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1.
The Apollo 16 soils have the largest low energy neutron fluences (up to 1017 n/cm2, E < 0.18eV) yet observed in lunar samples. Variations in the isotopic ratios 158Gd/157Gd and 150Sm/149Sm (up to 1.9% and 2.0% respectively) indicate that the low energy neutron fluence in the Apollo 16 drill stem increases with depth throughout the section sampled. Such a variation implies that accretion has been the dominant regolith “gardening” process at this location. The data may be fit by a model of continuous accretion of pre-irradiated material at a rate of ～70 g/(cm2 · 108yr) or by models involving as few as two slabs of material in which the first slab could have been deposited as long as 109 yr ago.The ratio of the number of neutrons captured per atom by Sm to the number captured per atom by Gd is lower than in previously measured lunar samples, which implies a lower energy neutron spectrum at this site. The variation of this ratio with chemical composition is qualitatively similar to that predicted by Lingenfelter, Canfield and Hampel.Variations are observed in the ratio 152Gd/160Gd which are fluence correlated and probably result from neutron capture by151Eu.  相似文献   

2.
The Gd isotopic composition in 19 lunar rock and soil samples from three Apollo sites is reported. The analytical techniques and the high precision mass spectrometric measurements are discussed. Enrichments in158GdO/157GdO due to neutron capture range up to 0.75%. Integrated ‘thermal’ neutron fluxes derived from the isotopic anomalies of Gd are compared with spallation Kr data from aliquot samples to construct a model which gives both average cosmic-ray irradiation depths and effective neutron exposure ages (Tn) for some rocks. In the case of rock 12053, this yields an average sample location of ∼300 g/cm2 below the lunar surface and an effective irradiation age of ∼230 my, compared to 99 my obtained by the81Kr-Kr method. Rock 14310 is the first lunar sample where Kr anomalies due to resonance neutron capture in Br are observed. A81Kr-Kr exposure age of 262 ± 7 my is calculated for this rock.  相似文献   

3.
Accurate determination of chlorine concentrations in terrestrial rocks is of importance for the interpretation of terrestrial in-situ cosmogenic 36Cl. Neutron capture by 35Cl, together with production from Ca and K, is one of the three major production pathways of 36Cl in rocks. Here, we present an inter-comparison of chlorine determinations by two procedures. The first approach is an independent Cl determination by prompt-gamma (neutron) activation analysis (PGAA). The second method is isotope-dilution based on isotopically enriched stable chlorine carrier added during chemical sample preparation for accelerator mass spectrometry (ID-AMS). Twenty-six (26) whole rock samples have been processed for PGAA and ID-AMS analysis. This study constitutes the first published inter-comparison for concentrations below 100 μgCl/g. Our results show no significant difference in Cl concentrations between methods. This agreement indicates good retention of chloride during the procedure we employ for whole rock sample dissolution. No significant loss of stable chlorine from either the spike or the sample occurs before isotopic equilibrium is reached, prior to AgCl precipitation. Uncertainties, which are <5% for both methods, affect the uncertainty of the total 36Cl production rate less than 2% for our samples.The Cl concentration measured by PGAA can be used to calculate the amount of isotopically enriched spike for AMS-ID sample preparation with the aim to optimize 36Cl analysis. Furthermore, PGAA offers an advance for the interpretation of 36Cl measurements. It allows measurement of concentrations of major, minor and trace elements including the elements for 36Cl production (Cl, K, Ca, Ti, and Fe), as well as of neutron absorbers and neutron moderators (H, B, Sm and Gd). These measurements are performed simultaneously and with a precision necessary for calculating the relative contributions to 36Cl production from the different mechanisms.  相似文献   

4.
Isotopic ratios and abundances of all REE in two Oklo ore samples have been measured. We have succeeded in developing a new method to estimate the neutron fluence, the conversion factor of uranium and the average neutron energy (or temperature) based on Gd and U isotopic ratios. This new calculation is found to be useful in evaluating those parameters for the natural nuclear reactors at Oklo. Comparison is made between the neutron fluence values evaluated by our new method employing Gd isotopes and a previous one [11] employing Sm and Nd isotopes. The relative agreement becomes better with the increase of fluence. A relationship between the abundances of fissiongenic nuclides of La, Ce, Nd, Sm, and Gd and their mass numbers is also presented.  相似文献   

