共查询到20条相似文献,搜索用时 31 毫秒
1.
Dominik J. Weiss Nicole Rausch Thomas F.D. Mason Jamie J. Wilkinson Liisa Ukonmaanaho Tiina M. Nieminen 《Geochimica et cosmochimica acta》2007,71(14):3498-3517
Zinc isotope ratios were measured in the top sections of dated ombrotrophic peat cores in Finland to investigate their potential as proxies for atmospheric sources and to constrain post depositional processes affecting the geochemical record. The peat deposits were located in Hietajärvi, a background site well away from any point pollution source and representing ‘background’ conditions, in Outokumpu, next to a mining site, and in Harjavalta, next to a smelter. Measured total concentrations, calculated excess concentrations and mass balance considerations suggest that zinc is subjected to important biogeochemical cycling within the peat. Significant isotopic variability was found in all three peat bogs, with heavier zinc in the deeper and lighter zinc in the upper sections. Isotope ratios and concentrations correlated in the two peats located next to dominant point sources, i.e. the smelting and mining site, suggesting that zinc isotopes trace pollution sources. Concentration and isotope peaks were offset from the period of mining and smelting activity, supporting migration of zinc down the profile. The δ66ZnJMC (where δ66Zn = [(66Zn/64Zn)sample/(66Zn/64Zn)JMC-standard − 1] × 103) of the top section sample at the remote Hietajärvi site was 0.9‰ and we suggest this represents the regional background isotope signature of atmospheric zinc. The deeper sections of the peat cores show isotopically heavier zinc than any potential atmospheric source, indicating that post depositional processes affected the isotopic records. The large variations encountered (up to 1.05‰ for δ66Zn) and Rayleigh modelling imply that multiple fractionation of zinc during diagenetic alterations occurs and nutrient recycling alone cannot explain the fractionation pattern.We propose that zinc isotopes are amenable to identify different atmospheric zinc sources, including zinc derived from anthropogenic activities such as mining and smelting, but multiple biogeochemical processes seriously affect the record and they need to be evaluated and assessed carefully if zinc isotopes are used in terrestrial paleorecords. 相似文献
2.
Interaction between zinc and freshwater and marine diatom species: Surface complexation and Zn isotope fractionation 总被引:1,自引:0,他引:1
This work is devoted to characterization of zinc interaction in aqueous solution with two marine planktonic (Thalassiosira weissflogii = TW, Skeletonema costatum = SC) and two freshwater periphytic species (Achnanthidium minutissimum = AMIN, Navicula minima = NMIN) by combining adsorption and electrophoretic measurements with surface complexation modeling and by assessing Zn isotopes fractionation during both long term uptake and short term adsorption on diatom cells and their frustules. Reversible adsorption experiments were performed at 25 and 5 °C as a function of exposure time (5 min to 140 h), pH (2 to 10), zinc concentration in solution (10 nM to 1 mM), ionic strength (I = 0.001 to 1.0 M) and the presence of light. While the shape of pH-dependent adsorption edge is almost the same for all four species, the constant-pH adsorption isotherm and maximal Zn binding capacities differ by an order of magnitude. The extent of adsorption increases with temperature from 5 to 25 °C and does not depend on light intensity. Zinc adsorption decreases with increase of ionic strength suggesting competition with sodium for surface sites. Cell number-normalized concentrations of sorbed zinc on whole cells and their silica frustules demonstrated only weak contribution of the latter (10-20%) to overall zinc binding by diatom cell wall. Measurements of electrophoretic mobilities (μ) revealed negative diatoms surface potential in the full range of zinc concentrations investigated (0.15-760 μmol/L), however, the absolute value of μ decreases at [Zn] > 15 μmol/L suggesting a change in surface speciation. These observations allowed us to construct a surface complexation model for Zn binding by diatom surfaces that postulates the constant capacitance of the electric double layer and considers Zn complexation with carboxylate and silanol groups. Thermodynamic and structural parameters of this model are based on previous acid-base titration and spectroscopic results and allow quantitative reproduction of all adsorption experiments. Although Zn adsorption constants on carboxylate groups are almost the same, Zn surface adsorption capacities are very different among diatom species which is related to the systematic differences in their cell wall composition and thickness. Measurements of Zn isotopic composition (66Zn/(64Zn)) performed using a multicollector ICP MS demonstrated that irreversible incorporation of Zn in cultured diatom cells produces enrichment in heavy isotope compared to growth media (Δ66Zn(solid-solution) = 0.27 ± 0.05, 0.08 ± 0.05, 0.21 ± 0.05, and 0.19 ± 0.05‰ for TW, SC, NMIN, and AMIN species, respectively). Accordingly, an enrichment of cells in heavy isotopes (Δ66Zn(solid-solution) = 0.43 ± 0.1 and 0.27 ± 0.1‰ for NMIN and AMIN, respectively) is observed following short-term Zn sorption on freshwater cells in nutrient media at pH ∼ 7.8. Finally, diatoms frustules are enriched in heavy isotopes compared to solution during Zn adsorption on silica shells at pH ∼ 5.5 (Δ66Zn(solid-solution) = 0.35 ± 0.10‰). Measured isotopes fractionation can be related to the structure and stability of Zn complexes formed and they provide a firm basis for using Zn isotopes for biogeochemical tracing. 相似文献
3.
