A direct measurement of the distribution in depth of 26Al in the Estacado meteorite     

J.C. Barton  A.H. Watson  A.G. Wright 《Geochimica et cosmochimica acta》1982,46(10):1963-1967

The two-dimensional distribution of ²⁶Al across a slice of the Estacado meteorite was measured using non-destructive γ-γ coincidence counting. The maximum activity was found to be 60 ± 2 dpm kg^?1 and the variation across the slice rather small. The intensity and distribution are in moderate agreement with existing theoretical models if the pre-atmospheric shape of the meteorite had been a sphere of radius 0.35 ± 0.05 m, but it was more likely to have been slightly non-spherical.  相似文献   

26Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites     

S. Sahijpal  M. A. Ivanova  L. L. Kashkarov  N. N. Korotkova  L. F. Migdisova  M. A. Nazarov  J. N. Goswami 《Journal of Earth System Science》1995,104(4):555-567

Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for²⁶Mg excess that may be attributed to the presence of the now-extinct radionuclide²⁶Al(τ ∼ 1 Ma) at the time of formation of these objects. A positive evidence for the presence of²⁶Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial²⁶Al/²⁷Al ratios, we have estimated the initial abundances of²⁶Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of²⁶Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of²⁶Al in the early solar system to serve as a significant heat source for the thermal metamorphism and melting of meteorite parent bodies (planetesimals) remains elusive.  相似文献   

Radionuclides produits par le rayonnement cosmique dans la météorite Saint-Séverin     

Roland Bibron  Concèle Leger  Jacques Tobailem  Yfji Yokoyama  Hisao Mabuchi  Noëlle Baillard 《Geochimica et cosmochimica acta》1974,38(2):197-205

]The activities of 18 radionuclides in 4 samples from the Saint-Séverin meteorite have been measured after chemical separation. The results show that the DI sample from the D piece has been less irradiated than the samples B₂, B₃ and B₄ from the B piece. This is based on activities of isotopes produced both by spallation as ²⁶Al, ⁵³Mn, ⁵⁴Mn, ⁵⁵Fe, ⁵⁷Co and by thermal neutron capture as ⁵⁹Ni and ⁶⁰Co.Comparison of experimental values to calculated production rates suggests that the D piece was connected to the main body of the meteorite by a small surface, and that the DI sample in this piece was opposite to the main body of the meteorite.  相似文献   

Density functional theory study and kinetic analysis of the formation mechanism of Al₃₀O₈(OH)₅₆(H₂O)₂₆ (Al₃₀) in aqueous solution     

Wenjing Yang 《Geochimica et cosmochimica acta》2010,74(4):1220-1032

The formation mechanism of Al₃₀O₈(OH)₅₆(H₂O)₂₆¹⁸⁺ (Al₃₀) has been investigated by the density functional theory based on the supermolecule model and kinetic analysis on the ²⁷Al nuclear magnetic resonance (NMR) experimental results in monitoring Al₃₀ synthesis process. The theoretical chemistry calculations on the four possible schemes show that δ-Na-Al₁₃ is the reasonable intermediate followed by the substitution of Na with Al to form δ-Al₁₄, and Na⁺ plays an important role in stabilizing the intermediate (δ-Na-Al₁₃) in the transformation. The kinetic analysis on the ²⁷Al NMR experimental data indicates that ε-Al₁₃ decomposes and isomerizes in the formation of Al₃₀, while Al monomers facilitate the decomposition of ε-Al₁₃ and so the isomerization of ε-isomers to δ-isomers effectively. The favorable formation mechanism of Al₃₀ includes three steps: (1) ε-Al₁₃ decomposes and rearranges into the isomer δ-Al₁₃; (2) Na⁺ reacts with δ-Al₁₃ to stabilize the intermediate δ-Na-Al₁₃, followed by Al monomers replacing Na to form δ-Al₁₄; (3) δ-Al₁₄ reacts with the Al monomers in the solution to finally form Al₃₀. Both Al monomers and Na⁺ are important in the transformation. Al monomers are the basic building units and helpful to the isomerization while Na⁺ can well stabilize the isomer δ-Al₁₃ to yield intermediate δ-Na-Al₁₃. The results also show that other isomers of ε-Al₁₃ (β-Al₁₃ and α-Al₁₃) form in the formation of Al₃₀, and their calculated ²⁷Al NMR tetrahedral resonance shifts are consistent with the experimental ²⁷Al NMR tetrahedral signals in the preparation process of Al₃₀.  相似文献   

