Silicon isotope fractionation during magmatic differentiation

The Si isotopic composition of Earth’s mantle is thought to be homogeneous (δ³⁰Si = −0.29 ± 0.08‰, 2 s.d.) and not greatly affected by partial melting and recycling. Previous analyses of evolved igneous material indicate that such rocks are isotopically heavy relative to the mantle. To understand this variation, it is necessary to investigate the degree of Si isotopic fractionation that takes place during magmatic differentiation. Here we report Si isotopic compositions of lavas from Hekla volcano, Iceland, which has formed in a region devoid of old, geochemically diverse crust. We show that Si isotopic composition varies linearly as a function of silica content, with more differentiated rocks possessing heavier isotopic compositions. Data for samples from the Afar Rift Zone, as well as various igneous USGS standards are collinear with the Hekla trend, providing evidence of a fundamental relationship between magmatic differentiation and Si isotopes. The effect of fractionation has been tested by studying cumulates from the Skaergaard Complex, which show that olivine and pyroxene are isotopically light, and plagioclase heavy, relative to the Si isotopic composition of the Earth’s mantle. Therefore, Si isotopes can be utilised to model the competing effects of mafic and felsic mineral fractionation in evolving silicate liquids and cumulates.At an average SiO₂ content of ∼60 wt.%, the predicted δ³⁰Si value of the continental crust that should result from magmatic fractionation alone is −0.23 ± 0.05‰ (2 s.e.), barely heavier than the mantle. This is, at most, a maximum estimate, as this does not take into account weathered material whose formation drives the products toward lighter δ³⁰Si values. Mass balance calculations suggest that removal of continental crust of this composition from the upper mantle will not affect the Si isotopic composition of the mantle.     

Redox-controlled iron isotope fractionation during magmatic differentiation: an example from the Red Hill intrusion, S. Tasmania

This study presents accurate and precise iron isotopic data for 16 co-magmatic rocks and 6 pyroxene–magnetite pairs from the classic, tholeiitic Red Hill sill in southern Tasmania. The intrusion exhibits a vertical continuum of compositions created by in situ fractional crystallisation of a single injection of magma in a closed igneous system and, as such, constitutes a natural laboratory amenable to determining the causes of Fe isotope fractionation in magmatic rocks. Early fractionation of pyroxenes and plagioclase, under conditions closed to oxygen exchange, gives rise to an iron enrichment trend and an increase in $ f_{{{\text{O}}_{2} }} $ of the melt relative to the Fayalite–Magnetite–Quartz (FMQ) buffer. Enrichment in Fe³⁺/ΣFe_melt is mirrored by δ⁵⁷Fe, where ^VIFe²⁺-bearing pyroxenes partition ⁵⁷Fe-depleted iron, defining an equilibrium pyroxene-melt fractionation factor of $ \Updelta^{57} {\text{Fe}}_{{{\text{px}} - {\text{melt}}}} \le - 0.25\,\permille \times 10^{6} /T^{2} $ . Upon magnetite saturation, the $ f_{{{\text{O}}_{2} }} $ and δ⁵⁷Fe of the melt fall, commensurate with the sequestration of the oxidised, ⁵⁷Fe-enriched iron into magnetite, quantified as $ \Updelta^{57} {\text{Fe}}_{{{\text{mtn}} - {\text{melt}}}} = + 0.20\,\permille \times 10^{6} /T^{2} $ . Pyroxene–magnetite pairs reveal an equilibrium fractionation factor of $ \Updelta^{57} {\text{Fe}}_{{{\text{mtn}} - {\text{px}}}} \approx + 0.30\,\permille $ at 900–1,000?°C. Iron isotopes in differentiated magmas suggest that they may act as an indicator of their oxidation state and tectonic setting.     

