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1.
Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases 36Cl could be determined as well. 36Cl-36Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic 3He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of 3He in the silicates is correlated with their mineralogical composition: feldspar has lost its 3He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high ratio in the metal, which is due to a high 39Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion).Production rates of 38,39Ar from Ca and 21,22Ne from Mg are given. The ratio is close to unity. The ratios vary between 20 and 50, and are not correlated with the absolute production rate of 38Ar from metal. The production ratio from Mg is found to be close to but below unity.Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10?10 cm3 STP 129Xe/g in the non-magnetic portion. 相似文献
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O Eugster N Grögler M.D Mendia P Eberhardt J Geiss 《Geochimica et cosmochimica acta》1973,37(9):1991-2003
He, Ne, Ar, Kr and Xe concentrations and isotopic abundances were measured in three bulk grain size fractions prepared from sample L-16-19, No. 120 (C level, 20–22 cm depth) returned by the Luna 16 mission. The expected anticorrelation between the concentrations of trapped solar wind noble gases and grain size is observed. Elemental abundances of solar wind trapped noble gases are similar to those previously found in corresponding grain size fractions of the Apollo 11 and 12 fines. The trapped ratio varies in the soils from different lunar maria due to diffusion losses. A rough correlation of with the proportion of ilmenite in these samples is apparent. The elemental and isotopic ratios of the surface correlated noble gases in Luna 16 resemble those previously found in Apollo fines. Based on 21Ne, 78Kr and 126Xe a cosmic ray exposure age of 360 my was determined. This age is similar to those obtained for the soils from other lunar maria. 相似文献
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15 ordinary chondrites for which unusually high spallogenic or ratios had been reported and one meteorite with marked shock characteristics were selected in order to investigate the relations between 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 towards the pre-atmospheric surface: for . Large deviations from this relationship may indicate that a meteorite experienced an abnormal flux of cosmic rays.For this trend continues but the data scatter more, probably because of the steadily increasing influence of pre-atmospheric size. ratios increase most rapidly in the outermost few centimeters according both to a plot of vs (recovered mass) and to track studies. The increase seems to derive from the enhanced importance of nuclear reactions on Si. 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 and Al26 in this region rules out the use of the ratio as a basis for a shielding correction to Al26. 相似文献
6.
Stephen P Smith 《Geochimica et cosmochimica acta》1984,48(5):1033-1041
Gabbro and diorite from the Skaergaard layered igneous intrusion contain noble gases which are mixtures of atmospheric and juvenile components. Atmospheric noble gases predominate in samples that have undergone extensive oxygen isotope exchange with meteoric-hydrothermal water. The source of the atmospheric noble gas component is inferred to be the hydrothermal circulation system. A juvenile component with and containing fission xenon is also present This component predominates in samples showing unaltered magmatic oxygen isotope compositions. Neon of atmospheric isotopic composition is associated with the juvenile radiogenic 40Ar and fission xenon. The source of this second noble gas component may be either the crustal country rock or the upper mantle. If the neon is juvenile primordial neon from a mantle source region, terrestrial primordial is the same as atmospheric to within 4%. However, subduction of atmospheric noble gases into the upper mantle may provide an alternate source of neon and other noble gases in the mantle. 相似文献
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B.M. Kennedy M.A. Lynch J.H. Reynolds S.P. Smith 《Geochimica et cosmochimica acta》1985,49(5):1251-1261
The Roving Automated Rare Gas Analysis (RARGA) lab of Berkeley's Physics Department was deployed in Yellowstone National Park for a 19 week period commencing in June, 1983. During this time 66 gas and water samples representing 19 different regions of hydrothermal activity within and around the Yellowstone caldera were analyzed on site. Routinely, the abundances of five stable noble gases and the isotopic compositions of He, Ne, and Ar were determined for each sample. In a few cases the isotopes of Kr and Xe were also determined and found to be of normal atmospheric constitution.Correlated variations in the isotopic compositions of He and Ar can be explained within the precision of the measurements by mixing of only three distinct components. The first component is of magmatic origin and is enriched in the primordial isotope 3He with times the air value. This component also contains radiogenic 40Ar and possible 36Ar with , resulting in a ratio ≥ 41,000 times the air value. The second component is assumed to be purely radiogenic 4He and 40Ar (). This component is the probable carrier of observed excesses of 211Ne, attributed to the α,n