David M. Borrok Richard B. Wanty W. Ian Ridley Paul J. Lamothe Briant A. Kimball Philip L. Verplanck Robert L. Runkel 《Applied Geochemistry》2009
Here the hydrogeochemical constraints of a tracer dilution study are combined with Fe and Zn isotopic measurements to pinpoint metal loading sources and attenuation mechanisms in an alpine watershed impacted by acid mine drainage. In the tested mountain catchment, δ56Fe and δ66Zn isotopic signatures of filtered stream water samples varied by ∼3.5‰ and 0.4‰, respectively. The inherent differences in the aqueous geochemistry of Fe and Zn provided complimentary isotopic information. For example, variations in δ56Fe were linked to redox and precipitation reactions occurring in the stream, while changes in δ66Zn were indicative of conservative mixing of different Zn sources. Fen environments contributed distinctively light dissolved Fe (<−2.0‰) and isotopically heavy suspended Fe precipitates to the watershed, while Zn from the fen was isotopically heavy (>+0.4‰). Acidic drainage from mine wastes contributed heavier dissolved Fe (∼+0.5‰) and lighter Zn (∼+0.2‰) isotopes relative to the fen. Upwelling of Fe-rich groundwater near the mouth of the catchment was the major source of Fe (δ56Fe ∼ 0‰) leaving the watershed in surface flow, while runoff from mining wastes was the major source of Zn. The results suggest that given a strong framework for interpretation, Fe and Zn isotopes are useful tools for identifying and tracking metal sources and attenuation mechanisms in mountain watersheds. 相似文献
4.
Redox processes are ubiquitous in Earth science and are often associated with large isotope fractionations. In a previous study, voltage-dependent amplification of stable isotope fractionation was observed for an Fe reduction process. Here, we describe experiments showing a similar effect for a second transition metal, zinc. After electrochemical reduction, the composition of plated Zn metal is enriched in the light isotope (64Zn) with respect to the Zn2+ leftover in solution, with a voltage-dependent fractionation factor. Results from voltage-dependent electroplating experiments are in good agreement with a second data set following equilibrium fractional isotope evolution of Zn isotopes during an electroplating process which stepwise removes most of the Zn from the aqueous reservoir. Taken together, the results indicate a voltage-dependent isotope fractionation (in permil) of 66Zn with respect to 64Zn to be equal to −3.45 to 1.71 V. The negative slope trend is in contrast with previously published results on iron isotope fractionation during electroplating which shows a positive slope. These results are interpreted using an extension of Marcus theory, which predicts isotope fractionations as a function of driving force in an electrochemical system. Taken together with observations of natural fractionation of redox-sensitive and non redox-active elements, our modified Marcus theory provides a framework for quantitatively predicting transition metal isotope geochemical signatures during environmentally relevant redox processes in terms of simple energetic parameters. 相似文献
5.