Calculation of oxygen isotope fractionation in magmatic rocks

The increment method is applied to calculation of oxygen isotope fractionation factors for common magmatic rocks. The ¹⁸O-enrichment degree of the different compositions of magmatic rocks is evaluated by the oxygen isotope indices of both CIPW normative minerals and normalized chemical composition. The consistent results are obtained from the two approaches, pointing to negligible oxygen isotope fractionation between rock and melt of the same compositions. The present calculations verify the following sequence of ¹⁸O-enrichment in the magmatic rocks: felsic rocks>intermediate rocks>mafic rocks>ultramafic rocks. Two sets of internally consistent fractionation factors are acquired for phenocryst–lava systems at the temperatures above 1000 K and rock–water systems in the temperatures range of 0–1200 °C, respectively. The present calculations are consistent with existing data from experiments and/or empirical calibrations. The obtained results can be used to quantitatively determine the history of water–rock interaction and to serve geological thermometry for various types of magmatic rocks (especially extrusive rocks).     

Electrochemically controlled iron isotope fractionation

Variations in the stable isotope abundances of transition metals have been observed in the geologic record and trying to understand and reconstruct the physical/environmental conditions that produced these signatures is an area of active research. It is clear that changes in oxidation state lead to large fractionations of the stable isotopes of many transition metals such as iron, suggesting that transition metal stable isotope signatures could be used as a paleo-redox proxy. However, the factors contributing to these observed stable isotope variations are poorly understood. Here we investigate how the kinetics of iron redox electrochemistry generates isotope fractionation. Through a combination of electrodeposition experiments and modeling of electrochemical processes including mass-transport, we show that electron transfer reactions are the cause of a large isotope separation, while mass transport-limited supply of reactant to the electrode attenuates the observed isotopic fractionation. Furthermore, the stable isotope composition of electroplated transition metals can be tuned in the laboratory by controlling parameters such as solution chemistry, reaction overpotential, and solution convection. These methods are potentially useful for generating isotopically-marked metal surfaces for tracking and forensic purposes. In addition, our studies will help interpret stable isotope data in terms of identifying underlying electron transfer processes in laboratory and natural samples.     

岩浆岩体系氧同位素分馏系数的理论计算

The increment method is applied tocalcuation of oxygen isotope fractionation factors for common magmatic rocks by usingoxygen isotope indices for known minerals.The results show that there are some differencesin the degree of 18O-enrichment for the different types of magmatic rocks,andtheir sequence of 18O-enrichment is reckoned as follows acid rocks >neutral rocks > basic rocks > ultrabasic rocks.Two sets of internally consistentfractionation factors for phenocryst-lava systems at temperacture above 1000K and forrock-water systems in the temperature range of 0 to 1200℃ are acquired, respectively.Thetheoretical calibrations are consistent with the data from hydrothermal experiments andempirical estimates.The present results can be used to quantitatively determine thehistory of water-rock exchange and to serve geological thermometry for various magmaticrocks (especially extrusive rocks containing phenocryst).     

Nitrogen in peridotite xenoliths: Lithophile behavior and magmatic isotope fractionation

In order to document the origin and speciation of nitrogen in mantle-derived rocks and minerals, the N and Ar contents and isotopic compositions were investigated for hydrous and anhydrous peridotite xenoliths from Ataq, Yemen, from Eifel, Germany, and from Massif Central, France. Nitrogen and Ar were extracted by stepwise combustion with a fine temperature resolution, followed by fusion in a platinum crucible. A large isotopic disequilibrium of up to 25.4‰ is observed within single peridotite xenoliths, with δ¹⁵N values as low as −17.3‰ in phlogopite whereas clinopyroxene and olivine show positive δ¹⁵N values. Identical Sr isotopic ratios of phlogopite, clinopyroxene and whole rock in this wehrlite sample are consistent with crystallization from a common reservoir, suggesting that the light N signature of phlogopite might be the result of isotopic fractionation during N uptake from the host magma. The nitrogen concentration is systematically high in phlogopite, (7.6-25.7 ppm), whereas that of bulk peridotite xenoliths is between 0.1 and 0.8 ppm. The high N content of phlogopite is at least partly due to host magma-mineral interaction, and may also suggest the occurrence of N as that substituted for K⁺ during mineral growth in mafic magmas. Such speciation is consistent with the fact that N and Rb contents correlate well for a set of samples from mantle regions that were affected by subduction-related metasomatism and magmatism. The N/Rb ratios of these samples are comparable with values estimated for subduction zone magmas, but are one order of magnitude lower than the N/Rb ratios characterizing subducting slabs. This difference suggests preferential release of N relative to alkalis in the forearc region. N/⁴⁰Ar^∗ ratios of minerals from analyzed mantle xenoliths are much higher than those of vesicles in MORBs and OIBs, requiring either the occurrence of nitrogen speciation in the mantle more compatible than Ar, significant loss of fluid phase during entrainment, or long residence time of volatile elements in the mantle source(s) of fluids to increase drastically the ⁴⁰Ar^∗ budget of the latter.     