A comparative study on ²⁸Al and ⁵³Mn in eighteen chondrites     

M Heimann  P.P Parekh  W Herr 《Geochimica et cosmochimica acta》1974,38(2):217-234

An investigation on the low energy cosmic ray products ²⁶Al and ⁵³Mn in 18 chondrites encompassed several side aspects in meteoritics. The ratio of observed to calculated specific activity of ²⁶Al in 13 chondrites with recovered masses ranging from ~ 3 to 2000 kg remains constant within ±15 per cent of a mean of 0.93. This is consistent with Fuse and Anders' observation of invariance of the ²⁶Al-production rate with depth and size. A plot of the ⁵³Mn specific production rate (dpm/kg Fe) versus recovered meteorite mass revealed that the compensation by the secondaries for the absorption of the primaries is also effective in the case of ⁵³Mn. Thus, the ⁵³Mn production rate remains rather constant within ±15 per cent of a mean of 520 dpm/kg Fe for most of the stones. Applying this information to two chondrites, Ladder Creek and Scurry, of known relatively short exposure and terrestrial ages, the ⁵³Mn half-life (T₅₃), computed from their ⁵³Mn_obs/²⁶Al_obs ratios, is found to be 3.85 ± 0.4 m.y. which agrees very well with the currently accepted value of T₅₃ = 3.8 ± 0.6 m.y. From the observed ⁵³Mn/⁵⁴Mn activity ratio in Allende, relative to the production rate ratio of 0.98, an exposure age of t_R = 4.2 m.y. is derived, in concordance with literature data. The result strongly suggests that this isotope pair holds promise as an index of absolute radiation ages of fresh falls.  相似文献   

Boron concentrations in carbonaceous chondrites     

Martha Riherd Weller  Marian Furst  T.A. Tombrello  D.S. Burnett 《Geochimica et cosmochimica acta》1978,42(7):999-1009

We have analyzed B in carbonaceous chondrites in order to clarify a factor of 100 difference between the solar system B abundance derived from the solar photosphere and that inferred from previous meteorite data. Consistent results were obtained from two instrumental methods for B analysis: (a) counting of the high energy betas from ¹²B produced by the ¹¹B(d,p) reaction, and (b) measurement of particle track densities from ¹⁰B(n,α)⁷Li in a plastic track detector affixed to a homogenized meteorite sample. Contamination is a major problem in B analyses, but extensive testing showed that our results were not seriously affected. Our B concentrations are typically 1–2 ppm and are a factor of 2–6 lower than previous carbonaceous chondrite measurements. Our data for the Cl chondrites Ivuna and Orgueil would indicate a solar system B/Si atomic abundance ratio of 58 × 10^?6, but this is still a factor of 2–10 higher than the photospheric estimates. It may be that B is depleted in the sun by thermonuclear processes; however, the similarity of photospheric and meteoritic Be abundances is a problem for this point of view. Alternatively, B may be enhanced in carbonaceous chondrites, but this would make B a cosmochemically unique element. A mm-sized (Fe,Mn,Mg)CO₃ crystal from Orgueil shows no B enrichment. We find ¹⁰B ≤ 10¹⁶ atoms/g in two Allende fine-grained inclusions suggesting that B is not a refractory element under solar nebula conditions. This ¹⁰B limit, when taken as a limit on ¹⁰Be when the inclusion formed, puts constraints on the possibility of a solar system synthesis of ²⁶Al. For a proton spectrum of E^?a, a must be ≥ 3 if a solar gas is irradiated or a ≥1.5 if dust of solar composition is irradiated.  相似文献   

Nonracemic isovaline in the Murchison meteorite: chiral distribution and mineral association     

Sandra Pizzarello  Michael Zolensky 《Geochimica et cosmochimica acta》2003,67(8):1589-1595

The enantiomeric and carbon-isotopic composition of the amino acid isovaline have been analyzed in several samples of the Murchison meteorite and one sample of the Murray meteorite. l-Enantiomeric excesses of the amino acid were found to range from 0 to 15.2%, varying significantly both between meteorite stones and at short distances within a single stone. The upper limit of this range is the largest enantiomeric excess measured to date for a biologically rare meteoritic amino acid and raises doubts that circularly polarized light irradiation could have been the sole cause of amino acids chiral asymmetry in meteorites. Individual d- and l-isovaline δ¹³C values ware found to be about +18‰, with no significant differences between the two enantiomers to suggest terrestrial contamination. The amino acid relative abundance also varied between samples, with isovaline/alanine ratios of 0.5 to 6.5. X-ray diffraction analyses of contiguous meteorite fragments suggest a possible correlation between isovaline and hydrous silicates abundances.  相似文献   

Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes     

M.A. Tyra  J. Farquhar  G.K. Benedix  T. Jackson 《Geochimica et cosmochimica acta》2007,71(3):782-795

The oxygen (δ¹⁸O, δ¹⁷O) and carbon (δ¹³C, FMOD¹⁴C-the fraction of modern ¹⁴C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ¹³C and FMOD¹⁴C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO₂ that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.  相似文献   

The Solar System's Earliest Chemistry: Systematics of Refractory Inclusions     

《International Geology Review》2012,54(10):865-894

Refractory inclusions, or CAIs (calcium-aluminium-rich inclusions) are a unique ingredient in chondritic meteorites. As the name suggests, they are enriched in refractory elements, essentially reflecting a condensation sequence of phases from a cooling gas of solar composition. However, the widespread preservation of diverse isotopic anomalies is not compatible with the inclusions having been in a gaseous form. Rather, the CAIs appear to represent mixtures of condensate and refractory residue materials. The condensates formed from cooling solar gas and fractionation of that gas produced variations in the abundances of refractory elements according to volatility. Solar condensate has isotopically normal Ca and Ti isotopic compositions and has ²⁶Al/²⁷Al of the canonical value for the solar system at 5 × 10?⁵. Residues of material falling in toward the Sun are probably aluminous oxides such as corundum and hibonite, and preserve diverse Ca and Ti isotopic anomalies. Meteoritic inclusions from the Murchison meteorite show the best polarization of these components. Spinel-hibonite-perovskite inclusions (SHIBs) predominantly have normal Ca and Ti isotopes, ²⁶Al/²⁷Al at 5 × 10?⁵, and ultrarefractory fractionated REE patterns. Single hibonite crystal fragments (PLACs) have diverse Ca and Ti isotopic compositions and low ²⁶Al/²⁷Al because of the initially high proportion of ²⁷Al in the residue. REE patterns in PLACs are variable in terms of the ultrarefractory fractionation of their REE patterns, as indicated by Tm/Tm?, but are dominated by depletion in the less refractory REE Eu and Yb. Both PLACs and SHIBs homogenized with ¹⁶O-rich gas, enriched relative to terrestrial O by up to 7%, thus removing any isotopic heterogeneity from the PLAC precursors. CAIs formed close to the Sun where condensation and re-evaporation of REE was possible, and were then ejected back to planetary radii where they were eventually accreted onto planetesimals.  相似文献   

High temperature heating of the Allende meteorite     

Kenji Notsu  Naoki Onuma  Norimasa Nishida  Hiroshi Nagasawa 《Geochimica et cosmochimica acta》1978,42(6):903-907

  相似文献   

NMR studies of chemical structural variation of insoluble organic matter from different carbonaceous chondrite groups   总被引：1，自引：0，他引：1  

George D. Cody  Conel M.O.’D. Alexander 《Geochimica et cosmochimica acta》2005,69(4):1085-1097

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., “CH_x,” and “CH_x(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   

53Mn-53Cr evolution of the early solar system     

《Geochimica et cosmochimica acta》1999,63(23-24):4111-4117

The model of Cr isotopic evolution presented here, relies on the relative volatility properties of the two elements: Mn-Cr in planetary formation processes. The Mn/Cr ratio of the respective parent bodies correlate in most cases with the K/U ratio. With the exception of Allende inclusions, the ⁵³Mn/⁵⁵Mn and ⁵³Cr/⁵²Cr isotopic ratios were homogeneous in the solar nebula. The Cr isotopic evolution of the bulk solar system corresponds to the C1 carbonaceous chondrites. In this figure the Earth is isolated within a few million years of the C1 formation, from the solar nebula before complete decay of ⁵³Mn. It has a Cr isotopic composition which is depleted in ⁵³Cr with respect to the solar system as a whole. The parent bodies of the different meteorite classes display various behaviour with no case of Mn enrichment relative to Cr when compared to C1. The ⁵³Mn-⁵³Cr isotopic system is a precise tool for the exploration of the early solar sytem history, bringing constraints both on time and processes in this phase of the evolution where the face of the planetary system was changing rapidly. The chronology deduced from Mn-Cr systematics, is generally in good agreement with other chronometers.  相似文献   

Diverse nucleosynthetic components in barium isotopes of carbonaceous chondrites: Incomplete mixing of s- and r-process isotopes and extinct Cs in the early solar system     