reaction on 18O. Its radiogenic character implies a crustal origin in U. Th, and Krich aquifer rocks. The third component, except for possible mass fractionation, is isotopically indistinguishable from the noble gases in the atmosphere. This component originates largely from infiltrating run-off water saturated with atmospheric gases.In addition to exhibiting nucleogenic 211Ne, Ne data show anomalies in the ratio , which correlate roughly with the anomalies for the most part, but not as would occur from simple mass fractionation. Some exaggerated instances of the anomaly occur which could be explained by combined mass fractionation of Ne and Ar isotopes to a severe degree coupled with remixing with normally isotopic gases. Otherwise exotic processes have to be invoked to explain the 20Ne data.Relative abundances of the non-radiogenic and non-nucleogenic noble gases (22Ne, 36Ar, 84Kr, and 132Xe) are highly variable but strongly correlated. High Xe/Ar ratios are always accompanied by low Ne/ Ar ratios and vice versa. Except for water from the few cold (T < 20°C) springs analyzed, none of the samples have relative abundances consistent with air saturated water and the observed variations are not readily explained by the distillation of air saturated water.In characterizing each area of hydrothermal activity by the highest ratio found for that area, we find that within the caldera this parameter is somewhat uniform at ~7 ± 1 times the air value. There are exceptions, most notably at Mud Volcano, an area located along a crest of recent and rapid uplift. Here the maximum ratio is ~ 16 times the air value. Also noteworthy is Gibbon Basin which is in the vicinity of the most recent rhyolitic volcanism and exhibits a ratio ~ 13 times the air value. Immediately outside the caldera the maximum ratio decreases rapidly to values < ~3 times the air value. 相似文献
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The Rameka Gabbro, emplaced 367 Ma ago, experienced a well documented reheating on intrusion of the Separation Point Batholith 114 Ma ago. age spectrum analyses of hornblende from the Rameka Gabbro show diffusion gradients which provide information on the 40Ar boundary concentration during reheating.Three samples of hornblende exhibit age spectra that conform to a model of 40Ar loss by diffusion, implying a zero 40Ar boundary concentration during heating. The calculated 40Ar loss from these samples, together with a model of heat flow in the aureole, provide estimates of diffusion coefficients of 40Ar in Mg-rich hornblende which correspond to an activation energy, E, of ~60 kcal-mol?1 and a frequency factor. D0, of ~ 10?3 cm2-sec?1. When combined with laboratory diffusion results, these data yield a well defined diffusion law (E = 63.3 ± 1.7 kcal-mol?1, ).The age spectra of the eight other samples record steep gradients of excess 40Ar over the first few percent of gas release. Although this effect causes high apparent conventional K-Ar ages, the plateau segments of many sampes still record the crystallization age of 367 ± 5 Ma. These measurements show that the excess 40Ar phase developed locally in the intergranular regions of the gabbro, following intrusion of the batholith. on time scales that varied from 104 to 106years. The minimum average ratio of this component was found to be 1300 ± 400. The partial pressure of Ar was at least 10?2 bars in some places.A single age spectrum analysis of plagioclase reveals a ‘saddle-shaped” release pattern with a minimum at 140 Ma.In conjunction with theoretical diffusion models and a diffusion law, age spectrum analysis of hornblende that has experienced a post-crystallization heating can provide close estimates of the maximum temperature of the thermal event as well as both age of crystallization and reheating. 相似文献
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Radioactivity measurements have shown evidence for long-lived 92Nb and in natural niobium. The specific activity of 94Nb was observed to be 0.32 ± 0.03 dis/min. kg Nb and that of 92Nb to be 0.058 ± 0.035 dis/min. kg Nb. With taken as ≈ 1.7 × 108yr, the isotopic abundance of 98Nb is 1.2 × 10?10 per cent. 相似文献
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R.K Moniot T.H Kruse C Tuniz W Savin G.S Hall T Milazzo D Pal G.F Herzog 《Geochimica et cosmochimica acta》1983,47(11):1887-1895
The 10Be contents of 28 stony meteorites with known 21Ne contents range from 0.97 to 23 dpm/kg and give an average 21Ne production rate (P21) of (0.28 ± 0.02) × 108 cm3 STP/g-Myr for shielding conditions corresponding to in an H-chondrite. Our agrees with others' P21 based on 22Na, 81Kr and 53Mn but not on 26A1. Temporal variations in the cosmic ray flux do not explain the disagreement satisfactorily; major errors in the radionuclide half-lives are not indicated. The discrepancy seems rooted in the data selection and the difficulties of making accurate corrections for shielding, chemical composition and other sources of variability. 相似文献