Oxygen isotope fractionation of dissolved oxygen during reduction by ferrous iron

The oxygen isotope fractionation factor of dissolved oxygen gas has been measured during inorganic reduction by aqueous FeSO₄ at 10−54 °C under neutral (pH 7) and acidic (pH 2) conditions, with Fe(II) concentrations ranging up to 0.67 mol L⁻¹, in order to better understand the geochemical behavior of oxygen in ferrous iron-rich groundwater and acidic mine pit lakes. The rate of oxygen reduction increased with increasing temperature and increasing Fe(II) concentration, with the pseudo-first-order rate constant k ranging from 2.3 to 82.9 × 10⁻⁶ s⁻¹ under neutral conditions and 2.1 to 37.4 × 10⁻⁷ s⁻¹ under acidic conditions. The activation energy of oxygen reduction was 30.9 ± 6.6 kJ mol⁻¹ and 49.7 ± 13.0 kJ mol⁻¹ under neutral and acidic conditions, respectively. Oxygen isotope enrichment factors (ε) become smaller with increasing temperature, increasing ferrous iron concentration, and increasing reaction rate under acidic conditions, with ε values ranging from −4.5‰ to −11.6‰. Under neutral conditions, ε does not show any systematic trends vs. temperature or ferrous iron concentration, with ε values ranging from −7.3 to −10.3‰. Characterization of the oxygen isotope fractionation factor associated with O₂ reduction by Fe(II) will have application to elucidating the process or processes responsible for oxygen consumption in environments such as groundwater and acidic mine pit lakes, where a number of possible processes (e.g. biological respiration, reduction by reduced species) may have taken place.     

Zirconium and hafnium fractionation in differentiation of alkali carbonatite magmatic systems

Zirconium and hafnium are valuable strategic metals which are in high demand in industry. The Zr and Hf contents are elevated in the final products of magmatic differentiation of alkali carbonatite rocks in the Polar Siberia region (Guli Complex) and Ukraine (Chernigov Massif). Early pyroxene fractionation led to an increase in the Zr/Hf ratio in the evolution of the ultramafic–alkali magmatic system due to a higher distribution coefficient of Hf in pyroxene with respect to Zr. The Rayleigh equation was used to calculate a quantitative model of variation in the Zr/Hf ratio in the development of the Guli magmatic system. Alkali carbonatite rocks originated from rare element-rich mantle reservoirs, in particular, the metasomatized mantle. Carbonated mantle xenoliths are characterized by a high Zr/Hf ratio due to clinopyroxene development during metasomatic replacement of orthopyroxene by carbonate fluid melt.     

High-temperature Si isotope fractionation between iron metal and silicate

The Si stable isotope fractionation between metal and silicate has been investigated experimentally at 1800, 2000, and 2200 °C. We find that there is a significant silicon stable isotope fractionation at high temperature between metal and silicate in agreement with Shahar et al. (2009). Further we find that this fractionation is insensitive to the structure and composition of the silicate as the fractionation between silicate melt and olivine is insignificant within the error of the analyses. The temperature-dependent silicon isotope fractionation is Δ³⁰Si_{silicate-metal} = 7.45 ± 0.41 × 10⁶/T². We also demonstrate the viability of using laser ablation MC-ICPMS as a tool for measuring silicon isotope ratios in high pressure and temperature experiments.     