Hiroshi Hidaka  Shigekazu Yoneda 《Geochimica et cosmochimica acta》2011,75(13):3687-3697

Barium isotopic compositions of chemical leachates from six carbonaceous chondrites, Orgueil (CI), Mighei (CM2), Murray (CM2), Efremovka (CV3), Kainsaz (CO3), and Karoonda (CK4), were determined using thermal ionization mass spectrometry in order to assess the chemical evolution in the early solar system.The Ba isotopic data from most of the leachates show variable ¹³⁵Ba excesses correlated with ¹³⁷Ba excesses, suggesting the presence and heterogeneity of additional nucleosynthetic components for s- and r-processes in the solar system. The isotopic deviations observed in this study were generally small (−1 < ε < +1) except in the case of the acid residues of CI and CM meteorites. Large deviations of ¹³⁵Ba (ε = −13.5 to −5.0) and ¹³⁷Ba (ε = −6.2∼−1.2) observed in the acid residues from one CI and two CM meteorites show significant evidence for the enrichment of s-process isotopes derived from presolar grains. Two models were proposed to estimate the ¹³⁵Cs isotopic abundances by subtraction of the s- and r-isotopic components from the total Ba isotopic abundances in the three CM meteorites, Mighei, Murchison (measured in a previous study), and Murray. The data points show individual linear trends between ¹³⁵Cs/¹³⁶Ba ratios and ¹³⁵Ba isotopic deviations for the three samples. Considering the different trends observed in the three CM meteorites, the Ba isotopic composition of the CM meteorite parent body was heterogeneous at its formation. Chronological information is unclear in the data for Murchison and Murray because of large analytical uncertainties imposed by error propagation. Only the Mighei meteorite data indicate the possible existence of presently extinct ¹³⁵Cs (¹³⁵Cs/¹³³Cs = (2.7 ± 1.6) × 10⁻⁴) in the early solar system. Another explanation of the data for the three CM meteorite is mixing of at least three components with different Ba isotopic compositions, although this is model-dependent.  相似文献   

Records of ancient cosmic radiation in extraterrestrial rocks     

Narendra Bhandari 《Journal of Earth System Science》1981,90(3):359-382

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of²⁶A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J _s,R _o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×10⁵ to 2×10⁶ years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (⁵³Mn,²⁸Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and²²Ne/²¹Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.²²Ne/²¹Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from⁶⁰Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.  相似文献   

Preliminary investigation of mineralogy,petrology and chemical composition of Qingzhen enstatite chondrite     

Daode Wang  Xiande Xie 《中国地球化学学报》1982,1(1):69-81

On September 13, 1976, the Qingzhen enstatite chondrite fell near Qingzhen County, Guizhou Province (26°32′N, 106°28′E). The total mass recovered was 2.6 kg. Phenomena of the meteorite fall were recorded as well. In the present paper some of the significant observations are described. X-ray diffraction analyses and petrologic studies revealed that it is mainly composed of enstatite/clinoenstatite, plagioclase, troilite, kamacite, taenite, quartz and oldhamite. X-ray electron microprobe analyses were performed on orthoenstatite and clinoenstatite. Chemical analysis gave: SiO₂-36.48; Al₂O₃-1.64; TiO₂-0.08; Cr₂O₃-0.28; FeO-0.30; MnO-0.25; MgO-18.19; CaO-1.14; Na₂O-1.06; K₂O-0.11; H₂O⁺-0.47; H₂O^?-0.10; P₂O₅-0.39; FeS-13.35; FeO-22.94; Ni-1.81; Co-0.093; C-0.37; Cu-0.022; Zn-0.027; CaS-0.60; total-99.70 (wt.%). This meteorite has been extremely reduced; all iron is present as FeS and metallic Fe-Ni. The Qingzhen enstatite chondrite is characterized by high ratio, Si/Mg as compared with ordinary chondrites, scarcity of olivine, and enrichment in sulfur and iron. In thin sections, it is observed that round chondrules approximately account for 10–15 per cent, being porphyritic, radial, excentroradial, or excentric fan-shaped. Devitrified chondrules composed of pyroxene were also identified. It is postulated from its mineralogy, chemistry and textural characteristics that the Qingzhen meteorite should be assigned toE ₄ orEH chondrites.  相似文献   

A study on cosmogenic nuclides in the Jilin meteorite and its irradiation history     