Stable Cu and Zn isotope ratios as tracers of sources and transport of Cu and Zn in contaminated soil 总被引:7,自引:0,他引:7
Copper and Zn metals are produced in large quantities for different applications. During Cu production, large amounts of Cu and Zn can be released to the environment. Therefore, the surroundings of Cu smelters are frequently metal-polluted. We determined Cu and Zn concentrations and Cu and Zn stable isotope ratios (δ65Cu, δ66Zn) in three soils at distances of 1.1, 3.8, and 5.3 km from a Slovak Cu smelter and in smelter wastes (slag, sludge, ash) to trace sources and transport of Cu and Zn in soils. Stable isotope ratios were measured by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) in total digests. Soils were heavily contaminated with concentrations up to 8087 μg g−1 Cu and 2084 μg g−1 Zn in the organic horizons. The δ65Cu values varied little (−0.12‰ to 0.36‰) in soils and most wastes and therefore no source identification was possible. In soils, Cu became isotopically lighter with increasing depth down to 0.4 m, likely because of equilibrium reactions between dissolved and adsorbed Cu species during transport of smelter-derived Cu through the soil. The δ66ZnIRMM values were isotopically lighter in ash (−0.41‰) and organic horizons (−0.85‰ to −0.47‰) than in bedrock (−0.28‰) and slag (0.18‰) likely mainly because of kinetic fractionation during evaporation and thus allowed for separation of smelter-Zn from native Zn in soil. In particular in the organic horizons large variations in δ66Zn values occur, probably caused by biogeochemical fractionation in the soil-plant system. In the mineral horizons, Zn isotopes showed only minor shifts to heavier δ66Zn values with depth mainly because of the mixing of smelter-derived Zn and native Zn in the soils. In contrast to Cu, Zn isotope fractionation between dissolved and adsorbed species was probably only a minor driver in producing the observed variations in δ66Zn values. Our results demonstrate that metal stable isotope ratios may serve as tracer of sources, vertical dislocation, and biogeochemical behavior in contaminated soil. 相似文献
6.
Farid Juillot Delphine Jouvin Philipe Telouk Philippe Ildefonse Steve Sutton Gordon E. Brown Jr. 《Geochimica et cosmochimica acta》2011,75(9):2295-2308
Zinc isotopes have been studied along two smelter-impacted soil profiles sampled near one of the largest Pb and Zn processing plants in Europe located in northern France, about 50 km south of Lille. Mean δ66Zn values along these two soil profiles range from +0.22 ± 0.17‰ (2σ) to +0.34 ± 0.17‰ (2σ) at the lowest horizons and from +0.38 ± 0.45‰ (2σ) to +0.76 ± 0.14‰ (2σ) near the surface. The δ66Zn values in the lowest horizons of the soils are interpreted as being representative of the local geochemical background (mean value +0.31 ± 0.38‰), whereas heavier δ66Zn values near the surface of the two soils are related to anthropogenic Zn. This anthropogenic Zn occurs in the form of franklinite (ZnFe2O4)-bearing slag grains originating from processing wastes at the smelter site and exhibiting δ66Zn values of +0.81 ± 0.20‰ (2σ). The presence of franklinite is indicated by EXAFS analysis of the topsoil samples from both soil profiles as well as by micro-XANES analysis of the surface horizon of a third smelter-impacted soil from a distant site. These results indicate that naturally occurring Zn and smelter-derived Zn exhibit significantly different δ66Zn values, which suggests that zinc isotopes can be used to distinguish between geogenic and anthropogenic sources of Zn in smelter-impacted soils. In addition to a possible influence of additional past sources of light Zn (likely Zn-sulfides and Zn-sulfates directly emitted by the smelter), the light δ66Zn values in the surface horizons compared to smelter-derived slag materials are interpreted as resulting mainly from fractionation processes associated with biotic and/or abiotic pedological processes (Zn-bearing mineral precipitation, Zn complexation by organic matter, and plant uptake of Zn). This conclusion emphasizes the need for additional Zn isotopic studies before being able to use Zn isotopes to trace sources and pathways of this element in surface environments. 相似文献
7.