5.
Measurements of143Nd/144Nd and147Sm/144Nd are reported for whole rocks and mineral separates from granulites of the Napier Complex at Fyfe Hills. Charnockites, leuconorites and gabbros yield a whole rock SmNd isochron age of3060 ± 160m.y. and an initial143Nd/144Nd ratio of0.50776 ± 10 (?Nd(3060m.y.) = ?2.0 ± 1.8). The negative ?Nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of Sm and Nd and partial re-equilibration of143Nd/144Nd ratios. This probably occurred during the upper granulite facies metamorphism which has also been dated at～ 3100m.y. by RbSr and UPb zircon studies [1]. Coexisting clinopyroxene, apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 ± 300 m.y. This array indicates that redistribution of Sm and Nd and re-equilibration of143Nd/144Nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at～ 2450m.y.Due to the resetting of the SmNd system on both whole rock and mineral scales, the primary crystallization age of the igneous protolith is not well constrained by the present data, although it is clearly3100m.y. If it is assumed that the complex was derived initially from a depleted mantle reservoir(?Nd(T) ? 2), evolution of the negative ?Nd value of ?2.0 with the observed Sm/Nd ratios requires a prehistory of～ 380m.y. This implies a primary age of～ 3480m.y. However, substantially older primary ages can be inferred if the source reservoirs had?Nd(T) > 2 and/or substantial reductions in the Sm/Nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m.y. Such an inferred reduction in the Sm/Nd ratio may have been the result of preferential loss of Sm relative to Nd, or introduction of a low Sm/Nd fluid with?Nd ≥ 0 during granulite facies metamorphism.  相似文献   

6.
Sm-Nd data for ten greenstone belt volcanics from Rhodesia define an age of2.64 ± 0.14AE, which is in agreement with other geochronological data and with observed field relationships. This age and the initial143Nd/144Nd ratio of0.50919 ± 0.00018 yield a time-integrated Sm/Nd ratio of0.302 ± 0.009 in the mantle source region, similar to that in chondrites. Sm/Nd ratios of some lavas are0.31 and imply that a small fraction of their source was removed prior to or as part of the main melting event.The utility of the Sm-Nd system for dating altered Archaean volcanic rocks is amply demonstrated by these data.  相似文献   

7.
The147Sm143Nd and146Sm142Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of (143Nd/144Nd)CHUR0 = 0.511847 and (147Sm/144Nd)CHUR0 = 0.1967. Most of the bulk chondrites analyzed have 143Nd/144Nd and 147Sm/144Nd within 0.5 ε-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an SmNd internal isochron age of T = 4.55 ± 0.33AE and an initial εNd = 0.11 ? 0.26. Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03AE and4.564 ± 0.037AE were obtained for these meteorites, respectively. The initial εNd values obtained for Moama and ADOR are 0.03 ? 0.25and0.14 ? 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial εNd data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial εNd values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the εNd vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods.146Sm142Nd systematics of ADOR and Moama are supportive of the hypothesis that146Sm was present in the early solar system and suggests a 146Sm/144Sm ratio of about 0.01 for the solar system ～ 4.56 AE ago. This inferred high146Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.  相似文献   

8.
The measurement of the176Lu/175Lu ratio in the Oklo uranium ore zones where nuclear reactions have been identified, can be related to the average equilibrium temperature of the neutrons at the time of the nuclear reactions; indeed, the effective neutron capture cross section of the isotope176Lu is particularly sensitive to temperature. A simple model has been developed to calculate the temperature. The results obtained on seven samples from three reactor zones are presented. From six samples an average value of280 ± 50°C is obtained, the corresponding integrated neutron flux τ varies from 0.06 × 1020 to 6 × 1020 n/cm2 and176Lu/175Lu from 0.0254 to 0.0054. From one sample a very high temperature is obtained:T > 1000°C, τ = 4.4 × 1020n/cm2,176Lu/175Lu= 0.0032. The validity of this result is sharply dependent on the relative stability of uranium and the rare earths during and after the nuclear reactions; so, it can be noticed that the two samples giving the highest temperatures present a surprisingly high enrichment in heavy rare earth elements.  相似文献   