Ziyuan Ouyang  G. Heusser 《中国地球化学学报》1982,1(4):421-434

44 samples of the Jilin meteorite were analyzed by various laboratories for spallogenic, radiogenic, and trapped rare gases. A non-uniform distribution has been found for the rare gases of different origins. There have been found correlations among the spallogenic rare gases, with apparent depth effects. 43 samples were analyzed for their cosmogenic radionuclides⁶⁰Co,²⁶Al,³⁶Cl,⁴⁰K,⁵³Mn,⁵⁴Mn, and²²Na. Correlations have been found between⁶⁰Co and⁵³Mn and between⁶⁰Co and²¹Ne. The Jilin meteorite has a complex history of cosmic-ray irradiation. According to the two-stage model and the other fragments have a burying depth two stages,T ₁=11 m.y. andT ₂=0.3 m.y. The burying depth of all samples in the parent body can be obtained by the content of²¹Ne. Jilin meteorite No. 1 is located 20–142 cm from the surface, No. 4 ranges from 106–134 cm, and the other fragments have a burying depth between 15 and 150 cm. The equation of the reference plane for the surface of the 1-stage Jilin meteoroid is 0.24x+0.81y+0.53z+0.5=0. Use can also be made of⁶⁰Co to determine the burying depth of all samples in the 2-stage Jilin meteoroid (under a geometry of 4π), and further to restore the preatmospheric form and size of the parent body. During its atmospheric passage, the Jilin had an ablation rate of ca. 30%. On the basis of the two-stage model and the concentration of²¹Ne and⁶⁰Co in the samples, we propose a scheme to restore the relative position of all samples in the parent body. 12 sample-distribution regions can be sketched out. Also can be restored the relative position of all the samples in the parent body during the two stages.  相似文献   

The stable carbon isotopes in enstatite chondrites and Cumberland Falls     

Peter Deines  Frans E. Wickman 《Geochimica et cosmochimica acta》1985,49(1):89-95

The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average ¹³C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest ¹³C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.  相似文献   

The distribution of U and Pu in the St. Severin chondrite     

J.H. Jones  D.S. Burnett 《Geochimica et cosmochimica acta》1979,43(12):1895-1905

A detailed study of the U distribution of the St. Severin chondrite has been made by fission track radiography in order to clarify the interpretation of fission Xe thermal release data in terms of the mineralogical location of the fission Xe within the meteorite. This is of importance because the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin has been widely adopted as the average solar system value. The U contents of the constituent minerals cannot account for the total rock U which, instead, appears to be primarily localized on grain boundaries. The greatest localizations of U are in olivine-poor, orthopyroxene-rich ‘clasts’. Our data coupled with those of Podosek (1970a) show that ²⁴⁴Pu in St. Severin was also located on grain boundaries and that the bulk of Pu and U are unfractionated within this meteorite. Due to recoil, the ²⁴⁴Pu fission Xe is found in 10 micron surface layers on major phases. Assuming that the grain boundaries (on which the Pu was located) was formed during metamorphism, the ${}^{244}\text{Pu}{}^{238}\text{U}$ ratio for St. Severin applies to a time subsequent to the textural recrystallization of the meteorite. Our data support the interpretation of Podosek and our best estimate of the solar system ${}^{244}\text{Pu}{}^{238}\text{U}$ is 0.015.  相似文献   

Do lunar and meteoritic archives record temporal variations in the composition of solar wind noble gases and nitrogen? A reassessment in the light of Genesis data     

