Isotope Fractionation Processes Research Articles

Ion-neutral collisions in a 22-pole trap at very low energies

This contribution concentrates on the use of an ion trapping technique which has been developed for studying collision processes and chemical reactions at low temperatures and which utilizes inhomogeneous radio frequency fields. Special emphasis is put onto recent progress achieved with a cryo cooled 22-pole ion trap. Experimental results, measured at a nominal temperature of 10 K, are presented for radiative association of CH3+ and C2H2+ with H2. Another group of results discussed in this paper deals with the dynamics of the growth of hydrogen clusters starting from H3+. In all cases, the influence of H2 rotation was examined by utilizing p-H2 and n-H2. Also the process of isotope fractionation in the presence of HD and D2 is briefly mentioned.

The Carbon and Nitrogen-Isotope Characteristics of the Argyle and Ellendale Diamonds, Western Australia

In investigations of diamond genesis, nitrogen can be used to place additional geochemical constraints on the origin of carbon. This study presents an examination of both the carbon and nitrogen stable-isotope ratios of statistically significant numbers of diamonds from the Argyle, Ellendale 4, and Ellendale 9 lamproites in Australia. The C and N stable-isotope data fall within the previously determined worldwide range. The stable-isotope ratios of these diamonds do not correlate with the physical properties of the diamond crystals, but do appear to be related to diamond paragenesis: both Iherzolitic and eclogitic parageneses define groups with different isotopic characteristics. Isotopic fractionation processes during or subsequent to diamond crystallization are not considered to be the cause of the observed isotopic variability. Instead, it is suggested that both Iherzolitic and eclogitic diamonds have a subduction signature: Iherzolitic diamonds reflect incorporation of marine carbonate, oceanic basalt...

Derivatization of organic compounds prior to gas chromatographic–combustion–isotope ratio mass spectrometric analysis: identification of isotope fractionation processes

An examination of the practice of derivatizing organic compounds such as fatty acids, sterols and amino acids in relation to subsequent analysis via gas chromatography–combustion–isotope ratio mass spectrometry is presented. Fractionation processes, such as kinetic isotope effects, which cause a deviation in the measured stable carbon isotope ratios (δ-values) of derivatized compounds from simple mass balance considerations are examined. Particular attention is paid to reactions that proceed by the cleavage of a carbon-containing bond and reactions that are likely to have kinetic isotope effects associated with them, such as acetylation and diazotization. Isotope fractionation processes other than those which are kinetic based are also discussed, as is the additional imprecision of the calculation of the δ-values of sample compounds inherent when derivative carbon is added. Failure to take this imprecision into account when comparing δ-values could lead to erroneous conclusions with respect to the magnitude of kinetic isotope effects caused by derivative reactions.

Comparison of Modeled and Observed Environmental Influences on the Stable Oxygen and Hydrogen Isotope Composition of Leaf Water in Phaseolus vulgaris L.

In this paper we describe how a model of stable isotope fractionation processes, originally developed by H. Craig and L. I. Gordon ([1965] in E Tongiorgi, ed, Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Paleotemperature, Spoleto, Italy, pp 9-130) for evaporation of water from the ocean, can be applied to leaf transpiration. The original model was modified to account for turbulent conditions in the leaf boundary layer. Experiments were conducted to test the factors influencing the stable isotopic composition of leaf water under controlled environment conditions. At steady state, the observed leaf water isotopic composition was enriched above that of stem water with the extent of the enrichment dependent on the leaf-air vapor pressure difference (VPD) and the isotopic composition of atmospheric water vapor (AWV). The higher the VPD, the larger was the observed heavy isotope content of leaf water. At a constant VPD, leaf water was relatively depleted in heavy isotopes when exposed to AWV with a low heavy isotope composition, and leaf water was relatively enriched in heavy isotopes when exposed to AWV with a large heavy isotope composition. However, the observed heavy isotope composition of leaf water was always less than that predicted by the model. The extent of the discrepancy between the modeled and observed leaf water isotopic composition was a strong linear function of the leaf transpiration rate.