9.
We have studied SmNd systematics in pyroxene and phosphate mineral separates of Angra dos Reis. A pyroxene-phosphate internal isochron age ofT2 = 4.55 ± 0.04AE is obtained, in excellent agreement with reported Pb-Pb ages.142Nd/144Nd ratios in pyroxene samples are systematically larger than those in phosphates by 6 parts in 105. This variation is tentatively assigned to a radiogenic contribution from extinct146Sm. Fission xenon components in pyroxene and phosphate separates are characterized by discrete ratios of fission/spallation and evidence is presented for a third ratio in celsian. It is shown that this characteristic is due to a close association of244Pu with the light REE. Computed ratios244Pu/Nd are the same in pyroxene and phosphate separates, but244Pu/238U and244Pu/232Th ratios are not. Taking the fission xenon retention age to be 4.55 AE, we obtain an abundance ratio244Pu/Nd= 1.5 × 10?4 (or an atomic ratio244Pu/150Nd= 1.6 × 10?3) at this time and in the region of the solar system where the Angra dos Reis parent body formed. The exposure age of Angra dos Reis, as obtained by the81Kr-83Kr method is55.5 ± 1.2m.y. Neutron capture during the 55.5-m.y. exposure to cosmic rays increased the ratio150Sm/149Sm in Angra dos Reis by 6 parts in 104.  相似文献   

10.
To constrain the depth-dependence of in situ 14C production we measured the cosmogenic 14C concentration of quartz separates along a quartzite core from the Leymon High site in northwest Spain. A total of 16 quartz samples were measured over a depth range of 1–1545 cm (3–4017 g cm−2). The obtained 14C profile was modeled using a neutron production rate, exponentially decreasing with depth, and a fast and negative muon production parameterized as a function of the local muon flux as derived by (Heisinger et al., 2002a, 2002b). This model yields a total negative muon capture probability of 1.72 (+0.22/−0.56) × 10−2 and a fast muon reaction cross section of 0 (+11.8/−0.00) μb. Rescaled to sea level high latitude using a Lal/Stone scaling scheme, these estimates yield a surface muon production rate of 3.31 (+0.43/−1.07) and 0 (+0.42/−0.00) at·g−1 yr−1 for negative muon capture and fast muons, respectively. This is the first muon production estimate for in situ 14C from a natural setting and is within uncertainty of the previous experimental estimates. The present contribution also provides new long-term blank and standard (PP-4, CRONUS-A & CRONUS-N) in situ 14C data from the ETH Zürich 14C extraction line.  相似文献   

11.
The depth profile of the neutron-induced fission rate of235U was directly measured to a depth of 350 g/cm2 by the Apollo 17 Lunar Neutron Probe Experiment. The fission rate rises sharply from the surface to a broad maximum from 110 to 160 g/cm2 and drops off at greater depths. The shape of theoretical depth profile of Lingenfelter et al. fits the measured capture rates well at all depths. The absolute magnitude of the experimental fission rates are (11±17)% lower than those calculated theoretically. The excellent agreement between theory and experiment implies that conclusions drawn previously by interpreting lunar sample data with the theoretical capture rates will not require revision. In particular lunar surface processes, rather than uncertainties in the capture rates, are required to explain the relatively low neutron fluences observed for surface soil samples compared to the fluences expected for a uniformly mixed regolith.  相似文献   

12.
Sm-Nd isotopic evolution of chondrites   总被引：8，自引：0，他引：8  
The143Nd/144Nd and147Sm/144Nd ratios have been measured in five chondrites and the Juvinas achondrite. The range in143Nd/144Nd for the analyzed meteorite samples is 5.3 ε-units (0.511673–0.511944) normalized to150Nd/142Nd= 0.2096. This is correlated with the variation of 4.2% in147Sm/144Nd (0.1920–0.2000). Much of this spread is due to small-scale heterogeneities in the chondrites and does not appear to reflect the large-scale volumetric averages. It is shown that none of the samples deviate more than 0.5 ε-units from a 4.6-AE reference isochron and define an initial143Nd/144Nd ratio at 4.6 AE of0.505828 ± 9. Insofar as there is a range of values of147Sm/144Nd there is no unique way of picking solar or average chondritic values. From these data we have selected a new set of self-consistent present-day reference values for CHUR (“chondritic uniform reservoir”) of (143Nd/144Nd)CHUR0 = 0.511836and(147Sm/144Nd)CHUR0 = 0.1967. The new147Sm/144Nd value is 1.6% higher than the previous value assigned to CHUR using the Juvinas data of Lugmair. This will cause a small but significant change in the CHUR evolution curve. Some terrestrial samples of Archean age show clear deviations from the new CHUR curve. If the CHUR curve is representative of undifferentiated mantle then it demonstrates that depleted sources were also tapped early in the Archean. Such a depleted layer may represent the early evolution of the source of present-day mid-ocean ridge basalts. There exists a variety of discrepancies with most earlier meteorite data which includes determination of all Nd isotopes and Sm/Nd ratios. These discrepancies require clarification in order to permit reliable interlaboratory comparisons. The new CHUR curve implies substantial changes in model ages for lunar rocks and thus also in the interpretation of early lunar chronology.  相似文献   