Iron isotope fractionation and atom exchange during sorption of ferrous iron to mineral surfaces

The application of stable Fe isotopes as a tracer of the biogeochemical Fe cycle necessitates a mechanistic knowledge of natural fractionation processes. We studied the equilibrium Fe isotope fractionation upon sorption of Fe(II) to aluminum oxide (γ-Al₂O₃), goethite (α-FeOOH), quartz (α-SiO₂), and goethite-loaded quartz in batch experiments, and performed continuous-flow column experiments to study the extent of equilibrium and kinetic Fe isotope fractionation during reactive transport of Fe(II) through pure and goethite-loaded quartz sand. In addition, batch and column experiments were used to quantify the coupled electron transfer-atom exchange between dissolved Fe(II) (Fe(II)_aq) and structural Fe(III) of goethite. All experiments were conducted under strictly anoxic conditions at pH 7.2 in 20 mM MOPS (3-(N-morpholino)-propanesulfonic acid) buffer and 23 °C. Iron isotope ratios were measured by high-resolution MC-ICP-MS. Isotope data were analyzed with isotope fractionation models. In batch systems, we observed significant Fe isotope fractionation upon equilibrium sorption of Fe(II) to all sorbents tested, except for aluminum oxide. The equilibrium enrichment factor, , of the Fe(II)_sorb-Fe(II)_aq couple was 0.85 ± 0.10‰ (±2σ) for quartz and 0.85 ± 0.08‰ (±2σ) for goethite-loaded quartz. In the goethite system, the sorption-induced isotope fractionation was superimposed by atom exchange, leading to a δ^56/54Fe shift in solution towards the isotopic composition of the goethite. Without consideration of atom exchange, the equilibrium enrichment factor was 2.01 ± 0.08‰ (±2σ), but decreased to 0.73 ± 0.24‰ (±2σ) when atom exchange was taken into account. The amount of structural Fe in goethite that equilibrated isotopically with Fe(II)_aq via atom exchange was equivalent to one atomic Fe layer of the mineral surface (∼3% of goethite-Fe). Column experiments showed significant Fe isotope fractionation with δ^56/54Fe(II)_aq spanning a range of 1.00‰ and 1.65‰ for pure and goethite-loaded quartz, respectively. Reactive transport of Fe(II) under non-steady state conditions led to complex, non-monotonous Fe isotope trends that could be explained by a combination of kinetic and equilibrium isotope enrichment factors. Our results demonstrate that in abiotic anoxic systems with near-neutral pH, sorption of Fe(II) to mineral surfaces, even to supposedly non-reactive minerals such as quartz, induces significant Fe isotope fractionation. Therefore we expect Fe isotope signatures in natural systems with changing concentration gradients of Fe(II)_aq to be affected by sorption.     

Sulfur Isotope Fractionation in Magmatic Systems：Models of Rayleigh Distillation and Selective Flux

The effect of Rayleigh distillation by outgassing of SO2 and H2S on the isotopic composition of sulfur remaining in silicate melts is quantitatively modelled.A threshold mole fraction of sulfur in sulfide component of the melts is reckoned to be of critical importance in shifting the δ^34S of the melts mith respect to the original magmas.The partial equilibrium fractionation in a magmatic system is evaluated by assuming that a non-equilibrium flux of sulfur occurs between magmatic volatiles and the melts,while an equilibrium fractionation is approached between sulfate and sulfide within the melts.The results show that under high fo2 conditions,the sulfate/sulfide ratio in a melt entds to increase,and the δ^34S value of sulfur in a solidified rock might then be shifted in the positive direction.This may either be due to Rayleigh outgassing in case the mole fraction of sulfide is less than the threshold,or due to a unidirectional increase in δ^34S value of the sulfate with decreaing temperature,Conversely,at low fo2,the sulfate/sulfide ratio tends to decrease and the δ^34S value of total sulfur could be driven in the negative direction,either because of the Rayleigh outgassing in case the mole fraction of sulfide is greater than the threshold,or because of a unidirectional decrease inδ^34S value of the sulfide.To establish isotopic equilibrium between sulfate and sulfide,the HM,QFM or WM buffers in the magmatic system are suggested to provide the redox couple that could simultaneously reduce the sulfate and oxidize the sulfide.CaO present in the silicatte Melts is also called upon to participate in the chemical equilibrium between sulfate and sulfide,Consequently,the δ^34S value of an igneous rock could considerably deviate from that of its original magma due to the influence of oxygen fugacity and temperature at the time of magma solidification.     

