Enrichment Of Heavy Isotopes Research Articles

The effect of core composition on iron isotope fractionation between planetary cores and mantles

We have conducted high-pressure, high-temperature isotope exchange experiments between molten silicate and molten Fe–Si–C-alloys to constrain the effect of Si on equilibrium Fe isotope fractionation during planetary core formation. The values of Δ57FeMetal-Silicate at 1850°C and 1 GPa determined by high-resolution MC-ICP-MS in this study range from −0.013±0.054‰ (2SE) to 0.072 ± 0.085‰ with 1.34–8.14 atom % Si in the alloy, respectively. These results, although not definitive on their own, are consistent with previous experimental results from our group and a model in which elements that substitute for Fe atoms in the alloy structure (i.e., Ni, S, and Si) induce a fractionation of Fe isotopes between molten silicate and molten Fe-alloys during planetary differentiation. Using in situ synchrotron X-ray diffraction data for molten Fe-rich alloys from the literature, we propose a model to explain this fractionation behavior in which impurity elements in Fe-alloys cause the nearest neighbor atomic distances to shorten, thereby stiffening metallic bonds and increasing the preference of the alloy for heavy Fe isotopes relative to the silicate melt. This fractionation results in the bulk silicate mantles of the smaller terrestrial planets and asteroids becoming isotopically light relative to chondrites, with an enrichment of heavy Fe isotopes in their cores, consistent with magmatic iron meteorite compositions. Our model predicts a bulk silicate mantle δ57Fe ranging from −0.01‰ to −0.12‰ for the Moon, −0.06‰ to −0.33‰ for Mars, and −0.08‰ to −0.33‰ for Vesta. Independent estimates of the δ57Fe of primitive mantle source regions that account for Fe isotope fractionation during partial melting agree well with these ranges for all three planetary bodies and suggest that Mars and Vesta have cores with impurity (i.e., Ni, S, Si) abundances near the low end of published ranges. Therefore, we favor a model in which core formation results in isotopically light bulk silicate mantles for the Moon, Mars, and Vesta. The processes of magma ocean crystallization, mantle partial melting, and fractional crystallization of mantle-derived melts are all likely to result in heavy Fe isotope enrichment in the melt phase, which can explain why basaltic samples from these planetary bodies have variable δ57Fe values consistently heavier than our bulk mantle estimates. Additionally, we find no clear evidence that Fe isotopes were fractionated to a detectable level by volatile depletion processes during or after planetary accretion, although it cannot be ruled out.

Lead, zinc, and copper redistributions in soils along a deposition gradient from emissions of a Pb-Ag smelter decommissioned 100 years ago

Sourcing and understanding the fate of anthropogenic metals in a historical contamination context is challenging. Here we combined elemental and isotopic (Pb, Zn, Cu) analyses with X-ray Absorption Spectroscopy (XAS) measurements (Zn) to trace the fate, in undisturbed soil profiles, of historical metal contamination emitted by a 167-year-old Pb-Ag smelter decommissioned 100 years ago located in the Calanques National Park (Marseilles, France). Lead isotopic measurements show that entire soil profiles were affected by 74 years of Pb emissions up to ~7 km from the smelter under the main NNW wind, and indicate particulate transfer down to 0.8 m at depth. This vertical mobility of anthropogenic Pb contrasts with previous studies where Pb was immobilized in surface horizons. The contribution of anthropogenic Pb to the total Pb concentration in soil was estimated at 95% in surface horizons, and 78% in the deepest horizons. Zinc isotopic signatures of past emissions that are enriched in light isotopes compared to the natural geological background (−0.70 ± 0.04‰ and −0.15 ± 0.02‰, respectively), were detected only in the surface horizons of the studied soils. Using XAS analyses, we showed that anthropogenic Zn was transformed and immobilized in surface horizons as Zn-Layered Double Hydroxide, thus favoring the enrichment in heavy isotopes in these surface horizons. No clear evidence of copper contamination by the smelter was found and Cu isotopes point to a bedrock origin and a natural distribution of Cu concentrations.

