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1.
Reasonable assumptions concerning activity coefficients allow the calculation of the relative volatility of the actinide elements under conditions expected during the early history of the solar system. Several of the light rare earths have volatilities similar to Pu and Cm and can be used as indicators of the degree of fractionation of these extinct elements. Uranium is considerably more volatile than either Pu or Cm, leading to fractionations of about a factor of 50 and 90 in the Pu/U and Cm/U ratio in the earliest condensates from the solar nebula. Ca, Al-rich inclusions from the Allende meteorite, including the coarse-grained inclusions, have a depletion of U relative to La of about a factor of three, suggesting that these inclusions may have been isolated from the nebular gas before condensation of U was complete. The inclusions, however, can be used to determine solar Pu/U and Cm/U ratios if the rare earth patterns are determined in addition to the other normal measurements.  相似文献   

2.
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   

3.
A87Rb-87Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ87Rb= 1.42 × 10?11yr?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the87Sr/86Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the87Rb-87Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the87Rb-87Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.  相似文献   

4.
Solar abundances based on recent laboratory oscillator strengths confirm the relationship between solar matter and carbonaceous chondrites. Within spectroscopic uncertainties (typically±40%) these meteorites contain refractory and volatile elements in solar proportions. Significant improvement of accuracy at present seems restricted to a few abundant elements having reliable quantum-mechanical oscillator strengths, and necessitates strictly differential spectrum analysis. Taking this into account, the solar abundance ratios Na/Ca and S/Ca have been determined to an accuracy of±15%. The results are:Na/Ca= 0.91and S/Ca= 6.8. These volatile/refractory ratios just match type 1 carbonaceous chondrites, but contrast with other types.These and related interstellar abundance features place constraints on the condensation process and a potential heterogeneity of the solar nebula. There is evidence that no drastic pre-solar separation of interstellar gas and grains has occurred, but minor imbalance may be a common mechanism co-determining stellar metal content.  相似文献   

5.
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   

6.
(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   

7.
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   

8.
A precise87Rb-87Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and87Sr/86Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(87Rb) = 1.42 × 10?11yr?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the87Rb/86Sr vs.87Sr/86Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and87Sr/86Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and87Sr/86Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a87Sr/86Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by26Al. An26Al/27Al ratio of 4–6 × 10?6 is enough for heating such a body. Further tests for this model are proposed.  相似文献   

9.
Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurböle (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurböle. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of129Xe, where we infer 129/Xe132Xe= 0.56 ? 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of129Xe from decay of129I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low129Xe seems to be to suppose that before decay to129Xe r-process production at mass 129 condensed into dust as129I, and that Arapahoe's parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped129Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.  相似文献   

10.
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   

11.
Isotopic anomalies in Mo and Zr have recently been reported for bulk chondrites and iron meteorites and have been interpreted in terms of a primordial nucleosynthetic heterogeneity in the solar nebula. We report precise Zr isotopic measurements of carbonaceous, ordinary and enstatite chondrites, eucrites, mesosiderites and lunar rocks. All bulk rock samples yield isotopic compositions that are identical to the terrestrial standard within the analytical uncertainty. No anomalies in 92Zr are found in any samples including high Nb/Zr eucrites and high and low Nb/Zr calcium-aluminum-rich inclusions (CAIs). These data are consistent with the most recent estimates of <10−4 for the initial 92Nb/93Nb of the solar system. There exists a trace of isotopic heterogeneity in the form of a small excess of r-process 96Zr in some refractory CAIs and some metal-rich phases of Renazzo. A more striking enrichment in 96Zr is found in acetic acid leachates of the Allende CV carbonaceous chondrite. These data indicate that the r- and s-process Zr components found in presolar grains were well mixed on a large scale prior to planetary accretion. However, some CAIs formed before mixing was complete, such that they were able to sample a population of r-process-enriched material. The maximum amount of additional r-process component that was added to the otherwise well-mixed Zr in the molecular cloud or disk corresponds to ∼0.01%.  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.
New measurements of mass-dependent calcium isotope effects in meteorites, lunar and terrestrial samples show that Earth, Moon, Mars, and differentiated asteroids (e.g., 4-Vesta and the angrite and aubrite parent bodies) are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution (± 0.07‰ SD for the 44Ca/40Ca ratio). In contrast, enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes (ca. + 0.5‰) and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes (ca. ? 0.5‰). The calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes. The isotopic variations probably survive from the earliest stages of nebular condensation, and indicate that condensation occurred under non-equilibrium (undercooled nebular gas) conditions. Some of this early high-temperature calcium isotope heterogeneity is recorded by refractory inclusions (Niederer and Papanastassiou, 1984) and survived in planetesimals, but virtually none of it survived through terrestrial planet accretion. The new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets; enstatite and carbonaceous chondrites are not. The enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction.  相似文献   