Studies of smectite membrane behavior Electrokinetic, osmotic, and isotopic fractionation processes at elevated pressures

A 1.087 m NaCl brine was forced through a day membrane 0.32 cm thick at 294 ± 1°K. The clay membrane was made by compacting the 0.5 to 2.0 micron size fraction of Cheto montmorillonite from Chambers, Arizona. A confining pressure of 34.5 MPa (5000 psi) and a mean hydraulic pressure of 15.9 MPa (2300 psi) were chosen to simulate a depth of about 1.5 km (5000 ft) in sedimentary basins. Differential hydraulic pressure across the day was 13.8 MPa (2000 psi). This differential hydraulic pressure setting was disturbed for only brief intervals during the experimental run. Hydraulic flow rate, electrical conductance, electrical potential across the clay, electroosmotic flow rate, and the chemical composition of the brine were determined periodically until constancy of both effluent chemical composition and electrical potential across the clay was achieved. Then, osmotic flow rate and stable oxygen isotopic compositions of both input and effluent reservoirs were determined before the run was terminated. The experimental results conform to the predictions of nonequilibrium thermodynamics under these simulated subsurface conditions. The electroviscous effect caused a significant reduction in the hydraulic flow rate but did not disturb the linearity between the flow rate and differential hydraulic pressure. Osmotic and electroosmotic effects may be responsible for driving large amounts of cross-formatkmal subsurface fluid flow in nature. A measured 13O fractionation of 0.25%. compares well with published field and laboratory data.

The source of sulphur in polymetallic deposits in the cretaceous magmatic arc, Chilean Andes

Polymetallic deposits occur in the different morphotectonic units of the Cretaceous magmatic arc of the Chilean Andes. A preliminary regional study shows that the sulphur isotope composition (δ 34S cdt) of galena samples from 16 mines in these units in the sector between 24°11'S and 32°11'S shows distinct ranges: −27.4 to −35.0%, 8.1 to −0.1% and −2.1 to −18.9% for margin, basin, and arc, respectively. This variation reflects the control of the tectonic setting on the source of sulphur and the sulphur isotope fractionation processes. The very low ratios in the marginal marine setting are typical of bacterial sulphate reduction processes. Chemical and petrographical features of the host rock affect the α 34S cdt within individual morphotectonic units. The deposits in the magmatic arc hosted by lavas have a relatively high and restricted range of compositions (−2.1 to −3.8%) reflecting dominantly a magnetic source. Breccia hosted deposits are intermediate (−6.4 to −10.4%), while the lowest ratio (−18.9%) occurs in deposits hosted by laminated tuffs, reflecting an increased contribution from bacterially reduced sulphur. Carbonate hosted basin deposits have high ratios (up to −0.1%). The variation in sulphur isotope composition stands in marked contrast to the homogeneity of the lead isotope composition throughout these deposits.

Mass-independent oxygen isotopic fractionation in a microwave plasma

The non-mass-dependent isotopic fractionation process in ozone synthesis in a 2450-MHz microwave discharge has been studied. The isotopic variation as a function of pressure of the ozone demonstrates that there are several isotopic fractionations occurring simultaneously. These processes are both mass-dependent and mass-independent and reflect homogeneous and heterogeneous processes. This study extends the electromagnetic regions which may produce the mass-independent isotopic fractionation. This is of considerable interest both to the early solar system chemistry and to the stratosphere where such mass-independent oxygen isotopic fractionations are observed.

The non-mass-dependent oxygen isotope effect in the electrodissociation of carbon dioxide: A step toward understanding NoMaD chemistry

A non-mass dependent (NoMaD) oxygen isotope effect is demonstrated in the dissociation of CO 2 similar to that observed in the electrosynthesis of ozone. The molecular oxygen produced carries the signature of two separate isotopic fractionation processes; a mass-dependent fractionation probably due to CO 2 + O isotopic exchange, and a secondary NoMaD fractionation ( δ 17 O = 0.97 ± 0.09 δ 18 O, with the O 2 depleted in 17O and 18O). It is suggested that the effect is due to either the formation or relaxation of ozone in an excited electronic state. This represents the latest advance in the understanding of chemical NoMaD effects which may be essential to the explanation of non-mass-dependent fractionations observed in meteorites.

The carbon isotopic ratio of atmospheric carbon dioxide at Tsukuba, Japan

To find out the secular and seasonal trends of the δ13C value and CO2 concentration in the surface air and the determination of the δ13C in the atmospheric CO2 collected at Tsukuba Science City was carried out during the period from July 1981 to October 1983. The monthly average of the δ13C value of CO2 in the surface air collected at 1400 LMT ranged from -7.52 to \s-8.45‰ with an average of -7.96±0.25‰ and the CO2 concentration in the air varied from 334.5 μl 1-1 to 359 μl 1-1 with an average of 347.2±6.3 μl 1-1. The δ13C value is high in summer and low in winter and is negatively correlated with the CO2 concentration. In general, the relationship between the δ13C and the CO2 concentration is explainable by a simple mixing model of two different constant carbon isotopic species but the relationship does not always follow the model. The correlation between the δ13C value and the CO2 concentration is low during the plant growth season and high at other times. The observed negative deviation of the δ13C value from the simple mixing model in the plant growth season is partly due to the isotopic fractionation process which takes place in the land biota.