Equilibrium and kinetic Fe isotope fractionation between aqueous ferrous and ferric species measured over a range of chloride concentrations (0, 11, 110 mM Cl−) and at two temperatures (0 and 22°C) indicate that Fe isotope fractionation is a function of temperature, but independent of chloride contents over the range studied. Using 57Fe-enriched tracer experiments the kinetics of isotopic exchange can be fit by a second-order rate equation, or a first-order equation with respect to both ferrous and ferric iron. The exchange is rapid at 22°C, ∼60-80% complete within 5 seconds, whereas at 0°C, exchange rates are about an order of magnitude slower. Isotopic exchange rates vary with chloride contents, where ferrous-ferric isotope exchange rates were ∼25 to 40% slower in the 11 mM HCl solution compared to the 0 mM Cl− (∼10 mM HNO3) solutions; isotope exchange rates are comparable in the 0 and 110 mM Cl− solutions.The average measured equilibrium isotope fractionations, ΔFe(III)-Fe(II), in 0, 11, and 111 mM Cl− solutions at 22°C are identical within experimental error at +2.76±0.09, +2.87±0.22, and +2.76±0.06 ‰, respectively. This is very similar to the value measured by Johnson et al. (2002a) in dilute HCl solutions. At 0°C, the average measured ΔFe(III)-Fe(II) fractionations are +3.25±0.38, +3.51±0.14 and +3.56±0.16 ‰ for 0, 11, and 111 mM Cl− solutions. Assessment of the effects of partial re-equilibration on isotope fractionation during species separation suggests that the measured isotope fractionations are on average too low by ∼0.20 ‰ and ∼0.13 ‰ for the 22°C and 0°C experiments, respectively. Using corrected fractionation factors, we can define the temperature dependence of the isotope fractionation from 0°C to 22°C as: where the isotopic fractionation is independent of Cl− contents over the range used in these experiments. These results confirm that the Fe(III)-Fe(II) fractionation is approximately half that predicted from spectroscopic data, and suggests that, at least in moderate Cl− contents, the isotopic fractionation is relatively insensitive to Fe-Cl speciation. 相似文献
8.
Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe2O3) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450 °C and pressures from saturated vapor pressure (Psat) to 600 bar. Precipitation experiments at 200 °C and Psat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (FeIII) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ57Fefluid-hematite = δ57Fefluid − δ57Fehematite = 0.01 ± 0.08‰ (2 × standard error, 2SE). In contrast, experiments at 300 °C and Psat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ57Fefluid-hematite = −0.54 ± 0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 °C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ57Fefluid-hematite values close to zero within errors, 0.15 ± 0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous FeII and FeIII fractions, and Fe isotope ratios in solution, and identical Δ57Fe values from dissolution and precipitation runs. Our measured equilibrium Δ57Fefluid-hematite values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid-hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ57Fe Fe(aq)-hematite value at 200 °C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite-saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions. 相似文献
9.
The origin of Zn isotope fractionation in sulfides 总被引:2,自引:0,他引:2
Toshiyuki Fujii Frédéric Moynier Francis Albarède 《Geochimica et cosmochimica acta》2011,75(23):7632-7643
Isotope fractionation of Zn between aqueous sulfide, chloride, and carbonate species (Zn2+, Zn(HS)2, , , ZnS(HS)−, ZnCl+, ZnCl2, , and ZnCO3) was investigated using ab initio methods. Only little fractionation is found between the sulfide species, whereas carbonates are up to 1‰ heavier than the parent solution. At pH > 3 and under atmospheric-like CO2 pressures, isotope fractionation of Zn sulfides precipitated from sulfidic solutions is affected by aqueous sulfide species and the δ66Zn of sulfides reflect these in the parent solutions. Under high PCO2 conditions, carbonate species become abundant. In high PCO2 conditions of hydrothermal solutions, Zn precipitated as sulfides is isotopically nearly unfractionated with respect to a low-pH parent fluid. In contrast, negative δ66Zn down to at least −0.6‰ can be expected in sulfides precipitated from solutions with pH > 9. Zinc isotopes in sulfides and rocks therefore represent a potential indicator of mid to high pH in ancient hydrothermal fluids. 相似文献
10.