Equilibrium Mg isotope fractionation among aqueous Mg2+, carbonates, brucite and lizardite: Insights from first-principles molecular dynamics simulations

Abstract Equilibrium Mg isotope fractionation properties between aqueous Mg2+ and minerals are the key for the applications of Mg isotopes in geochemistry. This study conducts first-principles molecular dynamics (FPMD) simulations for aqueous Mg2+ based on the density functional theory (DFT). Thirty-five snapshots are extracted from the FPMD trajectories after equilibration for the calculations of the reduced partition function ratio (β factor or 103lnβ). Combining with the β factors of minerals, we found that the β factor decreases in the sequence of lizardite > brucite > aqueous Mg2+ > dolomite > magnesite > calcite > aragonite. Our calculations also confirm the effect of Mg concentration on the β factor of calcite, and further show that the concentration effect is negligible when Mg/(Mg + Ca) (atomic ratio hereafter) is lower than 1/32. Our results depict significant equilibrium Mg isotope fractionations between minerals and aqueous Mg2+ (103lnαminerals-Mg_aq), which are dominantly controlled by the average force constant of Mg in these phases. Compared to aqueous Mg2+, carbonates are enriched in light Mg isotopes but brucite and lizardite show enrichment in heavy Mg isotopes. Among all minerals, 103lnαaragonite-Mg_aq is the largest, which is up to −14‰ at 300 K. Notably, 103lnαcalcite-Mg_aq is not only controlled by temperature, but also significantly affected by Mg content in calcite. 103lnαcalcite-Mg_aq is negatively correlated with Mg content in calcite. The experimentally measured Mg isotope fractionations between minerals (dolomite, magnesite, brucite) and solution at equilibrium are consistent with our predicted results, showing the accurate description of FPMD simulations for aqueous Mg2+ and the reliability of our calculations. Overall, the calculated equilibrium Mg isotope fractionations between minerals and aqueous Mg2+ provide a guideline for applications of Mg isotopes in aqueous geochemical processes.

Iron isotope fractionation in iron-organic matter associations: Experimental evidence using filtration and ultrafiltration

Colloids have been recognized as key vectors of pollutants in aqueous environment. Amongst them, those formed by iron (Fe) and organic matter (OM) are of major importance due to their ubiquity in the surface environment and strong affinity for metals. In the recent years, Fe stable isotopes have been increasingly used to elucidate the sources and biogeochemical cycling of Fe in Earth's surface environments. In this study, we aim to elucidate (i) the possible Fe isotopic signature resulting from the Fe/OM colloid formation and (ii) the mechanisms involved in the development of such isotopic signature. For this purpose, Fe-OM associations were synthesized through binding and titration experiments. Various pH levels were used in order to study the isotope behavior of Fe occurring as free species at pH 1, as Fe-OM complexes at pH 2 and as mixed Fe-oxyhydroxide/OM nanoaggregates or particles at pH 6.5. Organic matter-free, Fe-free and OM membrane-deposition experiments were also performed. These suspensions were (ultra)filtered at 0.2 µm, 30 kDa and 5 kDa to evidence the possible Fe isotope fractionation between fractions. This protocol allowed also testing the potential of (ultra)filtration techniques to generate isotope fractionation. The results provided evidence that abiotic Fe precipitation, (ultra)filtration techniques and OM deposition were not able to produce significant Fe isotope fractionation under the experimental conditions. However, at circum-neutral pH, the Fe-OM binding and titration experiments displayed a significant enrichment of heavy Fe isotopes in the 30 kDa and 30 kDa and 5kDa - <5kDa = -0.23 ± 0.08‰ suggesting that Fe heavy isotopes are preferentially bound to small humic OM molecules in the form of Fe monomers or small clusters. This study highlights the importance of organic matter for metals’ isotopic systems.

Near-equilibrium isotope fractionation during planetesimal evaporation

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controlled attendant isotope fractionations.Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by 26Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order <1 m that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥0.1% M⊕) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~700 km yield a steady-state far-field vapor pressure of 10−8 bar and a vapor pressure at the surface of 10−4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas.We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H2 on timescales of 104 to 105 years.

