Isotopic fractionation during ion filtration: I. Theory

Fractionation of the isotopes of H, O, and Cl during flow of aqueous solutions through semipermeable clay discs has been observed in the laboratory. The observed isotopic abundance trends of these elements in deep sedimentary basins have been attributed to similar phenomena. However, the mechanism responsible for such isotopic fractionation has not been identified. We propose that the fractionation of O and H isotopes in the water molecule results from increased activity of the heavy isotopic species in the membrane solution because high cation concentrations in the membrane pore fluid increase hydrationsphere fractionation effects. The heavy isotopic species thus diffuse away from the membrane and therefore are concentrated behind it. Predictions based on this theory satisfactorily model both laboratory and field observations.We propose that the fractionation of chlorine isotopes is a result of differences in the ionic mobility of ³⁵Cl⁻ and ³⁷Cl⁻. Negative charges on the membrane repel the isotopic species at slightly different velocities against the uniform advective motion of the bulk water and thus the more mobile ³⁵Cl⁻ is enriched behind the membrane. Predictions calculated from this theory compare well with laboratory and field data.     

Rare earth elements in tektites

Four moldavites and one sample each of an australite, billitonite, indochinite, philippinite, thailandite, Ivory Coast tektite, bediasite and a georgianite were analyzed using neutron activation analysis for La, Ce, Sm, Eu, Tb, Yb and Lu. The REE abundances resemble those of sedimentary rocks. Most of the tektites display a depletion of Eu, a characteristic feature of mature Phanerozoic continental sedimentary rocks. However, the Ivory Coast tektite and georgianite are relatively enriched in Eu, possibly due to the presence of plagioclase-rich source rocks.     

Isotopic fractionation between coexisting organic carbon—carbonate pairs in Precambrian sediments

δ¹³C_org and δ¹³C_carb values of 58 coexisting organic carbon-carbonate pairs covering the whole Precambrian have yielded means of ?24.7 ± 6.0%. [PDB] and +0.9 ± 2.7%. [PDB], respectively. Accordingly, isotopic fractionation between inorganic and organic carbon in Precambrian sediments is about the same as in geologically younger rocks (Δδ ? 25%.), a slight increase displayed by the Early Precambrian pairs (Δδ ? 28%.) being probably biassed by an over-representation in this age group of samples from one single locality (nevertheless, this value still lies within the range permitted for a possible deviation). It is reasonable to assume, therefore, that the overall isotope fractionation factor governing biological fixation of inorganic carbon has been virtually constant since some 3.3 × 10⁹ yr ago.     

Isotopic fractionation of Kr and Xe implanted in solids at very low energies

We report results on the implantation of Kr and Xe in W under closed system conditions at very low energies (50–500 eV). Investigation of the fraction of gas trapped as a function of time reveals the existence of competing trapping and release mechanisms and analysis of recovered trapped gas and residual gas phases shows that both elemental and isotopic fractionation result from these mechanisms. We determined the mass dependence for the overall implantation process to be at or near m¹, with heavier isotopes enriched in the implanted gas. This mass dependence is inferred to result from implantation and a combination of diffusive and gas sputtering release mechanisms. Our results reaffirm the conclusion of Bernatowicz and Fahey (1986) that previously observed isotopic fractionation of Kr and Xe in carbonaceous material synthesized in electrical discharges owes its origin to low energy ion implantation and also suggest that this process may be relevant to incorporation of noble gases in early solar system materials. We also discuss the implication of our results for noble gas mass spectrometry.     

低温环境下铜同位素分馏的若干重要过程

Cu同位素是一种新的地球化学示踪剂.正确运用这一同位素示踪技术的前提是对其同位素分馏机理和过程有足够的认识.本文报道了室温下CuSO4·5H2O结晶过程产生分馏的实验结果,并系统地总结了低温条件下Cu同位素分馏的一些重要过程,其中包括沉淀过程、还原过程、吸附过程、生物过程等.     

Exploration potential of Cu isotope fractionation in porphyry copper deposits

We examined the copper isotope ratio of primary high temperature Cu-sulfides, secondary low temperature Cu-sulfides (and Cu-oxides) as well as Fe-oxides in the leach cap, which represent the weathered remains of a spectrum of Cu mineralization, from nine porphyry copper deposits. Copper isotope ratios are reported as δ⁶⁵Cu‰ = ((⁶⁵Cu/⁶³Cu_sample/⁶⁵Cu/⁶³Cu_{NIST 976 standard}) − 1) ? 10³. Errors for all the analyses are ± 0.14‰ (determined by multiple analyses of the samples) and mass bias was corrected through standard-sample-standard bracketing. The overall isotopic variability measured in these samples range from − 16.96‰ to 9.98‰.     