Mercury isotope fractionation during liquid-vapor evaporation experiments

Liquid-vapor mercury isotope fractionation was investigated under equilibrium and dynamic conditions. Equilibrium evaporation experiments were performed in a closed glass system under atmospheric pressure between 0 and 22 °C, where vapor above the liquid was sampled at chemical equilibrium. Dynamic evaporation experiments were conducted in a closed glass system under 10⁻⁵ bar vacuum conditions varying (1) the fraction of liquid Hg evaporated at 22 °C and (2) the temperature of evaporation (22-100 °C). Both, residual liquid and condensed vapor fractions were analyzed using stannous chloride CV-MC-ICP-MS.Equilibrium evaporation showed a constant liquid-vapor fractionation factor (α_202/198) of 1.00086 ± 0.00022 (2SD, n = 6) within the 0-22 °C range. The 22 °C dynamic evaporations experiments displayed Rayleigh distillation fractionation behavior with liquid-vapor α_202/198 = 1.0067 ± 0.0011 (2SD), calculated from both residual and condensed vapor fractions. Our results confirm historical data (1920s) from Brönsted, Mulliken and coworkers on mercury isotopes separation using evaporation experiments, for which recalculated δ²⁰²Hg′ showed a liquid-vapor α_202/198 of 1.0076 ± 0.0017 (2SD). This liquid-vapor α_202/198 is significantly different from the expected kinetic α_202/198 value ((202/198)^0.5 = 1.0101). A conceptual evaporation model of back condensation fluxes within a thin layer at the liquid-vapor interface was used to explain this discrepancy. The δ²⁰²Hg′ of condensed vapor fractions in the 22-100 °C temperature range experiments showed a negative linear relationship with 10⁶/T², explained by increasing rates of exchange within the layer with the increase in temperature.Evaporation experiments also resulted in non-mass-dependent fractionation (NMF) of odd ¹⁹⁹Hg and ²⁰¹Hg isotopes, expressed as Δ¹⁹⁹Hg′ and Δ²⁰¹Hg′, the deviation in ‰ from the mass fractionation relationship with even isotopes. Liquid-vapor equilibrium yielded Δ¹⁹⁹Hg′/Δ²⁰¹Hg′ relationship of 2.0 ± 0.6 (2SE), which is statistically not different from the one predicted for the nuclear field shift effect (Δ¹⁹⁹Hg/Δ²⁰¹Hg ≈ 2.47). On the other hand, evaporation under dynamic conditions at 22 °C led to negative anomalies in the residual liquid fractions that are balanced by positive anomalies in condensed vapors with lower Δ¹⁹⁹Hg′/Δ²⁰¹Hg′ ratios of 1.2 ± 0.4 (2SD). This suggests that either magnetic isotope effects may have occurred without radical chemistry or an unknown NMF process on odd isotopes operated during liquid mercury evaporation.     

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.     