Chromium speciation and its stable isotopic signature in the dolomite–terra rossa weathering system

We present the results of a study of chromium (Cr) content, Cr speciation and its stable isotope composition in a terra rossa weathering profile, which includes ferromanganese concretions (FMCs) and gibbsite spots, developed on dolomite in the Yunnan–Guizhou Plateau in south central China. Sequential extraction results show that the Cr in all samples is present mainly within the residual fraction and oxidizable fraction, and that the Cr content of the exchangeable fraction and reducible fraction is very low. The significantly high Cr content (mean of 21.0 ppm) of the oxidizable fraction is a potential environmental hazard. The Cr in the terra rossa and gibbsite spots exhibits absolute accumulation throughout the profile; correspondingly, the FMCs are significantly enriched in most trace elements, except for Cr. Re-oxidation of Cr(III) to soluble Cr(VI) by MnO2 is likely responsible for the relative depletion of Cr in the FMCs. In the bulk dolomite, the δ53Cr values of the carbonate component and detrital component are 0.447‰ and −0.437‰, respectively. The δ53Cr signal in both fractions is heterogeneous, while both δ53Cr signals are significantly different from that of bulk silicate Earth (BSE; −0.124 ± 0.101‰). Our results provide a proof of concept that the allogenic Cr-isotopic composition of terrigenous minerals in carbonate rocks, can provide new insights into paleo-terrestrial oxidative weathering. Overall, the terra rossa is characterized by the enrichment of heavy Cr isotopes compared to the detrital component of dolomite. Hence, the isotopic signature of Cr does not support the idea that the detrital component in dolomite is the sole parent material of the terra rossa. The absolute accumulation of Cr and heavy Cr isotope enrichment in terra rossa indicate that the extraneous input of dissolved Cr(VI) with a heavy isotopic signature occurred during dolomite weathering and terra rossa formation.

Pervasively anoxic surface conditions at the onset of the Great Oxidation Event: New multi-proxy constraints from the Cooper Lake paleosol

Oceanic element inventories derived from marine sedimentary rocks place important constraints on oxidative continental weathering in deep time, but there remains a scarcity in complementary observations directly from continental sedimentary reservoirs. This study focuses on better defining continental weathering conditions near the Archean-Proterozoic boundary through the multi-proxy (major and ultra-trace element, Fe and Cr stable isotopes, µ-XRF elemental mapping, and detrital zircon U-Pb geochronology) investigation of the ca. 2.45 billion year old (giga annum, Ga) Cooper Lake paleosol (saprolith) developed on a sediment-hosted mafic dike within the Huronian Supergroup (Ontario, Canada).Throughout the variably altered Cooper Lake saprolith, ratios of immobile elements (Nb, Ta, Zr, Hf, Th, Al, Ti) are constant, indicating a uniform pre-alteration dike composition, lack of extreme pH weathering conditions, and no major influence from ligand-rich fluids during weathering or burial metasomatism/metamorphism. The loss of Mg, Fe, Na, Sr, and Li, a signature of albite and ferromagnesian silicate weathering, increases towards the top of the preserved profile (unconformity) and dike margins. Coupled bulk rock behaviour of Fe-Mg-Mn and co-localization of Fe-Mn in clay minerals (predominantly chlorite) indicates these elements were solubilized primarily in their divalent state without Fe/Mn-oxide formation. A lack of a Ce anomaly and immobility of Mo, V, and Cr further support pervasively anoxic weathering conditions. Subtle U enrichment, if primary, is the only geochemical evidence that could be consistent with oxidative element mobilization. The leaching of ferromagnesian silicates was accompanied by variable mobility and depletion of transition metals with a relative depletion order of Fe ≈ Mg ≈ Zn > Ni > Co > Cu (Cu being significantly influenced by secondary sulfide formation). Mild enrichment of heavy Fe isotopes (δ56/54Fe from 0.169 to 0.492‰) correlating with Fe depletion in the saprolith indicates open-system loss of isotopically light aqueous Fe(II). Minor REE + Y fractionation with increasing alteration intensity, including a decreasing Eu anomaly and Y/Ho ratio, is attributed to albite breakdown and preferential scavenging of HREE > Y by clay minerals, respectively. Younger metasomatism resulted in the addition of several elements (K, Rb, Cs, Be, Tl, Ba, Sn, In, W), partly or wholly obscuring their earlier paleo-weathering trends.The behavior of Cr at Cooper Lake can help test previous hypotheses of an enhanced, low pH-driven continental weathering flux of Cr(III) to marine reservoirs between ca. 2.48–2.32 Ga and the utility of the stable Cr isotope proxy of Mn-oxide induced Cr(III) oxidation. Synchrotron µ-XRF maps and invariant Cr/Nb ratios reveal complete immobility of Cr despite its distribution amongst both clay-rich groundmass and Fe-Ti oxides. Assuming a pH-dependent, continental source of Cr(III) to marine basins, the Cr immobility at Cooper Lake indicates either that signatures of acidic surface waters were localized to uppermost and typically unpreserved regolith horizons or were geographically restricted to acid-generating point sources. However, given detrital pyrite preservation in overlying fluvial sequences, it is probable that the oxidative sulfide corrosion required to drive surface pH < 4 lagged behind in this region relative to other early Proterozoic sequences. The entire saprolith exhibits a consistently light stable Cr isotope composition (δ53/52Cr: −0.321 ± 0.038‰, 2sd, n = 34) that cannot be linked to Cr(III) oxidation and is instead interpreted to have a magmatic origin.The combined paleosol chemical signatures and preservation of detrital pyrite/uraninite indicate low atmospheric O2 during weathering at ca. 2.45 Ga in the rift-related environment of the Lower Huronian. The aqueous flux from the reduced weathering of mafic rocks was presumably characterized by a greater transition metal (Fe, Mn, Zn, Co, Ni) load with isotopically light Fe(II) compared to modern environments, as well as higher Eu/Eu* and Y/Ho than the source rock. In most models of Precambrian ocean element inventories, hydrothermal fluids are viewed as the main supplier of several metals (e.g., Fe, Zn), although the results herein suggest that a riverine metal supply may have been substantial and that using Eu-excess as a strict proxy for hydrothermal flux may be misleading in near-shore marine sedimentary deposits.