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Noble gas data are reported for 12 E-chondrites. Combined with literature data, they show that K-Ar ages are >4 Æ for 14 out of 18 meteorites, yet U, Th-He ages are often shorter, perhaps due to late, mild reheating. Cosmic-ray exposure ages differ systematically between types 4 and 6, with E4's mostly below 16 Myr and E6's above 30 Myr. This may mean that the E-chondrite parent body contains predominantly a single petrologic type on the (~ 1 km) scale of individual impacts, in contrast to the more thoroughly mixed parent bodies of the ordinary chondrites.The heavy noble gases consist of at least two primordial components: the usual planetary component () and a less fractionated, ‘subsolar’ component (). The latter is found in highest concentration in the E4 chondrite South Oman (36Ar = 760 × 10?8cc/g, ). The isotopic compositions of both components are similar to typical planetary values, indicating that some factor other than mass controlled the noble gas elemental ratios. The heavy Xe isotopes occasionally show some of the lowest and ratios measured in bulk chondrites, suggestive of nearly fission-free Xe (e.g. ). Amounts of planetary gas in E4 E6 chondrites fall in the range for ordinary chondrites of types 4–6, but, in contrast to the ordinary chondrites. fail to correlate with petrologic type or volatile trace element contents. Another unusual feature of E-chondrites is that primordial Ne is present even in most 4's and 5's (20Nep ~ 1 to 7 × 10?8cc/g). with an isotopic composition consistent with planetary Ne.Analyses of mineral separates show that the planetary gases are concentrated in an HF- and HCl-insoluble mineral similar to phase Q, the poorly characterized, HNO3-soluble carrier of primordial gases in carbonaceous and ordinary chondrites. The subsolar gases, on the other hand, are located in an HCl- and HNO3-resistant phase, possibly enstatite or a minor phase included in enstatite. Much of the 129Xer (50% for E4's, > 70% for E6's) is in HCl-resistant but HF-soluble sites, suggestive of a silicate.A similar subsolar component may be responsible for the high ratios of some C3's, unequilibrated ordinary chondrites, and the unique aubrite Shallowater. The planet Venus also has a high ratio, well above the planetary range, and hence may have acquired its noble gases from an E-chondrite-like material, similar to South Oman. 相似文献
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age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma?1) through the temperature zone of partial radiogenic 40Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of 39Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma?1. Diffusion parameters calculated from the 39Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol?1, and a frequency factor/grain size parameter, . This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol?1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess 40Ar upon diffusion profiles of the kind described here. 相似文献
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He3, He4, Ne21 and Ar38 contents were determined in 18 metal, troilite, sehreibersite and graphite inclusions of 9 iron meteorites, by total outgassing and stepwise heating. The ratio in metal phase ranges from 3.85 to 4.65, but in non-metallic samples, from 6.70 to 30.5. The results for cosmogenic isotopes of helium, neon and argon disagree appreciably with data on accelerator-irradiated targets. It should be noted, however, that some inclusions have lost considerable amounts of gas by diffusion.Uranium contents of 22 troilite and sehreibersite samples were determined by the fission track technique. The average uranium content of troilite is 0.4-0.7 ppb. Excess He4 of unknown origin was observed in troilite inclusions. If one assumes that the excess He4 was produced by uranium decay in situ, then the apparent U-He4 age is at least 5.9 × 109 yr. 相似文献
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The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods.Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10?2?1 × 10?2, Na/Ca ratios from 3.6 × 10?3 to 9.2 × 10?3, values between 1.5 and 7%. and from ?7.7 to 3.4%..The distribution coefficient of Na+ between aragonite and aqueous solutions, , is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator.Sr/Ca ratios and values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%).Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m.The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation.Na/Ca ratios in aragonite, which correlate well with values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan. 相似文献
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F.A Podosek J.C Huneke A.J Gancarz G.J Wasserburg 《Geochimica et cosmochimica acta》1973,37(4):887-904
Ages were determined by the 40Ar-39Ar method on two metaclastic rocks returned from the lunar highlands north of Mare Fecunditatis by the Luna 20 probe. Both samples gave very well-defined argon retention ages of 3.90 ± 0.04 AE which are indistinguishable from each other within a resolution of 0.02 AE. Both fragments, 22006 and 22007, are highly recrystallized polymict breccias; there is no evidence for loss of radiogenic 40Ar, and the age almost surely dates the time of recrystallization. The cosmic ray exposure ages of these fragments are similar and high: 900 million years for 22006, 1300 million years for 22007. 22007 also contains substantial trapped argon with a high ratio.The Luna 20 results greatly extend the area of the Moon's surface exhibiting a well-defined record of metamorphism at 3.9 AE. So far, lunar history in the interval 4.6?3.9 AE is not preserved in the ages of surface rocks. This obliteration suggests lunar-wide metamorphic conditions occurring or terminating at this time as a result of major impacts. 相似文献
