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1.
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
2.
Since Nb and Zr are only little fractionated during magmatic processes, the 92Nb-92Zr relative chronometer has the potential of dating the formation of the planetary bodies through their differentiation. Thus, we have analyzed the isotopic composition of zirconium in lunar, meteoritic and old and recent terrestrial samples.No isotopic variation has been found. However, the ratio of 3.8 Ga. old zircons from the Isua acid conglomerate is on the lower limit of the range of the standard measurements. If considered an anomaly, it would correspond to a +1.5 × 10?492Zr relative deficiency or to a 3.0 × 10?494Zr relative excess.Our data constrain the isotopic ratio at the time of formation of the solar system to be less than 0.007, so that the maximum sensitivity of the 92Nb-92Zr relative chronometer for the formation of planetary bodies is around 107 a. A discussion of some possible nuclear reactions indicates that zirconium isotopic variations in zircons are not easily produced, and that the 92Nb and 94Nb natural activities (Apt et al., 1974) cannot be explained by any single one of the processes proposed so far. 相似文献
3.
The inert gases have been measured in six size fractions covering the range below 500 μm, in a single feldspathic fragment weighing 523 μg, and in an agglutinate particle weighing 465 μg. The two size fractions between 125 and 250 μm as well as 250 and 500 μm were separated into magnetic and non-magnetic portions, which were measured separately. Like the Apollo and Luna 16 fines, the terra fines represented by Luna 20 are very rich in trapped solar-wind gases, but they contain relatively less He4 and Ne20, which is revealed by their average ratio of 35 and ratio of 2.9. Obviously the terra materials are less retentive for solar-wind He and Ne than typical mare fines such as 10084. Whether this is due to the relatively small TiO2 or the relatively large plagioclase content of the former is not resolved. ( and ( ratios are relatively large; the average values are 2800 and 14400, respectively. The apparent Ne21 radiation ages of all the size fractions are in the range 209–286 × 106 yr; the average is 260 × 106 yr. This is in the range of values known for the Apollo and Luna 16 fines. The feldspathic fragment has a much greater apparent Nec21 age of 780 × 106 yr. The Ar40-Ar36 systematic reveals the presence of two Ar40 components, because Ar40 = (1.41 ± 0.076)Ar36 + (0.490 ± 0.130) × 10?4 (cm3 STP/g). The slope of 1.41 is not inconsistent with an origin of the sample from a relatively old terra region. 相似文献
4.
Xanthates are used in the flotation of sulfide ores although their aqueous solutions are not stable under certain conditions. Their stability in acidic and weakly acidic aqueous solutions was therefore investigated, as these media are required for some processes.The peak absorbances of ethylxanthate ion and carbon disulfide were first determined in aqueous solution. The decomposition of ethylxanthate ion was analyzed by measuring variations in absorbance (at 301 nm) and pH with respect to time. A pH regulation system was then used while measuring variations in absorbance and productions of protons caused by xanthate decomposition.The results concerning xanthate half-lives show good agreement with the literature, but the kinetic results deviate substantially. The following relation was obtained for half-life: We established that ethylxanthate decomposition at pH 4 is a first order reaction with respect to ethylxanthate concentration, and postulating this order to the other pH values, the following kinetic relation was found: where v is the rate of decomposition (mol l?1 min?1), and [EtX?]∞ is the ethylxanthate concentration when the decomposition equilibria are reached (mol l?1). The better concentration was found to obey the law: 相似文献
5.
Darrell Kirk Nordstrom 《Geochimica et cosmochimica acta》1977,41(12):1835-1841
The redox potential of ZoBell's solution, consisting of molar K4Fe(CN)6, molar K3Fe(CN)6 and 0.10 molar KCl, has been measured by a polished platinum electrode vs a saturated KCl, Ag/AgCl reference electrode. Measurements in the temperature range 8–85°C fit the equation where is in degrees Celsius. Evaluation of literature data was necessary to obtain a reliable value for the Ag/AgCl half-cell reference potential as a function of temperature. Combining the measurements from this study with the literature evaluation of the Ag/AgCl reference potential yields the temperature dependent potential for ZoBell's solution: relative to the standard hydrogen potential. From these data the enthalpy, entropy, free energy and heat capacity for the ferro-ferricyanide redox couple have been calculated. The temperature equation for the potential of ZoBell's solution may be used for checking potentiometric equipment in the determination of the redox potential of natural waters. 相似文献
6.