Here we compare new experimental studies with theoretical predictions of equilibrium iron isotopic fractionation among aqueous ferric chloride complexes (Fe(H2O)63+, FeCl(H2O)52+, FeCl2(H2O)4+, FeCl3 (H2O)3, and FeCl4-), using the Fe-Cl-H2O system as a simple, easily-modeled example of the larger variety of iron-ligand compounds, such as chlorides, sulfides, simple organic acids, and siderophores. Isotopic fractionation (56Fe/54Fe) among naturally occuring iron-bearing species at Earth surface temperatures (up to ∼3‰) is usually attributed to redox effects in the environment. However, theoretical modeling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar magnitude due to non-redox changes in speciation (i.e., ligand bond strength and coordination number). In the present study, fractionations are measured in a series of low pH ([H+] = 5 M) solutions of ferric chloride (total Fe = 0.0749 mol/L) at chlorinities ranging from 0.5 to 5.0 mol/L. Advantage is taken of the unique solubility of FeCl4- in immiscible diethyl ether to create a separate spectator phase, used to monitor changing fractionation in the aqueous solution. Δ56Feaq-eth = δ56Fe (total Fe remaining in aqueous phase)−δ56Fe (FeCl4- in ether phase) is determined for each solution via MC-ICPMS analysis.Both experiments and theoretical calculations of Δ56Feaq-eth show a downward trend with increasing chlorinity: Δ56Feaq-eth is greatest at low chlorinity, where FeCl2(H2O)4+ is the dominant species, and smallest at high chlorinity where FeCl3(H2O)3 is dominant. The experimental Δ56Feaq-eth ranges from 0.8‰ at [Cl-] = 0.5 M to 0.0‰ at [Cl-] = 5.0 M, a decrease in aqueous-ether fractionation of 0.8‰. This is very close to the theoretically predicted decreases in Δ56Feaq-eth, which range from 1.0 to 0.7‰, depending on the ab initio model.The rate of isotopic exchange and attainment of equilibrium are shown using spiked reversal experiments in conjunction with the two-phase aqueous-ether system. Equilibrium under the experimental conditions is established within 30 min.The general agreement between theoretical predictions and experimental results points to substantial equilibrium isotopic fractionation among aqueous ferric chloride complexes and a decrease in 56Fe/54Fe as the Cl-/Fe3+ ion ratio increases. The effects on isotopic fractionation shown by the modeling of this simple iron-ligand system imply that ligands present in an aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due to other speciation effects (such as iron-sulfide systems or iron bonding with organic ligands), and must be considered when interpreting iron isotope fractionation in the geological record. 相似文献
11.
Ammonia (NH3) is the major intermediate phase in the pathway of nitrogen (N) transfer from the fixed N phases (e.g., in crustal material) to free N2 (e.g., in natural gas reservoirs and volcanic gases). Yet the N isotopic behavior during these N-cycling processes remains poorly known. In an attempt to contribute to the understanding of N cycling using N isotopes, we carried out laboratory experiments to investigate the N isotopic effect associated with thermal decomposition of ammonia (2NH3 → N2 + 3H2). Pure NH3 (with initial δ15NNH3 of ∼ −2‰, relative to air standard) was sealed into quartz tubes and thermally decomposed at 600, 700 or 800 °C from 2 hours to 500 days. With the progress of the reaction, the δ15N of the remaining NH3 and the accumulated N2 increased from −2 to +35‰ and from −20 to −2‰, respectively. The differences of the N-isotope fractionations at the three temperatures are not significant. Modeling using the Rayleigh distillation model yielded similar kinetic N-isotope fractionation factors (αN2-NH3) of 0.983 ± 0.002 for 600, 700 and 800 °C. Applied to geological settings, this significant isotope discrimination (∼17‰) associated with partial decomposition of NH3/NH4+ from crustal sources (δ15Naverage ∼ +6.3‰) can produce mantle-like (i.e. ∼ −5‰) or even lower δ15N values of N2. This may explain the large variation of δ15N (−20 to +30‰) of N2 in natural gas reservoirs. It can also possibly explain the extreme 15N-depletion of N2 in some volcanic gases. This possibility has to be carefully considered when using N isotopes to trace geological N cycling across subduction zones by analysis of volcanic N2. 相似文献
12.