Isotopic fractionation: A kinetic model for crystals growing from magmatic melts

Recent experiments in the fields of crystal growth, crystal-melt element partitioning, and diffusion in magmatic melts make it possible to estimate disequilibrium partitioning for many species between liquid and crystals and to compute quantitative models which take into account partition coefficients, diffusivities and rates of crystal growth. A slight difference in the diffusivities of two isotopes should lead to selective depletion (or enrichment) in the crystals as they grow, resulting in varying isotopic ratios as crystallization proceeds. Reasonable hypotheses permit under general kinetic conditions an estimation of magnitude of possible effects. The resulting isotopic fractionation for major elements (like oxygen) may exceed the per ml level; for trace elements the effects may be more significant (5–10 per ml), especially in the case of low partition coefficients, but analytical difficulties are formidable at present.     

Lithium in tektites and natural glasses

Lithium concentrations in various tektites and natural glass samples have been determined using RNAA. For most tektites Li abundances range from 40 to 60 ppm, with the exception of bediasites (23 ppm). In other natural glasses of presumed impact origin, the Li values are considerably lower (6–11 ppm). It is suggested that the Li concentrations reflect mostly the original abundance of this element in parent materials. A terrestrial origin for tektites is supported by the Li range overlap between tektites and sedimentary rocks     

Isotopic fractionation of the major elements of molten basalt by chemical and thermal diffusion

Samples produced in piston cylinder experiments were used to document the thermal isotopic fractionation of all the major elements of basalt except for aluminum and the fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic fractionations are summarized in terms of a parameter Ω_i defined as the fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report Ω_Ca = 1.6, Ω_Fe = 1.1, Ω_Si = 0.6, Ω_O = 1.5. In an earlier paper we reported Ω_Mg = 3.6. These fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt-rhyolite diffusion couple showed significant fractionation that are parameterized in terms of a parameter β_Fe = 0.03 when the ratio of the diffusion coefficients D₅₄ and D₅₆ of ⁵⁴Fe and ⁵⁶Fe is expressed in terms of the atomic mass as D₅₄/D₅₆ = (56/54)^β_Fe. This value of β_Fe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., β_Li = 0.215, β_Ca = 0.05, β_Mg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic fractionations driven by either or both chemical and thermal gradients.     

Stable Cu isotope fractionation in soils during oxic weathering and podzolization

Copper stable isotope ratios are fractionated during various biogeochemical processes and may trace the fate of Cu during long-term pedogenetic processes. We assessed the effects of oxic weathering (formation of Cambisols) and podzolization on Cu isotope ratios (δ⁶⁵Cu). Two Cambisols (oxic weathered soils without strong vertical translocations of soil constituents) and two Podzols (soils showing vertical translocation of organic matter, Fe and Al) were analyzed for Cu concentrations, partitioning of Cu in seven fractions of a sequential extraction and δ⁶⁵Cu values in bulk soil. Cu concentrations in the studied soils were low (1.4-27.6 μg g⁻¹) and Cu was mainly associated with strongly bound Fe oxide- and silicate-associated forms. Bulk δ⁶⁵Cu values varied between −0.57‰ and 0.44‰ in all studied horizons. The O horizons had on average significantly lighter Cu isotope compositions (−0.21‰) than the A horizons (0.13‰) which can either be explained by Cu isotope fractionation during cycling through the plants or deposition of isotopically light Cu from the atmosphere. Oxic weathering without pronounced podzolization in both Cambisols and a weakly developed Podzol (Haplic Podzol 2) caused no significant isotope fractionation in the single profiles, while a slight tendency to lower δ⁶⁵Cu values with depth was visible in all four profiles. This is the opposite depth distribution of δ⁶⁵Cu values to that we observed in hydromorphic soils (soils which show indication of redox changes because of the influence of water saturation) in a previous study. In a more pronounced Podzol (Haplic Podzol 1), δ⁶⁵Cu values and Cu concentrations decreased from Ah to E horizons and increased again deeper in the soil. Humus-rich sections of the Bhs horizon had higher Cu concentrations (2.8 μg g⁻¹) and a higher δ⁶⁵Cu value (−0.18‰) than oxide-rich sections (1.9 μg g⁻¹, −0.35‰) suggesting Cu translocation between E and B horizons as organo-Cu complexes. The different depth distributions in oxic weathered and hydromorphic soils and the pronounced vertical differences in δ⁶⁵Cu values in Haplic Podzol 1 indicate a promising potential of δ⁶⁵Cu values to improve our knowledge of the fate of Cu during long-term pedogenetic processes.     