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Roy S. Lewis Leo Alaerts Jan Hertogen Marie-Josée Janssens Herbert Palme Edward Anders 《Geochimica et cosmochimica acta》1979,43(6):897-903
A black inclusion from the Krymka LL3 chondrite was analyzed for 20 trace elements and five noble gases, by radiochemical neutron activation and mass spectrometry. The trace element pattern somewhat resembles that of C1 or C2 chondrites, but with several unique features. Elements of nebular condensation T ? 1000 K (U, Re, Os, Ir, Ni, Pd, Au, Sb and Ge) are essentially undepleted, as in C1 chondrites, but is 1.49 × higher than the characteristic Cl value. Among elements condensing below 1000 K, Cs, Se, Te, and In are depleted to approximately C2 levels (~0.6 × C1), whereas Ag, Bi, Tl are enriched to ~ 1.6 × C1. Such enrichments are thought to be characteristic of late nebular condensates.The noble-gas pattern also is unique. Gas contents are higher than in C1s, by factors of 2.6 to 19 for Ne through Xe. The ratio of 500 is higher than mean values for C1s or C2s (109 or 89) and exceeds even the highest value seen in C3Os, 420, whereas the ratio of 62 is much lower than the values for C1s and C2s (200–370). The and ratios of 1.040 and 0.320 resemble those of C1 chondrites, and seem to imply typical proportions of radiogenic Xe129 and ‘fissiogenic’ xenon.It appears that the inclusion represents a new primitive meteorite type, similar to C-chondrites, but probably a late condensate from a region of higher nebular pressure. 相似文献
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A direct-sampling, mass-spectrometric technique has been used to measure simultaneously the solubilities of He, Ne, Ar, Kr, and Xe in fresh water and NaCl brine (0 to 5.2 molar) from 0° to 65 °C, and at 1 atm total pressure of moist air. The argon solubility in the most concentrated brines is 4 to 7 times less than in fresh water at 65 °C and 0°C, respectively. The salt effect is parameterized using the Setschenow equation. where M is NaCl moiarity, βio(T) and βi(T) the Bunsen solubility coefficients for gas i in fresh water and brine, and KiM(T) the empirical salting coefficient. Values of KiM(T) are calculated using volumetric concentration units for noble gas and NaCl content and are independent of NaCl molarity. Below about 40°C, temperature coefficients of all KiM are negative. The value of KHeM is a minimum at 40°C. KArM decreases from about 0.40 at 0°C to 0.28 at 65 °C. The absolute magnitudes of the differences in salting coefficients (relative to KArM) decrease from 0° to 65°C. Over the range of conditions studied, all noble gases are salted out, and KHeM ? KNeM < KArM < KKrM < KXeM.From the solubility data, we calculated , , and for the transfer of noble gases from fresh water to 1 molar NaCl solutions. At low temperatures , is positive, but decreases and becomes negative at temperatures ranging from about 25°C for He to 45°C for Xe. At low temperatures, the dissolved electrolyte apparently interferes with the formation of a cage of solvent molecules about the noble gas atom. At higher temperatures, the local environment of the gas atom in the brine appears to be slightly more ordered than in pure water, possibly reflecting the longer effective range of the ionic fields at higher temperature.The measured solubilities can be used to model noble gas partitioning in two-phase geothermal systems at low temperatures. The data can also be used to estimate the temperature and concentration dependence of the salt effect for other alkali halides. Extrapolation of the measured data is not possible due to the incompletely-characterized minima in the temperature dependence of the salting coefficients. The regularities in the data observed at low temperatures suggest relatively few high-temperature data will be required to model the behavior of noble gases in high-temperature geothermal brines. 相似文献
19.
K.A. Foland Jeffrey S. Linder Thomas E. Laskowski Norman K. Grant 《Chemical Geology》1984,46(3):241-264
Nine glauconite samples with relatively high K concentrations and which appear to be well crystallized using normal X-ray diffraction techniques have been studied using the method. The glauconite apparent ages exceed their KAr, RbSr and, in most cases, stratigraphic ages by substantial amounts. release spectra sometimes yield plateaus but these apparent ages have no geological significance. The results indicate that 39Ar is lost by recoil from mineral grains during neutron irradiation, consistent with previously reported observations. The amount of 39Ar loss was measured by isotope dilution for four samples and varied from 29% to 17%. In contrast, radiogenic 40Ar is quantitatively retained during irradiation. The very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39Ar loss and unsuitable for dating. 相似文献
20.
Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced 3He, 21Ne, 22Ne and 38Ar indicate that all eight samples experienced less than average cosmic ray shielding. 3He and 21Ne exposure ages were calculated using shielding corrected chondritic production rates and the measured . Exposure ages calculated from and ratios and constant relative production rates show a bias between the two ages due to variations in . Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the and production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe. 相似文献