Isotopic variations of dissolved copper and zinc in stream waters affected by historical mining 总被引:4,自引:0,他引:4
Zinc and Cu play important roles in the biogeochemistry of natural systems, and it is likely that these interactions result in mass-dependent fractionations of their stable isotopes. In this study, we examine the relative abundances of dissolved Zn and Cu isotopes in a variety of stream waters draining six historical mining districts located in the United States and Europe. Our goals were to (1) determine whether streams from different geologic settings have unique or similar Zn and Cu isotopic signatures and (2) to determine whether Zn and Cu isotopic signatures change in response to changes in dissolved metal concentrations over well-defined diel (24-h) cycles.Average δ66Zn and δ65Cu values for streams varied from +0.02‰ to +0.46‰ and −0.7‰ to +1.4‰, respectively, demonstrating that Zn and Cu isotopes are heterogeneous among the measured streams. Zinc or Cu isotopic changes were not detected within the resolution of our measurements over diel cycles for most streams. However, diel changes in Zn isotopes were recorded in one stream where the fluctuations of dissolved Zn were the largest. We calculate an apparent separation factor of ∼0.3‰ (66/64Zn) between the dissolved and solid Zn reservoirs in this stream with the solid taking up the lighter Zn isotope. The preference of the lighter isotope in the solid reservoir may reflect metabolic uptake of Zn by microorganisms. Additional field investigations must evaluate the contributions of soils, rocks, minerals, and anthropogenic components to Cu and Zn isotopic fluxes in natural waters. Moreover, rigorous experimental work is necessary to quantify fractionation factors for the biogeochemical reactions that are likely to impact Cu and Zn isotopes in hydrologic systems. This initial investigation of Cu and Zn isotopes in stream waters suggests that these isotopes may be powerful tools for probing biogeochemical processes in surface waters on a variety of temporal and spatial scales. 相似文献
13.
Equilibrium chlorine-isotope (37Cl/35Cl) fractionations have been determined by using published vibrational spectra and force-field modeling to calculate reduced partition function ratios for Cl-isotope exchange. Ab initio force fields calculated at the HF/6-31G(d) level are used to estimate unknown vibrational frequencies of 37Cl-bearing molecules, whereas crystalline phases are modeled by published lattice-dynamics models. Calculated fractionations are principally controlled by the oxidation state of Cl and its bond partners. Molecular mass (or the absence of C-H bonds) also appears to play a role in determining relative fractionations among simple Cl-bearing organic species. Molecules and complexes with oxidized Cl (i.e., Cl0, Cl+, etc.) will concentrate 37Cl relative to chlorides (substances with Cl−). At 298 K, ClO2 (containing Cl4+) and [ClO4]− (containing Cl7+) will concentrate 37Cl relative to chlorides by as much as 27‰ and 73‰, respectively, in rough agreement with earlier calculations. Among chlorides, 37Cl will be concentrated in substances where Cl is bonded to +2 cations (i.e., FeCl2, MnCl2, micas, and amphiboles) relative to substances where Cl is bonded to +1 cations (such as NaCl) by ∼2 to 3‰ at 298 K; organic molecules with C-Cl bonds will be even richer in 37Cl (∼5 to 9‰ at 298 K). Precipitation experiments, in combination with our results, provide an estimate for Cl-isotope partitioning in brines and suggest that silicates (to the extent that their Cl atoms are associated with nearest-neighbor +2 cations analogous with FeCl2 and MnCl2) will have higher 37Cl/35Cl ratios than coexisting brine (by ∼2 to 3‰ at room temperature). Calculated fractionations between HCl and Cl2, and between brines and such alteration minerals, are in qualitative agreement with both experimental results and systematics observed in natural samples. Our results suggest that Cl-bearing organic molecules will have markedly higher 37Cl/35Cl ratios (by 5.8‰ to 8.5‰ at 295 K) than coexisting aqueous solutions at equilibrium. Predicted fractionations are consistent with the presence of an isotopically heavy reservoir of HCl that is in exchange equilibrium with Cl−aq in large marine aerosols. 相似文献
14.