Carbon isotope fractionation in wood during carbonization

A significant uncertainty exists as to whether δ¹³C values in charcoal meaningfully represent the stable isotopic content of the original material, with studies suggesting variable responses to both natural and laboratory heating. An extensive study was undertaken using fully homogenised samples of wood taken from Eucalyptus spp., Quercus robur and Pinus radiata. The results demonstrate that the duration of heating had no tangible effect on the final composition of the charred material, with the δ¹³C and carbon content of wood fixed after 30 min of heating. Furthermore, all three wood types become progressively depleted in ¹³C with increasing temperature. The results demonstrate that even at temperatures commonly reached in natural fires (<450 °C) isotopic fractionation of up to 1.3‰ can take place indicating that the absolute values obtained from charcoal extracted for paleoenvironmental reconstruction must be interpreted with caution.     

低温环境下铁同位素分馏的若干重要过程

详细了解同位素分馏的过程与机理是运用稳定同位素体系解决科学问题的关键.本文对沉淀、溶解、吸附、氧化、还原、生物等过程中的Fe同位素分馏研究结果进行了系统总结.在沉淀过程中,优先沉淀轻同位素;在吸附过程中,Fe(Ⅲ)矿物优先吸附重同位素;氧化还原过程中,Fe的化合价越高,Fe同位素组成越重.     

Iron isotope fractionation during hydrothermal ore deposition and alteration

Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources or fluid histories. The former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, and the latter allows the reconstruction of precipitation and redox processes. These processes take place during ore formation or alteration. The aim of this contribution is to investigate the suitability of this new isotope method as a probe of ore-related processes. For this purpose 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, the ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300 °C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100 °C). Measured iron isotope compositions are variable and cover a range in δ⁵⁶Fe between −2.3‰ and +1.3‰. Primary hematite (δ⁵⁶Fe: −0.5‰ to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ⁵⁶Fe: −0.5‰) leached from the crystalline basement. Occasional input of CO₂-rich waters resulted in precipitation of isotopically light siderite (δ⁵⁶Fe: −1.4 to −0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitating from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethites from oolitic sedimentary iron ores were also analyzed. Their compositional range appears to indicate oxidative precipitation from relatively uniform Fe dissolved in coastal water. This comprehensive iron isotope study illustrates the potential of the new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.     

Inverse kinetic isotope fractionation during bacterial nitrite oxidation

Natural abundance stable isotopes in nitrate (), nitrite (), and nitrous oxide (N₂O) have been used to better understand the cycling of nitrogen in marine and terrestrial environments. However, in order to extract the greatest information from the distributions of these isotopic species, the kinetic isotope effects for each of the relevant microbial reactions are needed. To date, kinetic isotope effects for nitrite oxidation and anaerobic ammonium oxidation (anammox) have not been reported. In this study, the nitrogen isotope effect was measured for microbial nitrite oxidation to nitrate. Nitrite oxidation is the second step in the nitrification process, and it plays a key role in the regeneration of nitrate in the ocean. Surprisingly, nitrite oxidation occurred with an inverse kinetic isotope effect, such that the residual nitrite became progressively depleted in ¹⁵N as the reaction proceeded. Three potential explanations for this apparent inverse kinetic isotope effect were explored: (1) isotope exchange equilibrium between nitrite and nitrous acid prior to reaction, (2) reaction reversibility at the enzyme level, and (3) true inverse kinetic fractionation. Comparison of experimental data to ab initio calculations and theoretical predictions leads to the conclusion that the fractionation is most likely inverse at the enzyme level. Inverse kinetic isotope effects are rare, but the experimental observations reported here agree with kinetic isotope theory for this simple N-O bond-forming reaction. Nitrite oxidation is therefore fundamentally different from all other microbial processes in which N isotope fractionation has been studied. The unique kinetic isotope effect for nitrite oxidation should help to better identify its role in the cycling of nitrite in ocean suboxic zones, and other environments in which nitrite accumulates.     