Increased Mg release rates and related Mg isotopic signatures during bacteria-phlogopite interactions

Phlogopite weathering experiments were carried out under aerobic conditions in a closed system to study Mg isotopic fractionation during the dissolution and processes involved in the presence of bacterial strains. Four different bacterial strains with different metabolisms were chosen. Biotic experiments were performed in batch reactors at 24 °C for 12 days. In parallel, abiotic phlogopite weathering experiments were also performed using organic and nitric acids. Citric and gluconic acids were used to model the effects of chelating agents produced by bacteria, and nitric acid was used to model the effects of acidifying agents. The results indicate that both decreases in pH and the production of metabolites during heterotrophic bacterial activities significantly accelerate the release of Mg and Si in solution. Also, at a given pH, the fraction of elements released in solution is greater in the presence of bacteria or citric acid compared to that released in the presence of nitric and gluconic acids. Magnesium isotopic analyses indicate that the solutions obtained using nitric acid and acidifying bacterial strains display, on average, δ26Mg values that are close to or slightly heavier (by 0.4‰) than those of fresh phlogopite (−1.2‰ ± 0.08). In contrast, in experiments performed with citric acid, the Mg leached into solution is slightly enriched in light isotopes by −0.3‰ relative to the initial phlogopite, while its binding to organics is expected to be related to its preferential enrichment in heavy isotopes. Despite its small range of variations, the δ26Mg values of solutions vary inversely with pH, thus suggesting that secondary phases preferentially enriched in 26Mg at the highest pH values may play a key role.

Mercury Stable Isotope Fractionation During Coal Combustion in Coal-Fired Boilers: Reconciling Atmospheric Hg Isotope Observations with Hg Isotope Fractionation Theory.

Mercury (Hg) stable isotope is a useful tool to understand the transformation of atmospheric Hg. The observation on the enrichment of heavier isotopes in gaseous elemental Hg (GEM) relative to oxidized HgII species in atmosphere cannot be convincingly explained by isotope fractionation of Hg redox processes. This review shows that the large Hg isotope mass dependent fractionation (MDF) in coal-fired boilers is one of the underlying reasons. The reported Hg isotope data of feed coals and their combustion products are first summarized to give a general overview of how Hg isotopes fractionate before Hg discharge from coal-fired boilers. Then, predictive MDF models are discussed to simulate δ202Hg values of different Hg species in coal combustion flue gases. The discharged GEM is predicted to have the highest δ202Hg followed by gaseous HgII and particulate-bound HgII, which is in consistent with the observed MDF pattern of atmospheric Hg species.

SIP-Metaproteomics: Linking Microbial Taxonomy, Function, and Activity.