The range in 56Fe/54Fe isotopic compositions measured in naturally occurring iron-bearing species is greater than 5‰. Both theoretical modeling and experimental studies of equilibrium isotopic fractionation among iron-bearing species have shown that significant fractionations can be caused by differences in oxidation state (i.e., redox effects in the environment) as well as by bond partner and coordination number (i.e., nonredox effects due to speciation).To test the relative effects of redox vs. nonredox attributes on total Fe equilibrium isotopic fractionation, we measured changes, both experimentally and theoretically, in the isotopic composition of an Fe2+-Fe3+-Cl-H2O solution as the chlorinity was varied. We made use of the unique solubility of FeCl4− in immiscible diethyl ether to create a separate spectator phase against which changes in the aqueous phase could be quantified. Our experiments showed a reduction in the redox isotopic fractionation between Fe2+- and Fe3+-bearing species from 3.4‰ at [Cl−] = 1.5 M to 2.4‰ at [Cl−] = 5.0 M, due to changes in speciation in the Fe-Cl solution. This experimental design was also used to demonstrate the attainment of isotopic equilibrium between the two phases, using a 54Fe spike.To better understand speciation effects on redox fractionation, we created four new sets of ab initio models of the ferrous chloride complexes used in the experiments. These were combined with corresponding ab initio models for the ferric chloride complexes from previous work. At 20 °C, 1000 ln β (β = 56Fe/54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 6.39‰ to 5.42‰ for Fe(H2O)62+, 5.98‰ to 5.34‰ for FeCl(H2O)5+, and 5.91‰ to 4.86‰ for FeCl2(H2O)4, depending on the model. The theoretical models predict ferric-ferrous fractionation about half as large (depending on model) as the experimental results.Our results show (1) oxidation state is likely to be the dominant factor controlling equilibrium Fe isotope fractionation in solution and (2) nonredox attributes (such as ligands present in the aqueous solution, speciation and relative abundances, and ionic strength of the solution) can also have significant effects. Changes in the isotopic composition of an Fe-bearing solution will influence the resultant Fe isotopic signature of any precipitates. 相似文献
15.
Laurie S. Balistrieri David M. Borrok W. Ian Ridley 《Geochimica et cosmochimica acta》2008,72(2):311-328
Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO3 electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (65Cu/63Cu) and Zn (66Zn/64Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (αsoln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δsoln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II). 相似文献
16.
Carbonate clumped isotope thermometry of deep-sea corals and implications for vital effects 总被引:1,自引:0,他引:1
Here we calibrate the carbonate clumped isotope thermometer in modern deep-sea corals. We examined 11 specimens of three species of deep-sea corals and one species of a surface coral spanning a total range in growth temperature of 2-25 °C. External standard errors for individual measurements ranged from 0.005‰ to 0.011‰ (average: 0.0074‰) which corresponds to ∼1-2 °C. External standard errors for replicate measurements of Δ47 in corals ranged from 0.002‰ to 0.014‰ (average: 0.0072‰) which corresponds to 0.4-2.8 °C. We find that skeletal carbonate from deep-sea corals shows the same relationship of Δ47 (the measure of 13C-18O ordering) to temperature as does inorganic calcite. In contrast, the δ13 C and δ18O values of these carbonates (measured simultaneously with Δ47 for every sample) differ markedly from equilibrium with seawater; i.e., these samples exhibit pronounced ‘vital effects’ in their bulk isotopic compositions. We explore several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence of a vital effect despite having large conventional isotopic vital effects. 相似文献
17.
Experimental determination of oxygen isotope fractionations between CO2 vapor and soda-melilite melt
We report results of experiments constraining oxygen isotope fractionations between CO2 vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000.lnαCO2-Na melilitic melt, is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na2O). We combine these data with the known reduced partition function ratio of CO2 to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO2-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts. 相似文献
18.