Isotope fractionation

A rash of new controversy has emerged around the subject of mass‐independent isotope fractionation effects, particularly in the case of the oxygen isotopes. To be sure, the controversy has been around for awhile, but it has been given new impetus by the results of a recent study by Mark H. Thiemens and John E. Heidenreich III of the University of California, San Diego (Science, March 4, 1983).Gustav Arrhenius has been trying to convince the planetary science community that chemical effects in isotope fractionation processes could explain observations in meteorites that appear to be outside of the traditionally understood mass‐dependent fractionations (G. Arrhenius, J . L. McCrumb, and N. F. Friedman, Astrophys. Space Sci, 65, 297, 1974). Robert Clayton had made the basic observations of oxygen in carbonaceous chondrites that the slope of the δ17 versus δ18 line was 1 instead of the slope of ½ characteristic of terrestrial rocks and lunar samples (Ann. Rev. Nucl. Part. Sci., 28, 501, 1978). The mass‐independent effects were ascribed to the apparent contribution of an ancient presolar system component of O16.

A non-mass-dependent isotope effect in the production of ozone from molecular oxygen

Experimental demonstration of a new, non-mass-dependent isotopic fractionation process is presented. The effect is analytically demonstrated by mass spectrometric analysis of 16O, 17O, and 18O distributions in the reactants and products in the electrosynthesis of ozone from molecular oxygen. The effect appears to be a result of differences in reaction kinetics as a result of symmetries of the isotopically substituted molecules. The direction, magnitude, and abundance distribution of the oxygen isotopes has a direct bearing on theories for the evolution of the solar system.

Isotope fractionation in secondary ion mass spectrometry

We report a systematic ion microprobe study of the isotope fractionation effects produced during the sputtering of pure metal targets. The metals studied were B, Mg, Si, Cr, Ni, Cu, Ge, Mo, Ag, Sb, Re, Tl, and Pb; isotopic ratios were determined with typical precisions of 0.1%–0.3%. Detailed studies show that the data are free of possible instrumental biases, interference effects, etc. In all cases, the secondary ions produced by sputtering are enriched in the lighter isotope, and the degree of this enrichment is a function both of the spatial location and the energy of the extracted ions relative to the sputtering site. The maximum observed L/H enrichment factors vary from 6.5%/amu for B to 0.6%/amu for Pb, and are observed to show an approximate MH/ML [rather than a (MH/ML)1/2 ] dependence on mass. For masses greater than 50, the mass dependence of this fractionation can be closely described by an ionization model modified after Schroeer et al. [Surf. Sci. 34, 571 (1973)]; it is improbable that the observed fractionation is related to differential sputtering effects. Use of ion microprobes for isotopic research in geochemistry/cosmochemistry will require careful control of this isotopic fractionation process if a precision of better than 1%–2% is sought.

Isotope Input into Runoff Systems from Melting Snow Covers

Stable environmental isotope techniques contribute to reasonable separations of the direct runoff component in snowmelt hydrographs. For the separation procedure, the actual isotope input from snow cover outflows is required. In order to study the effects of isotope fractionation and exchange processes in the snow cover on the isotopic outflow concentrations several cold room experiments have been carried out with isotopically homogeneous and stratified snow columns. To simulate natural conditions the columns were treated with different heat supply and rainwater at the surface, and the outflows analysed for 2H and 18O contents. Some fundamental results are discussed with respect to the more complex natural situation. Finally, the hydrological application of such experience is demonstrated for a natural environment.

Equation of Isotope Fractionation Between Ice and Water in A Melting Snow Column with Continuous Rain and Percolation

AbstractPartial differential equations are derived to describe isotope fractionation between ice and water phases in temperate snow caps and glaciers. Numerical solutions are obtained and shown to be consistent with laboratory experiments and measurements of deuterium concentrations in temperate glaciers. The process of isotope fractionation as described is manifested by application of the model to tritium fractionation in temperate snow caps.

Equation of Isotope Fractionation Between Ice and Water in A Melting Snow Column with Continuous Rain and Percolation

Abstract Partial differential equations are derived to describe isotope fractionation between ice and water phases in temperate snow caps and glaciers. Numerical solutions are obtained and shown to be consistent with laboratory experiments and measurements of deuterium concentrations in temperate glaciers. The process of isotope fractionation as described is manifested by application of the model to tritium fractionation in temperate snow caps.

