe excess is inherent to the plume sources.Possible sources of the primordial component of mantle plume reservoirs—and possibly also the MORB mantle—could be mantle reservoirs that remained relatively isolated over most of Earth's history (“blobs”, a deep abyssal layer, or the D” layer), but these need a considerable concentration of primordial gases to compensate U, Th, K decay over 4.5 Ga. Earth's core is evaluated as an alternative viable source feeding primordial nuclides into mantle reservoirs: even low metal-silicate partitioning coefficients allow sufficient primordial noble gases to be incorporated into the early forming core, as the undifferentiated proto-Earth was initially gas-rich. Massive mantle degassing soon after core formation then provides the opposite concentration gradient that allows primordial noble gases reentering the mantle at the core-mantle boundary, probably via partial mantle melts. Another possible source of primordial noble gases in Earth's mantle are subducted sediments containing extraterrestrial dust with solar He and Ne, but this supply mechanism crucially depends on largely unconstrained parameters. The latter two scenarios do not require the preservation of a “primitive” mantle reservoir over 4.5 Ga, and can potentially better reconcile increasing geochemical evidence of recycled lithospheric components in mantle plumes and seismic evidence for whole mantle convection.  相似文献   

20.
Allan Hills (ALHA) 77005 is a newly discovered, unique achondrite from Antarctica. Petrologic similarities with the shergottites in terms of mineralogy, oxidation state, inferred source region composition, shock metamorphic effects and shock ages suggest a genetic relationship. Volatile to involatile element ratios (e.g. K/U, Rb/U, Cs/U, Tl/U) and abundances of other trace elements support this hypothesis. ALHA 77005 may be a cumulate that crystallized from a liquid parental to those from which the shergottites crystallized; alternatively it may be a sample of the type of source peridotite from which shergottite parent liquids were derived by partial melting. Chemical similarities with terrestrial ultramafic rocks suggest that this unique meteorite provides an additional sample of the only other solar system body known to have basalt source regions chemically similar to the upper mantle of the Earth.  相似文献   

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