The ratios measured in 27 Southern Africa diamond stones, four from Premier Mine and the rest of unidentified origin, range from 4.2 × 10?8 to 3.2 × 10?4, with three stones above 1 × 10?4. We conclude that the initial helium isotopic ratio ( in the earth was significantly higher than that of the planetary helium-A (), but close to the solar helium ().The apparent K-Ar ages for the twelve diamonds of unidentified origin show enormously old age, indicating excess argon-40. evolution in diamonds suggests that the diamonds with the high ratio (>2 × 10?4) may be as old as the earth.Noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number.This paper discusses the implications of these observations on the early solar system and the origin of diamonds. 相似文献
7.
W.F Slawson 《Geochimica et cosmochimica acta》1976,40(1):117-121
Sixty-three samples from the granitic core of the Vredefort ring structure have been analyzed for K, Rb, and Sr. The ratios were determined for thirteen of these. An isochron yielding an age of 2.85 × 109 yr was obtained. The and ratios exhibit a ‘bulls-eye’ pattern in the granite suggesting the central portion has been extruded from depth. Diapiric flow and subsequent erosion has exposed a section of the continental crust within the Vredefort core. 相似文献
8.
The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, ( ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, ); porphyry (361.2 ± 1.4 Ma, ); using λ87Rb = 1.42 × 10?11yr?1.A suite of Meguma country rock samples showed a variation of at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known. 相似文献
9.
D.W. Davis J. Gray G.l. Gumming H. Baadsgaard 《Geochimica et cosmochimica acta》1977,41(12):1745-1749
The decay constant 87Rb has been redetermined by measuring the amount of radiogenic 87Sr produced over a period of 19 years, in 20 g samples of purified RbClO4, using isotope dilution techniques. The rubidium sample was spiked with 84Sr and the nanogram quantities of strontium separated by coprecipitation with Ba(NO3)2. Analyses were carried out on a 25cm, 90° sector mass spectrometer equipped with a Spiraltron electron multiplier. Measurement of three independent ratios permitted continuous monitoring of the ion beam fractionation. The average of nine determinations gives a value for the decay constant of . [.] 相似文献
10.
Silver in the metal phases of Cape York (IIIA) and Grant (IIIB) has been determined after an extensive surface cleaning process. The was found to be enriched over that found in terrestrial Ag by ~7%. to 19%., demonstrating the presence of excess 107Ag (107Ag1) in this class of meteorites. An effort was made to find schreibersite with a distinctive 108Pd/109Ag ratio in order to establish a three-point isochron, but the results are not markedly different from those obtained for the bulk metal. The Ag isotopic ratio of sulfides from the same meteorites were nearly normal in composition. These results demonstrate correlations of with between coexisting phases of two iron meteorites that are associated with planetary differentiation processes. The ratios were found to be 1.7 × 10?5 and 1.2 × 10?5 for Cape York and Grant, respectively. These observations are in support of the widespread presence of 107Pd in the early solar system. The difference in isotopic composition between metal and sulfide phases demonstrates that silver diffusion was small (over 6.5 × 106 y) indicating a cooling rate much greater than 150°C/my for meteorites which have been attributed to small planetary cores. Uranium determinations were carried out on the metal phases and concentrations of ~ g U/g and 2 × 10?10g U/g were found for Cape York and Grant, respectively. The Pb in these meteorites was determined using the improved cleaning procedures and chemical separations with low blank levels. The results confirm the presence of variable proportions of radiogenic Pb in both the metal and sulfide phases of iron meteorites. No simple explanation for the presence of radiogenic lead is apparent; while terrestrial contamination may appear to be the obvious explanation, it is possible that this effect could result from relatively recent metamorphism in the meteorite parent body. 相似文献
11.
Krystyna W. Semkow Ronald A. Rizzo Larry A. Haskin David J. Lindstrom 《Geochimica et cosmochimica acta》1982,46(10):1879-1889
Cyclic voltammetry has been done for Ni2+, Co2+, and Zn2+ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are , , and . The electrode reactions are rapid, with first order rate constants of the order of 10?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10?6 cm2/sec for Ni2+, 3.4 × 10?6 cm2/sec for Co2+, and 3.8 × 10?6 cm2/sec for Zn2+ at 1500°C. The value of (, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of ) was not observed to vary with composition over the range CaO · MgO · 2SiO2 to CaO·MgO·3SiO2 or from 1.67 CaO·0.33MgO·2SiO2 to 0.5 CaO·1.5MgO·2SiO2. The value for the diffusion coefficient for Ni2+ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO2 to CaO · MgO · 3SiO2. This is consistent with a mechanism by which Ni2+ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni2+ species discharging at the cathode regardless of the SiO2 concentration in the melt. 相似文献
12.