Bestrahlungsalter und Herkunft der Eisenmeteorite

Cosmic ray exposure ages of more than 60 iron meteorites have been determined using the 41K/40-K-method. Isotope fractionation processes and the reproducibility of age values have been studied. Iron meteorites with ages of 900, 700, 600, 400 × 106 y (and hexahedrites with ages of 10 to 100 × 106 y according to WÄNKE) are abundant, such with ages exceeding 1 × 109 y are rare. Correlations have been found between exposure ages and structural classes as well as Ga—Ge-groups. These prove that the remarkable groupings of iron meteorites according to some of their structural and compositional features are indeed due to the fact that it were only a few distinct parent iron masses which essentially contributed to the production of iron meteorites. Some of these parent masses supplied the meteorites mainly in one or two “catastrophic” collisions and break-up events, others in a quasi-continuous series of events over a period of many hundreds of million years. More specified, the results are as follows: Medium and fine octahedrites of the Ga—Ge-group III have, essentially, been produced in two major collisions 600 and 700 × 106 y ago, fine octahedrites of the Ga—Ge-group IV a in one collision about 400 × 106 y ago. These events alone supplied already about 50% of all iron meteorites. Hexahedrites, coarse octahedrites of the Ga—Ge-group I and nickel-rich ataxites reveal, on the other hand, age distributions which appear rather continuous. Mean interplanetary life times deduced from these distributions are about 100 × 106 y for hexahedrites (according to WANKE), 500 × 106 y for nickel-rich ataxites and 700 × 106 y for coarse octahedrites of the Ga — Ge-group I. The differences in the statistics of meteorite production events for different meteorite classes indicate that fundamental dissimilarities may exist in the parent iron masses and parent celestial bodies. Accordingly, one model is proposed for explaining the data on hexahedrites, coarse octahedrites of the Ga—Ge-group I, and ataxites, another one for explaining the data on the medium and fine octahedrites of the Ga—Ge-groups III and IV a: The hexahedrites, coarse octahedrites and ataxites are fragments of a multiplicity of nickel-iron inclusions (“pools” according to URET) within the surface layers of Moon, Mars and asteroids (Mars-asteroids), respectively. These fragments, together with stone meteorites, have been produced in a multiplicity of crater-forming impacts by comets on Moon, Mars or asteroids. Rather compact and large nickel-iron masses, perhaps fragments of asteroidal nickel-iron cores are considered as being the parent masses of the fine octahedrites and medium octahedrites of the Ga—Ge-groups III und IV a. The meteorites of each group have been produced either when such a nickel-iron mass as a projectile hit another asteroid or when a comet hit the nickel-iron mass. Mainly surface material is expected to survive the catastrophe in form of structurally undisturbed meteorites, of which a considerable portion may have been exposed to cosmic rays to a measurable degree prior to the collision. Possible conclusions from a comparison of 41K/40K-age values with 36Ar/36Cl- and similar ages are reconsidered.

Isotope effects in solid Li2SO4 at 600-790 °C

Three different isotope fractionation processes have been studied in the same experiment, when solid Li 2 SO 4 was electrolysed in a Vycor cell using a liquid zinc cathode and a gold anode. The main object was to study the enrichment of 7 Li at the anode due to the difference in ion mobility. For this electromigration process the mass effect (i. e. relative difference in ion mobility/relative difference in mass) was µ = - 0,14, independent of temperature. The mass effect is thus about the same as in molten lithium halides. The principal difference between electromigration experiments in solid and molten salts is that in the former the salt lattice forms the « separation column », which is gradually consumed during the run, while for a melt the hydrostatic forces produce a counterflow which keeps the separation column filled with salt.The two other isotope effects were found for the competing cathode reactions. Thus a lithium-zinc alloy was formed due to a deposition reaction while, simultaneously, reduction reactions gave a mixture of lithium oxide, sulfide, and also some sulfite. As to the deposition reactions, 6 Li was enriched in the alloy with a separation factor of 1,01. The detection of an isotope effect for the reduction reactions was complicated by the fact that the reaction products were thoroughly mixed with remaining sulfate. However, a fraction of the products was insoluble in water, and a significant difference in isotope abundance was established between the soluble and the insoluble material.

Theoretical effect of diffusion on isotopic abundance ratios in rocks and associated fluids

Diffusion is considered as a possible process of isotope fractionation taking place throughout geologic time. Both diffusion in solids and diffusion in liquids are taken as possible mechanisms, the latter being more important. Arguments are presented to show that if significant fractionation takes place within a crystal by outward diffusion under solid-state conditions, enrichment will be evident only in elements of minor concentration. Similar conclusions are inferred for solid-state diffusion across a boundary or for diffusion in liquids. No isotopic enrichment can be expected in relatively large bodies of diffusion transported material. Although the necessary data to confirm these conclusions are scanty, it seems worth while to undertake further work in this direction.