《Chemie der Erde / Geochemistry》2016,76(4):463-480

Since about half a century samples from the lunar and asteroidal regoliths been used to derive information about elemental and isotopic composition and other properties of the present and past solar wind, predominantly for the noble gases and nitrogen. Secular changes of several important compositional parameters in the solar wind were proposed, as was a likely secular decrease of the solar wind flux. In 2004 NASA’s Genesis mission returned samples which had been exposed to the solar wind for almost 2.5 years. Their analyses resulted in an unprecendented accuracy for the isotopic and elemental composition of several elements in the solar wind, including noble gases, O and N. The Genesis data therefore also allow to re-evaluate the lunar and meteorite data, which is done here. In particular, claims for long-term changes of solar wind composition are reviewed.Outermost grain layers from relatively recently irradiated lunar regolith samples conserve the true isotopic ratios of implanted solar wind species. This conclusion had been made before Genesis based on the agreement of He and Ne isotopic data measured in the aluminum foils exposed to the solar wind on the Moon during the Apollo missions with data obtained in the first gas release fractions of stepwise in-vacuo etch experiments. Genesis data allowed to strengthen this conclusion and to extend it to all five noble gases. Minor variations in the isotopic compositions of implanted solar noble gases between relatively recently irradiated samples (<100 Ma) and samples irradiated billions of years ago are very likely the result of isotopic fractionation processes that happened after trapping of the gases rather than indicative of true secular changes in the solar wind composition. This is particularly important for the ³He/⁴He ratio, whose constancy over billions of years indicates that hardly any ³He produced as transient product of the pp-chains has been mixed from the solar interior into its outer convective zone. The He isotopic composition measured in the present-day solar wind therefore is identical to the (D + ³He)/⁴He ratio at the start of the suns’s main sequence phase and hence can be used to determine the protosolar D/H ratio.Genesis settled the long-standing controversy on the isotopic composition of nitrogen in lunar regolith samples. The ¹⁵N/¹⁴N ratio in the solar wind as measured by Genesis is lower than in any lunar sample. This proves that nitrogen in regolith samples is dominated by non-solar sources. A postulated secular increase of ¹⁵N/¹⁴N by some 30% over the past few Ga is not tenable any longer. Genesis also provided accurate data on the isotopic composition of oxygen in the solar wind, invaluable for cosmochemisty. These data superseded but essentially confirmed one value – and disproved a second one – derived from lunar regolith samples shortly prior to Genesis.Genesis also confirmed prior conclusions that lunar regolith samples essentially conserve the true elemental ratios of the heavy noble gases in the solar wind (Ar/Kr, Kr/Xe). Several secular changes of elemental abundances of noble gases in the solar wind had been proposed based on lunar and meteoritic data. I argue here that lunar data – in concert with Genesis – provide convincing evidence only for a long-term decrease of the Kr/Xe ratio by almost a factor of two over the past several Ga. It appears that the enhancement of abundances of elements with a low first ionisation potential in the solar wind (FIP effect) changed with time.Finally, Genesis allows a somewhat improved comparison of the present-day flux of solar wind Kr and Xe with the total amount of heavy solar wind noble gases in the lunar regolith. It remains unclear whether the past solar wind flux has been several times higher on average than it is today.  相似文献   

Variability of the Al²⁶ production rate in ordinary chondrites     

Gregory F Herzog  Philip J Cressy 《Geochimica et cosmochimica acta》1974,38(12):1827-1841

15 ordinary chondrites for which unusually high spallogenic $\text{Ne}{}^{22}\text{Ne}{}^{21}$ or $\text{He}{}^3\text{Ne}{}^{21}$ ratios had been reported and one meteorite with marked shock characteristics were selected in order to investigate the relations between $\text{Ne}{}^{22}\text{Ne}{}^{21}$ ratios, Al²⁶ contents and depth. We report Al²⁶ and K contents of 13 samples from 11 of these and-noble gas contents of 30 samples from all of these stones.A decrease in the Al²⁶ production rate accompanies the increase of $\text{Ne}{}^{22}\text{Ne}{}^{21}$ towards the pre-atmospheric surface: $\text{Al}{}_{\mathrm{obs}}{}^{26}\text{Al}{}_{\mathrm{calc}}{}^{26}\text{= 3.2?2.0}\text{Ne}{}^{22}\text{Ne}{}^{21}$ for $\text{1.08 ≤}\text{Ne}{}^{22}\text{Ne}{}^{21}\text{≤ 1.2}$. Large deviations from this relationship may indicate that a meteorite experienced an abnormal flux of cosmic rays.For $\text{Ne}{}^2\text{Ne}{}^{21}\text{> 1.2}$ this trend continues but the data scatter more, probably because of the steadily increasing influence of pre-atmospheric size. $\text{Ne}{}^{22}\text{Ne}{}^{21}$ ratios increase most rapidly in the outermost few centimeters according both to a plot of $\text{Ne}{}^{22}\text{Ne}{}^{21}$ vs (recovered mass)^{$\text{1}\text{3}$} and to track studies. The increase seems to derive from the enhanced importance of nuclear reactions on Si.$\text{Ne}{}^{22}\text{Ne}{}^{21}\text{< 1.08}$ defines a region where the Al²⁶ production rates are less sensitive to depth and vanish in the limit of large shielding; the weak correlation between $\text{Ne}{}^{22}\text{Ne}{}^{21}$ and Al²⁶ in this region rules out the use of the $\text{Ne}{}^{22}\text{Ne}{}^{21}$ ratio as a basis for a shielding correction to Al²⁶.  相似文献