The performance characteristics of PANURGE, a modified CAMECA IMS3F ion microprobe, have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching. The techniques used for this type of measurement are described. Using this approach, the isotopic composition of Mg and Si and the atomic ratio of in minerals from the Allende inclusion WA and the Allende FUN inclusion Cl have been measured with the ion microprobe at high mass resolving power. Enrichments in 26Mg of up to 260%. have been found. Mg and measurements on cogenetic spinel inclusions and host plagioclase crystals yield Mg-Al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry. The measurements confirm the presence of substantial excess 26Mg in WA () and its near absence in Cl (). In WA plagioclase, data for which define a linear array with and with initial composition 30%. greater than in high Mg phases. This suggests a metamorphic reequilibration of Mg in Allende plagioclase at least 0.6 my after WA formation. There were no variations in detected in WA plagioclase associated with concentration of 26Mg1 into isolated clusters. We have confirmed by ion probe measurements that the Mg composition in Allende Cl is highly fractionated and is uniform among pyroxene, melilite, plagioclase, spinel crystals and spinel included in melilite and plagioclase crystals. Likewise, the Si composition is mass fractionated and is the same in pyroxene, melilite and plagioclase. 相似文献
13.
The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10?3 molal. The rate of the reaction: was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law: where mFe3+ is the molal concentration of uncomplexed ferric iron, k is the rate constant and is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10?4 to 2.7 × 10?4 sec?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater ratio, they appear to be more reactive per unit mass than the higher temperature types. 相似文献
14.
Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The ratios of hydrogen in the organic matter vary from 8 × 10?5 to 170 × 10?5 (δD ranges from ? 500‰ to 10,000‰) as compared to 16 × 10?5 for terrestrial hydrogen and 2 × 10?5 for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose ratios are greater than 30 × 10?5. If the ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having ratios above 50 × 10?5 would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed ratios in organic compounds in interstellar clouds are typically 180 × 10?5 and range between about 40 × 10?5 and 5000 × 10?5. The values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their ratios.In contrast, the ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10?5; very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H2O with H2 in the solar nebula at temperatures of about 200 K and higher.The ratios of organic matter, irrespective its ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the ratio of interstellar carbon five billion years ago was similar to the present terrestrial value.Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials. 相似文献
15.
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
16.
Howard M. May Philip A. Helmke Marion L. Jackson 《Geochimica et cosmochimica acta》1979,43(6):861-868
Solubility curves were determined for a synthetic gibbsite and a natural gibbsite (Minas Gerais, Brazil) from pH 4 to 9, in 0.2% gibbsite suspensions in 0.01 M NaNO3 that were buffered by low concentrations of non-complexing buffer agents. Equilibrium solubility was approached from oversaturation (in suspensions spiked with Al(NO3)3 solution), and also from undersaturation in some synthetic gibbsite suspensions. Mononuclear Al ion concentrations and pH values were periodically determined. Within 1 month or less, data from over-and undersaturated suspensions of synthetic gibbsite converged to describe an equilibrium solubility curve. A downward shift of the solubility curve, beginning at pH 6.7, indicates that a phase more stable than gibbsite controls Al solubility in alkaline systems. Extrapolation of the initial portion of the high-pH side of the synthetic gibbsite solubility curve provides the first unified equilibrium experimental model of Al ion speciation in waters from pH 4 to 9.The significant mononuclear ion species at equilibrium with gibbsite are Al3+, AlOH2+, Al(OH)+2 and Al(OH)?4, and their ion activity products are , , and . The calculated standard Gibbs free energies of formation (ΔG°f) for the synthetic gibbsite and the A1OH2+, Al(OH)+2 and Al(OH)?4 ions are ?276.0, ?166.9, ?216.5 and ?313.5 kcal mol?1, respectively. These ΔG°f values are based on the recently revised ΔG°f value for Al3+ (?117.0 ± 0.3 kcal mol?1) and carry the same uncertainty. The ΔG°f of the natural gibbsite is ?275.1 ± 0.4 kcal mol?, which suggests that a range of ΔG°f values can exist even for relatively simple natural minerals. 相似文献
17.
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
18.
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
19.
20.