Stable isotope probing combined with metaproteomics enables the detection and characterization of active key species in microbial populations under near-natural conditions, which greatly helps to understand the metabolic functions of complex microbial communities. This is achieved by providing growth substrates labeled with heavy isotopes such as 13C, which will be assimilated into microbial biomass. After subsequent extraction of proteins and proteolytic cleavage into peptides, the heavy isotope enrichment can be detected by high-resolution mass spectrometric analysis, and linked to the functional and taxonomic characterization of these biomarkers. Here we provide protocols for obtaining isotopically labeled proteins and for downstream SIP-metaproteomics analysis.

Spatial and Seasonal Variation of O and H Isotopes in the Jiulong River, Southeast China

The stable isotope technique of oxygen and hydrogen (δ18O and δ2H) and deuterium excess (d-excess) was used to investigate distribution characteristics in June 2017 and January 2018 in the Jiulong River, southeast China. The results revealed that (1) seasonal isotopic composition was mainly controlled by precipitation. It enriched lighter water isotopes in winter more than in summer because of the aggravating effect of low temperature and great rainfall. (2) Spatial distribution of the North, West, and South River showed increasing enrichment of heavy isotopes in that order. In the high-flow season, the continuous high-flow made δ18O and δ2H homogeneous, despite increasing weak evaporation along water-flow paths in the West and South River. In the low-flow season, there was a decreasing trend in the middle and lower reaches of the North and West main stream and an increasing trend in the South River. (3) O and H isotopic geochemistry exhibited natural and anthropogenic influence in hydrological process, such as heavy rainfall and cascade reservoirs. The results showed that O and H isotopes are indeed useful tracers of the water cycle.

Iron isotopic fractionation in mineral phases from Earth's lower mantle: Did terrestrial magma ocean crystallization fractionate iron isotopes?

Iron is the most abundant transition metal in the Earth's interior, yet considerable uncertainties remain as to why mantle-derived rocks have diverse iron isotopic compositions. In particular, the isotopic fractionation behavior of iron in the lower-mantle minerals bridgmanite and ferropericlase are largely unexplored. The reason is that it is challenging to study isotopic fractionation at the high pressures relevant to the deep mantle. Here we report in situ measurements of the mean force constants of iron bonds in these minerals pressurized in diamond anvil cells using the technique of nuclear resonant inelastic X-ray scattering (NRIXS). We find that the transition from high- to low-spin iron in ferropericlase ((Mg0.75Fe0.25)O) at approximately 60 GPa drastically stiffens its iron bonds in the low-spin state. The mean force constant of iron bonds in both Fe-bearing and (Fe,Al)-bearing bridgmanite exhibits softening by 21% at approximately 40–60 GPa, which seems to be associated with changes in the iron local environment during the transition from low to high quadrupole splitting states. These results indicate that in the lower mantle, low-spin ferropericlase is enriched in heavy iron isotopes relative to bridgmanite and metallic iron by +0.15‰ and +0.12‰, respectively. Based on these results, we investigate whether terrestrial magma ocean crystallization could have fractionated iron isotopes. We conclude that this process cannot be responsible for the heavy iron isotope enrichment measured in terrestrial basalts.

Mercury Stable Isotope Fractionation during Abiotic Dark Oxidation in the Presence of Thiols and Natural Organic Matter.

Mercury (Hg) stable isotope fractionation has been widely used to trace Hg sources and transformations in the environment, although many important fractionation processes remain unknown. Here, we describe Hg isotope fractionation during the abiotic dark oxidation of dissolved elemental Hg(0) in the presence of thiol compounds and natural humic acid. We observe equilibrium mass-dependent fractionation (MDF) with enrichment of heavier isotopes in the oxidized Hg(II) and a small negative mass-independent fractionation (MIF) owing to nuclear volume effects. The measured enrichment factors for MDF and MIF (ε202Hg and E199Hg) ranged from 1.10‰ to 1.56‰ and from -0.16‰ to -0.18‰, respectively, and agreed well with theoretically predicted values for equilibrium fractionation between Hg(0) and thiol-bound Hg(II). We suggest that the observed equilibrium fractionation was likely controlled by isotope exchange between Hg(0) and Hg(II) following the production of the Hg(II)-thiol complex. However, significantly attenuated isotope fractionation was observed during the initial stage of Hg(0) oxidation by humic acid and attributed to the kinetic isotope effect (KIE). This research provides additional experimental constraints on interpreting Hg isotope signatures with important implications for the use of Hg isotope fractionation as a tracer of the Hg biogeochemical cycle.