Goethite (Ax-2) from Axel Heiberg Island (∼80°N) on the margin of the Arctic Ocean is the dominant mineral in a sample of “petrified” Eocene wood, but U, Th, and He measurements suggest that the goethite (α-FeOOH) crystallized in the latest Miocene/Pliocene (ca. 5.5 to 2.8 Ma). Measured δD and δ18O values of Ax-2 are −221 (±6)‰ and −9.6 (±0.5)‰, respectively. The inferred δD and δ18O values of the ancient water were about −139‰ and −18.6‰, respectively, with a calculated temperature of crystallization of 3 (±5)°C, which compares with the modern summer (J-J-A) temperature of 3 °C and contrasts with a modern MAT of −19 °C. Published results from various biological proxies on nearby Ellesmere Island indicate a Pliocene (∼4 Ma) MAT of either −6 or −0.4 °C and corresponding seasonal amplitudes of about 18 or 13 °C. A conductive heat flow model suggests that a temperature of 3 °C could represent goethite crystallization at depths of ∼100-200 cm (for MAT = −6 °C) or ∼250-450 cm (for MAT = −0.4 °C) over seasonally restricted intervals of time.The δ18O value of the Ax-2 water (−18.6‰) is more positive than the modern J-J-A precipitation (−22‰). In combination, the paleotemperatures and δ18O values of ancient waters (from Ax-2 and published results from three Eocene or Pliocene proxy sites on Axel Heiberg and Ellesmere Islands) are consistent with a warm season bias in those isotopic proxies. The results are also consistent with higher proportions of J-J-A precipitation in the annual total. If so, this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates, and suggests that the Arctic hydrologic cycle, as expressed in the seasonal distribution and isotopic composition of precipitation (perhaps modified by a warmer Arctic Ocean), differed from modern.The δ13C value of the Fe(CO3)OH component in the Ax-2 goethite is +6.6‰, which is much more positive than expected if crystallizing goethite incorporated CO2 derived primarily from oxidation of relict Eocene wood with δ13C values of about −24‰. This apparent paradox may be resolved if the goethite is a product of oxidation of 13C-rich siderite, which had previously replaced wood in an Eocene methanogenic burial environment. Thus, the goethite retains a carbon isotope “memory” of a diagenetic Eocene event, but a δD and δ18O record of the latest Miocene/Pliocene Arctic climate. 相似文献
19.
Joost Hoek Anna-Louise Reysenbach Donald E. Canfield 《Geochimica et cosmochimica acta》2006,70(23):5831-5841
The fractionation of sulfur isotopes by the thermophilic chemolithoautotrophic Thermodesulfatator indicus was explored during sulfate reduction under excess and reduced hydrogen supply, and the full temperature range of growth (40-80 °C). Fractionation of sulfur isotopes measured under reduced H2 conditions in a fed-batch culture revealed high fractionations (24-37‰) compared to fractionations produced under excess H2 supply (1-6‰). Higher fractionations correlated with lower sulfate reduction rates. Such high fractionations have never been reported for growth on H2. For temperature-dependant fractionation experiments cell-specific rates of sulfate reduction increased with increasing temperatures to 70 °C after which sulfate-reduction rates rapidly decreased. Fractionations were relatively high at 40 °C and decreased with increasing temperature from 40-60 °C. Above 60 °C, fractionation trends switched and increased again with increasing temperatures. These temperature-dependant fractionation trends have not previously been reported for growth on H2 and are not predicted by a generally accepted fractionation model for sulfate reduction, where fractionations are controlled as a function of temperature, by the balance of the exchange of sulfate across the cell membrane, and enzymatic reduction rates of sulfate. Our results are reproduced with a model where fractionation is controlled by differences in the temperature response of enzyme reaction rates and the exchange of sulfate in and out of the cell. 相似文献
20.
Two types of laboratory experiments were used to quantify magnesium isotopic fractionations associated with chemical and thermal (Soret) diffusion in silicate liquids. Chemical diffusion couples juxtaposing a molten natural basalt (SUNY MORB) and a molten natural rhyolite (Lake County Obsidian) were run in a piston cylinder apparatus and used to determine the isotopic fractionation of magnesium as it diffused from molten basalt to molten rhyolite. The thermal diffusion experiments were also run in a piston cylinder apparatus but with a sample made entirely of molten SUNY MORB displaced from the hotspot of the assembly furnace so that the sample would have a temperature difference of about 100-200 °C from one end to the other. The chemical diffusion experiments showed fractionations of 26Mg/24Mg by as much as 7‰, which resulted in an estimate for the mass dependence of the self-diffusion coefficients of the magnesium isotopes corresponding to D26Mg/D24Mg=(24/26)β with β = 0.05. The thermal diffusion experiments showed that a temperature difference of about 100 °C resulted in the MgO, CaO, and FeO components of the basalt becoming slightly enriched by about 1 wt% in the colder end while SiO2 was enriched by several wt% in the hotter end. The temperature gradient also fractionated the magnesium isotopes. A temperature difference of about 150 °C produced an 8‰ enrichment of 26Mg/24Mg at the colder end relative to the hotter end. The magnesium isotopic fractionation as a function of temperature in molten basalt corresponds to 3.6 × 10−2‰/°C/amu. 相似文献