Evidence for equilibrium iron isotope fractionation by nitrate-reducing iron(II)-oxidizing bacteria

Iron isotope fractionations produced during chemical and biological Fe(II) oxidation are sensitive to the proportions and nature of dissolved and solid-phase Fe species present, as well as the extent of isotopic exchange between precipitates and aqueous Fe. Iron isotopes therefore potentially constrain the mechanisms and pathways of Fe redox transformations in modern and ancient environments. In the present study, we followed in batch experiments Fe isotope fractionations between Fe(II)_aq and Fe(III) oxide/hydroxide precipitates produced by the Fe(III) mineral encrusting, nitrate-reducing, Fe(II)-oxidizing Acidovorax sp. strain BoFeN1. Isotopic fractionation in ⁵⁶Fe/⁵⁴Fe approached that expected for equilibrium conditions, assuming an equilibrium Δ⁵⁶Fe_{Fe(OH)3-Fe(II)aq} fractionation factor of +3.0‰. Previous studies have shown that Fe(II) oxidation by this Acidovorax strain occurs in the periplasm, and we propose that Fe isotope equilibrium is maintained through redox cycling via coupled electron and atom exchange between Fe(II)_aq and Fe(III) precipitates in the contained environment of the periplasm. In addition to the apparent equilibrium isotopic fractionation, these experiments also record the kinetic effects of initial rapid oxidation, and possible phase transformations of the Fe(III) precipitates. Attainment of Fe isotope equilibrium between Fe(III) oxide/hydroxide precipitates and Fe(II)_aq by neutrophilic, Fe(II)-oxidizing bacteria or through abiologic Fe(II)_aq oxidation is generally not expected or observed, because the poor solubility of their metabolic product, i.e. Fe(III), usually leads to rapid precipitation of Fe(III) minerals, and hence expression of a kinetic fractionation upon precipitation; in the absence of redox cycling between Fe(II)_aq and precipitate, kinetic isotope fractionations are likely to be retained. These results highlight the distinct Fe isotope fractionations that are produced by different pathways of biological and abiological Fe(II) oxidation.     

Adsorption as a cause for iron isotope fractionation in reduced groundwater

Iron isotopes were used to investigate iron transformation processes during an in situ field experiment for removal of dissolved Fe from reduced groundwater. This experiment provided a unique setting for exploring Fe isotope fractionation in a natural system. Oxygen-containing water was injected at a test well into an aquifer containing Fe(II)-rich reduced water, leading to oxidation of Fe(II) and precipitation of Fe(III)(hydr)oxides. Subsequently, groundwater was extracted from the same well over a time period much longer than the injection time. Since the surrounding water is rich in Fe(II), the Fe(II) concentration in the extracted water increased over time. The increase was strongly retarded in comparison to a conservative tracer added to the injected solution, indicating that adsorption of Fe(II) onto the newly formed Fe(III)(hydr)oxides occurred. A series of injection-extraction (push-pull) cycles were performed at the same well. The δ⁵⁷Fe/⁵⁴Fe of pre-experiment background groundwater (−0.57 ± 0.17 ‰) was lighter than the sediment leach of Fe(III) (−0.24 ± 0.08 ‰), probably due to slight fractionation (only ∼0.3 ‰) during microbial mediated reductive dissolution of Fe(III)(hydr)oxides present in the aquifer. During the experiment, Fe(II) was adsorbed from native groundwater drawn into the oxidized zone and onto Fe(III)(hydr)oxides producing a very light groundwater component with δ⁵⁷Fe/⁵⁴Fe as low as −4 ‰, indicating that heavier Fe(II) is preferentially adsorbed to the newly formed Fe(III)(hydr)oxides surfaces. Iron concentrations increased with time of extraction, and δ⁵⁷Fe/⁵⁴Fe linearly correlated with Fe concentrations (R² = 0.95). This pattern was reproducible over five individual cycles, indicating that the same process occurs during repeated injection/extraction cycles. We present a reactive transport model to explain the observed abiotic fractionation due to adsorption of Fe(II) on Fe(III)(hydr)oxides. The fractionation is probably caused by isotopic differences in the equilibrium sorption constants of the various isotopes (K_ads) and not by sorption kinetics. A fractionation factor α_57/54 of 1.001 fits the observed fractionation.