Cu isotopic fractionation in the supergene environment with and without bacteria

The isotopic composition of dissolved Cu and solid Cu-rich minerals [δ⁶⁵Cu (‰) = (⁶⁵Cu/⁶³Cu_sample/⁶⁵Cu/⁶³Cu_std) - 1)*1000] were monitored in batch oxidative dissolution experiments with and without Thiobacillus ferrooxidans. Aqueous copper in leach fluids released during abiotic oxidation of both chalcocite and chalcopyrite was isotopically heavier (δ⁶⁵Cu = 5.34‰ and δ⁶⁵Cu = 1.90‰, respectively, [±0.16 at 2σ]) than the initial starting material (δ⁶⁵Cu = 2.60 ± 0.16‰ and δ⁶⁵Cu = 0.58 ± 0.16‰, respectively). Isotopic mass balance between the starting material, aqueous copper, and secondary minerals precipitated in these experiments explains the heavier isotopic values of aqueous copper. In contrast, aqueous copper from leached chalcocite and chalcopyrite inoculated with Thiobacillus ferrooxidans was isotopically similar to the starting material. The lack of fractionation of the aqueous copper in the biotic experiments can best be explained by assuming a sink for isotopically heavy copper present in the bacteria cells with δ⁶⁵Cu = 5.59 ± 0.16‰. Consistent with this inference, amorphous Cu-Fe oxide minerals are observed surrounding cell membranes of Thiobacillus grown in the presence of dissolved Cu and Fe.Extrapolating these experiments to natural supergene environments implies that release of isotopically heavy aqueous Cu from oxidative leach caps, especially under abiotic conditions, should result in precipitates in underlying enrichment blankets that are isotopically heavy. Where iron-oxidizing cells are involved, isotopically heavy oxidized Cu entrained in cellular material may become associated with leach caps, causing the released aqueous Cu to be less isotopically enriched in the heavy isotope than predicted for the abiotic system. Rayleigh fractionation trends with fractionation factors calculated from our experiments for both biotic and abiotic conditions are consistent with large numbers of individual abiotic or biotic leaching events, explaining the supergene chalcocites in the Morenci and Silver Bell porphyry copper deposits.     

Isotopic fractionation of noble gases by diffusion in liquid water: Molecular dynamics simulations and hydrologic applications

Despite their great importance in low-temperature geochemistry, diffusion coefficients of noble gas isotopes in liquid water (D) have been measured only for the major isotopes of helium, neon, krypton and xenon. Data on the diffusion coefficients of minor noble gas isotopes are essentially non-existent and so typically have been estimated by a kinetic-theory model in which D varies as the inverse square root of the isotopic mass (m): D ∝ m^−0.5. To examine the validity of the kinetic-theory model, we performed molecular dynamics (MD) simulations of the diffusion of noble gases in ambient liquid water. Our simulation results agree with available experimental data on the solvation structure and diffusion coefficients of the major noble gas isotopes and reveal for the first time that the isotopic mass-dependence of all noble gas self-diffusion coefficients has the power-law form D ∝ m^−β with 0 < β < 0.2. Thus our results call into serious question the widespread assumption that the ‘square-root’ model can be applied to estimate the kinetic fractionation of noble gas isotopes caused by diffusion in ambient liquid water. To illustrate the importance of this finding, we used the diffusion coefficients determined in our MD simulations to reconsider the geochemical modeling of ²⁰Ne/²²Ne and ³⁶Ar/⁴⁰Ar isotopic ratios in three representative hydrologic studies. Our new modeling results indicate that kinetic isotopic fractionation by diffusion may play a significant role in noble gas transport processes in groundwater.     

Mass spectrometric evidence for organic constituents in tektites

Since the physical and chemical events leading to the formation of organic molecules in numerous carbon-containing meteorites (e.g. paraffins, alcohols and aromatics have been identified) have been studied in considerable detail, it becomes important to compare and relate similar studies with tektites. For this purpose, various tektite specimens were heated to temperatures up to 1500°C and the vaporizing molecular species identified with a mass spectrometer. In this technique, volatile constituents are selectively vaporized from the sample, ionized to electrically charged positive particles by bombardment with high energy electrons, and separated in an electric and magnetic field according to their respective molecular weights. Mass spectra for the direct volatilization of individual tektites from the Philippine Islands, Viet Nam, Thailand, Australia, Czechoslovakia and Texas were compared with those obtained from the carbonaceous Pueblito de Allende chondrite and terrestrial obsidian.     

Chemical systematics among the moldavite tektites

The compositional variations that occur among the moldavite tektites are caused principally by incomplete mixing of two components during fusion. With the possible exception of silica, there is no evidence for significant losses of volatile species by fractional vaporization. Chemical constraints have been calculated for the two source-materials that contributed to the moldavites. If these tektites were formed by impact fusion, as is commonly believed, then the compositional systematics preserved within the moldavites suggest that hypersonic flow and ejection of impact melts are orderly processes. Insights gained from the study of tektites should prove useful in interpreting the chemistries of impact glasses from other bodies in the solar system.     