13.
Calculated univariant equilibria and oxygen isotope compositions of silicates and carbonates support the proposal that the “Mottled Zone Event” is a low-pressure (1–25 atm), high-temperature (200° < T < 1300°C) metamorphism of calcareous siliceous sediments in which the thermal energy is provided by combustion of organic matter. δ18O of silicates decreases systematically with increasing metamorphic grade from averages of 18.1‰ in protolith shales, to 16.6‰ in grossular-diopside-zeolite rocks, 15.6‰ in wollastonite and anorthite-diopside-gehlenite-grossular fels, 14.1‰ in spurrite-brownmillerite marbles and 11.7‰ in the highest-grade larnite-gehlenite-brownmillerite assemblages. Decarbonation is the principal mechanism influencing the oxygen isotope compositions. The progressive decrease of δ18O in silicates can be modelled as a Rayleigh distillation of CO2 approximately 16‰ enriched in 18O relative to whole rock assemblages i.e., of initial isotopic composition 8.5‰ heavier than the parent carbonates. The mineral assemblage of one sample with an unusual granoblastic texture is in apparent isotopic equilibrium at a temperature of 540°C.  相似文献   

14.
Kr and Xe were measured by a stepwise heating technique in three samples of a drill core in the “Minnesota” fragment of the Estherville mesosiderite. The cosmogenic78Kr/83Kr decreased from the “top” sample to the “bottom” sample(“top” = 0.163 ± 0.005, “bottom” = 0.151 ± 0.005) while the cosmogenic131Xe/126Xe ratio increased(“top” = 5.58 ± 0.35, “bottom” = 6.92 ± 0.17). Cosmic-ray track studies have shown that the “top” sample was indeed closer to the preatmospheric surface than the “bottom” sample by ～ 10 cm. This is the first direct evidence, in a sample of known geometry, that the cosmogenic131Xe/126Xe ratio increases as a function of depth, and as such, confirms the hypothesis that the lunar131Xe anomaly is a bona fide depth effect due to resonance neutron capture in130Ba.  相似文献   

15.
The electrical impedance of a sintered forsterite sample containing 1 mol percent SiO2 in excess has been studied as a function of frequency (1–105 Hz) and temperature (400–900°C). The electrical properties are strongly frequency-dependent, requiring both conduction and displacement current for their interpretation. The electrical conductivity is thermally activated with a mean activation energy of 1.15 eV. It is interpreted as being ionic and caused by migration of magnesium ions via a vacancy mechanism. Dielectric data deduced from impedance measurements vary as ωn?1 (0 < n < 1) at constant temperature. The value of n is different below and above a critical frequency ωc, which is thermally activated and interpreted as the jump frequency of the migrating species.  相似文献   

16.
First-principles calculations have been used to determine the equation of state of Fe3C in both its low-pressure magnetically ordered and high-pressure non-magnetically ordered states; at 0 K the ferromagnetic transition was found to occur at about 60 GPa. In the high pressure, non-magnetically ordered regime at 0 K the material may be described by a Birch-Murnaghan third-order equation of state with V0=8.968(7) Å3 per atom, K0=316.62(2) GPa and K′=4.30(2). At atmospheric pressure the ferromagnetic phase transition in Fe3C occurs at ∼483 K; preliminary measurements of the thermal expansion by powder neutron diffraction show that this transition produces a large effect on thermoelastic properties. The volumetric thermal expansion coefficient in the paramagnetic phase was found to be 4.34×10−5 K−1 at T∼550 K. By applying a thermal expansion correction to the calculated equation of state at 0 K, predicted values for the density and adiabatic incompressibility of this material at core pressures and temperatures were obtained. These results appear to be sufficiently different from seismological data so as to preclude Fe3C as the major inner core-forming phase.  相似文献   