14.
A small particle (ca. 10?6 g) was magnetically separated from a Ca,Al-rich inclusion of the Allende meteorite. By using instrumental neutron activation analysis it was found that the elements Os, W, Re, Ir, Mo, Ru and Pt were enriched by a mean factor of about 7000 relative to Cl chondrites.A polished section of the grain showed that it consisted mainly of silicates, with a rounded particle of metal and sulfide (20 μm across) attached to it.Concentrations of up to 11% Pt were determined with the microprobe in the Ni-Fe center of the particle. Furthermore, Rh was for the first time measured in an Allende inclusion. It is enriched in about the same degree as Pt, Ir and W. The Ni-Fe center was surrounded by troilite. Mo was concentrated in the sulfide, while Os and Ru were inhomogeneously distributed over the metal + sulfide phases. The particle is interpreted as direct product of metal condensation of the solar nebula. The sulfide phase formed at lower temperatures and caused redistribution of the refractory siderophile elements. Condensation calculations for a metal alloy show that Fe and Ni are expected to be already present at higher temperatures than the condensation temperatures of pure Fe. Pt and Rh, having lower condensation temperatures than pure Fe should also be completely condensed above the condensation temperature of pure Fe. Kinetic considerations show that minimum times to grow this kind of particle should be of the order of 500 years at 10?3 atm.  相似文献   

15.
Nitrogen contents range from a few parts per million in ordinary chondrites and achondrites to several hundred parts per million in enstatite chondrites and carbonaceous chondrites. Four major isotopic groups are recognized: (1) C1 and C2 carbonaceous chondrites have δ15N of+30to+50%.; (2) enstatite chondrites have δ15N of?30to?40‰; (3) C3 chondrites have low δ15N with large internal variations; (4) ordinary chondrites have δ15N of?10to+20‰. The major variations are primary, representing isotopic abundances established at the time of condensation and accretion. Secondary processes, such as spallation reactions, solar wind implantation and metamorphic loss may cause small but observable isotopic variations in particular cases. The large isotopic difference between enstatite chondrites and carbonaceous chondrites cannot be accounted for by equilibrium condensation from a homogeneous nebular gas, and requires either unusually large kinetic effects, or a temporal or spatial variation of isotopic composition of the nebula. Nitrogen isotopic heterogeneity in the nebula due to nuclear processes has not been firmly established, but may be required to account for the large variations found within the Allende and Leoville meteorites. The unique carbonaceous chondrite, Renazzo, has δ15N of+170%., which is well beyond the range of all other data, and also requires a special source. It is not yet possible, from the meteoritic data, to establish the mode of accretion of nitrogen onto the primitive Earth.  相似文献   

16.
The Ca-Al-rich inclusions within Allende are described as quickly frozen non-equilibrated partial melts arising from energetic collisions between centimeter-sized mechanical accumulations of cold presolar grains. The resulting minerals are refractory-rich because refractory supernova condensates are the most persistent components of the preheated accumulates. The shock heating drives off most of the more volatile matrix that had accumulated cold around the refractory cores, which quickly recrystallize while picking up isotopically homogenized trace elements. This picture is advanced to account for the isotopic anomalies in those elements for which fractionation of stardust from gas also fractionates a special isotope whose stellar condensation history can be expected to have been special.I call the anomaly that would have existed before the special component was added anisotopic ghost. These ghosts can be larger than the special anomaly surviving today in meteorites and planets. I argue that ghosts in26Mg/24Mg,87Sr/86Sr, and206,207Pb/204Pb have caused erroneous cosmoradiogenic estimates of large age differences between meteorites, their special phases, and even the Moon.  相似文献   

17.
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   

18.
Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium–aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation–evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ?17O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ?17O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal–silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ?17O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and/or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies.  相似文献   

19.
Particle-reactive radionuclides were determined in sediments from the inner New York Bight to trace transport and storage of fine-grained sediments and associated reactive materials. Seven sediment ? cores 20–50 cm in length were analyzed for water content, loss on ignition (LOI) and excess210Pb; three of these were also analyzed for239,240Pu. Excepting some depth horizons in a core from a dredge-spoil dumpsite, every sample analyzed contained excess210Pb. Variations in the concentration of excess210Pb with depth in the sediment at all stations correlated strongly with LOI, which apparently traces that fraction of the sediment which is active in removing reactive elements from the water column. In the cores analyzed for239,240Pu, every sample contained finite Pu, and Pu concentrations correlated strongly with excess210Pb.The radionuclide distributions may be simply viewed as products of steady-state sediment accumulation or of mixing. Geochemically reasonable accumulation rates are very high (0.5–2.6 g/cm2 y) and could probably only be sustained by offshore transport of dumped materials. At the other extreme the relationships between excess210Pb and LOI are compatible with rapid mixing of a210Pb carrier phase (traced by LOI) into the pre-existing substrate with little or no actual accumulation. Other non-steady-state processes, such as sediment gravity flow, could also explain the observed distributions.Measured sediment inventories (dpm/cm2) of excess210Pb and Pu at these stations are greatly in excess of those supportable by direct atmospheric deposition: lateral supply is required. Incorporation of sedimentary fines into the sand substrate could make the inner New York Bight an important repository of reactive materials.  相似文献   

20.
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   

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