Isotopic fractionation and reaction kinetics between Cr(III) and Cr(VI) in aqueous media

The redox-sensitive stable isotope geochemistry of chromium bears the potential to monitor the attenuation of chromate pollution and to investigate changes in environmental conditions in the present and the past. The use of stable Cr isotope data as a geo-environmental tracer, however, necessitates an understanding of the reaction kinetics and Cr fractionation behaviour during redox transition and isotope exchange. Here, we report stable chromium isotope fractionation data for Cr(VI) reduction, Cr(III) oxidation and isotopic exchange between soluble Cr(III) and Cr(VI) in aqueous media. The reduction of Cr(VI) to Cr(III) with H₂O₂ under strongly acidic conditions shows a near-equilibrium isotope fractionation of Δ^53/52Cr_{(Cr(III)-Cr(VI))} of −3.54 ± 0.35‰. At pH neutrality, however, the reduction experiments show a kinetic isotope fractionation Δ^53/52Cr_{(Cr(III)-Cr(VI))} of −5‰ for the extent of reduction of up to 85% of the chromium. The oxidation of Cr(III) to Cr(VI) in alkaline media, using H₂O₂ as the oxidant, cannot be explained by a single, unidirectional reaction. Our experiments indicate that the involvement of the unstable intermediates Cr(IV) and Cr(V) and their disproportionation during redox reactions between Cr(III) and Cr(VI) influence the overall fractionation factor, depending on the prevailing pH conditions and the reaction rates. No detectable isotope exchange between soluble Cr(VI) and Cr(III) species at pH values of 5.5 and 7 was revealed over a timescale of days to weeks. This means that, at least within such a time frame, the isotopic composition of Cr(VI) in a natural system will not be influenced by equilibration with any Cr(III) and thus reveal the true extent of reduction, given that the Cr isotope composition of the source Cr(VI) and the fractionation factor for the prevailing conditions are known.     

中条山篦子沟铜矿辉钼矿铼-锇同位素年龄及其地质意义

篦子沟铜矿位于山西中条山胡-篦型铜矿田,矿体与地层产状一致,呈层状、似层状分布于中条群篦子沟组和余元下组中,经历了多期次多阶段成矿作用。为限定篦子沟铜矿后期热液脉状矿化的形成时代,对篦子沟铜矿区方解石-石英脉中与黄铜矿共生的辉钼矿样品进行了Re-Os同位素测定。结果表明,6件辉钼矿样品Re-Os同位素模式年龄为(1 539±26)~(1 616±26)Ma,加权平均年龄为(1 577±31)Ma(MSWD=5.5),等时线年龄为(1 522±180)Ma(MSWD=9.6),表明热液脉状矿化形成于中元古代长城纪晚期。据此篦子沟铜矿的热液脉状矿化可分为3个成矿期次,分别为古元古代晚期发生的微细脉浸染状矿化期、脉状矿化期和中元古代早期热液脉状辉钼矿矿化期。此次事件可能为中元古代伸展-裂解事件岩浆侵位产生的热液对原来的篦子沟铜矿进行再次的矿化、富集。结合已有资料,证明篦子沟以及中条山地区在中元古代早期存在热液矿化事件。     

Isotopic fractionation associated with biosynthesis of fatty acids by a marine bacterium under oxic and anoxic conditions

Shewanella putrefaciens (Strain MR-4), a gram negative facultative marine bacterium, was grown to stationary phase under both aerobic and anaerobic conditions using lactate as the sole carbon source. Aerobically-produced cells were slightly enriched in ¹³C (+1.5‰) relative to the lactate carbon source, whereas those from anaerobic growth were depleted in ¹³C (−2.2‰). The distribution of fatty acids produced under aerobic conditions was similar to that resulting from anaerobic growth, being dominated by C_16:1 ω7 and C_16:0 fatty acids with a lesser amount of the C_18:1 ω7 component. Low concentrations of saturated even numbered normal fatty acids in the C₁₄ to C₁₈ range, and iso-C_15:0 were synthesized under both conditions. Fatty acids from anaerobic cultures (average δ¹³C=−37.8‰) were considerably depleted in ¹³C relative to their aerobically-synthesized counterparts (−28.8‰). The distinct differences in isotopic composition of both whole cells and individual fatty acid components result from differences in assimilation pathways. Under aerobic conditions, the primary route of assimilation involves the pyruvate dehydrogenase enzyme complex which produces acetyl-CoA, the precursor to lipid synthesis. In contrast, under anaerobic conditions formate, and not acetate, is the central intermediate in carbon assimilation with the precursors to fatty acid synthesis being produced via the serine pathway. Anaerobically-produced bacterial fatty acids were depleted by up to 12‰ relative to the carbon source. Therefore, detection of isotopically depleted fatty acids in sediments may be falsely attributed to a terrestrial origin, when in fact they are the result of bacterial resynthesis.