17.
Pb, Nd and Sr isotope analyses together with U, Pb, Sm, Nd, Rb and Sr concentrations have been obtained for separated phases of lherzolite and bulk rock mafic granulite xenoliths in Recent volcanics from Tanzania. A garnet lherzolite from the Lashaine vent has yielded the least radiogenicPb(206Pb/204Pb= 15.55) and Nd(143Nd/144Nd= 0.51127; ?Nd0 = ?26.7) isotope compositions recorded so far for an ultramafic xenolith, and 87Sr/86Sr= 0.83604. The Pb isotope compositions of the mafic granulites are variable 15.77<206Pb/204Pb<17.50 and some show evidence for depletion of U relative to Pb up to 2.0 Ga ago. Overall the isotope results suggest that the mantle part of the continental lithosphere beneath Tanzania has components that have undergone a complex history that includes major chemical fractionations ca. 2.0 Ga ago. A phlogopite-amphibole vein from the Pello Hill sample has Sr, Nd and Pb isotope compositions similar to those of mid-ocean ridge basalts, indicating both a young emplacement age for the vein material and a source which had an isotopic signature characteristic of depleted mantle.The Sr, Nd and Pb isotope systematics of ultramafic xenoliths do not conform with those of MORB, particularly in terms of their PbSr, and NdPb relationships. In this regard they are similar to some ocean islands and could be a viable source material for some ocean island basalts at least. The mantle part of the continental lithosphere is as likely to contain recycled components derived from the continental crust as are other regions of mantle. If the mantle part of continental lithosphere is invoked as a source for ocean islands, it does not negate the possibility that substantial recycled components are involved.  相似文献   

18.
The ranges of δ18O and δ17O in components of the Murchison (C2) chondrite exceed those in all other meteorites analyzed. Previous authors have proposed that C2 chondrites are the products of aqueous alteration of anhydrous silicates. A model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes. The minimum number (two) of initial isotopic reservoirs is assumed. Two major stages of reservoir interaction are required: one at high temperature to produce the16O-mixing line observed for the anhydrous minerals, and another at low temperature to produce the matrix minerals. The isotopic compositions severely constrain the conditions of the low-temperature process, requiring temperatures < 20°C and volume fractions of water > 44%. Extension of the model to the data on C1 chondrites requires aqueous alteration in a warmer, wetter environment.  相似文献   

19.
We report the results of thermal-release argon analyses of neutron-irradiated green glass spherules separated from lunar sample 15426. The gas-retention age, as determined by the40Ar39Ar method, is (3.38 ± 0.06) X 109yr. This age is similar to those of local mare basalts and distinct from the ages of Appenine Front samples recovered from the same region as 15426. Trapped argon is present in near-surface regions of the spherules, and can be resolved into at least two components requiring separate origins, a shallow (0.1 μ) component with40Ar/39Ar > 30, and a deeper (2 μ) component with 40Ar/36Ar= 2.9. The ratio of trapped40Ar to36Ar is higher than found in any lunar soil and suggests that the trapped gas was implanted early in the spherules' history. The cosmic-ray exposure age is 300 my.  相似文献   

20.
We present the new 14C extraction line at ETH Zürich. This system is designed to extract in situ-produced cosmogenic 14C from terrestrial quartz samples, and to obtain pure CO2 gas for analysis with a gas ion source Accelerator Mass Spectrometry (AMS) system. Samples are degassed at 1550–1600 °C without the use of a fluxing agent. Gas purification is achieved by a series of cryogenic traps and passage through hot Ag and Cu wool/mesh. Graphitization and, thus, sample dilution is not required. Tests to determine the CO2 recovery after gas extraction and cleaning yielded consistently good recovery rates of >99.8% (n = 7). The 14C blank contribution from the all-metal tubing system is negligible. Our preliminary procedural blank estimate – deriving mostly from the hot extraction furnace – is <5 × 105 14C atoms. Extraction tests on two quartz samples by stepped-heating show a quantitative separation of atmospheric 14C at ≤500 °C from the in situ component above 1200 °C. Based on these data, we estimate to achieve a complete 14C extraction from a quartz sample.  相似文献   

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