The behavior of nickel isotopes at the biogeochemical interface between ultramafic soils and Ni accumulator species

Ultramafic derived soils are characterized by low nutrient soils, a low Ca:Mg ratio, and high metal contents such as Ni, Co and Cr. The vegetation growing on these soils is highly adapted and includes both Ni hyperaccumulator and accumulator species. Today, approximately 530 Ni hyperaccumulator species are listed worldwide and the Ni concentration can be extremely high, e.g. up to 25% in latex from Pycnandra acuminata (Sapotaceae), a tree found in New Caledonia. The aim of this study is to identify the potential role of Ni hyperaccumulator plants in the Ni biogeochemical cycle at the soil surface by using Ni isotopes. A set of Ni hyperaccumulator and Ni accumulator plants as well as topsoils were sampled on the Barro Alto and Niquelândia ultramafic complexes (Goiás State, Brazil). Three Ni hyperaccumulator plants were collected: Justicia lanstyakii, Heliotropium aff. salicoides, Cnidoscolus aff. urens, as well as one Ni accumulator plant, Manihot sp. The isotopic compositions of the whole plants were determined and compared to those of the bulk topsoils and DTPA-extractable Ni. The topsoils exhibited δ60Ni values ranging from −0.30 ± 0.06‰ to 0.16 ± 0.05‰. The DTPA-extractable Ni in the topsoils ranged from 94 to 623 mg kg−1, i.e. 0.9–4.9% of the total soil Ni and was found to be isotopically heavier than the corresponding topsoil (from −0.30 ± 0.05‰ to 0.34 ± 0.08‰). The δ60Ni values for the Ni accumulator plants showed an enrichment in heavy Ni isotopes in the aerial parts of the plant compared to the roots, whereas similar δ60Ni values for the roots, stems and aerial parts suggested that no significant fractionation results from Ni uptake and translocation in Ni hyperaccumulator plants. Moreover, the aerial parts (i.e. leaves and flowers) from all of the plants analyzed showed the highest Ni concentrations and the heaviest δ60Ni values up to 1.21 ± 0.05‰. The enrichment in heavy Ni isotopes in the leaves (0.09 ± 0.06‰ < Δ60Nileaves-soil < 1.06 ± 0.03‰) may result in a heavy Ni input in the litter during organic matter restitution. There is a non-negligible amount of Ni uptake by Ni accumulator and Ni hyperaccumulator plants and this may modify both the Ni isotope composition at the soil-plant interface and the overall cycle of Ni in surface soils.

Molybdenum stable isotope fractionation during the precipitation of powellite (CaMoO4) and wulfenite (PbMoO4)

Laboratory batch experiments were conducted to study Mo isotope fractionation during the precipitation of powellite (CaMoO4) and wulfenite (PbMoO4). These minerals act as important sinks of dissolved Mo, especially in alkaline-pH mine drainage. Their precipitation might be traceable through Mo isotope analyses, provided that it induces Mo isotope fractionation. Experiments involved the presence/absence of dissolved sulfate and inorganic carbon (DIC) to better mimic the aqueous chemistry of mine drainage, as these constituents are characteristic of mine waters.Molybdate minerals preferentially incorporated light Mo isotopes from aqueous solution with fractionation patterns that are linked to the saturation state. The Rayleigh fractionation model was most suitable to describe powellite precipitation under supersaturated conditions. The average Rayleigh isotope separation factor for powellite precipitation was −0.82 ± 0.12‰, with no significant effect from the presence/absence of sulfate and DIC. Upon approaching chemical equilibrium, bidirectional exchange of molybdate between dissolved Mo and powellite led to a pronounced release of isotopically light Mo to solution, which overrode the Rayleigh fractionation pattern formed at early time. Molybdenum isotope data during wulfenite precipitation could reasonably be fitted using the Rayleigh and equilibrium fractionation models, which yielded isotope separation factors of −0.25 ± 0.10‰ and −0.75 ± 0.07‰, respectively. These results contribute to a growing knowledge of Mo isotope fractionation during processes of dissolved Mo removal (e.g., adsorption and precipitation), thus far all of which show that an enrichment in heavy Mo isotopes in solution is indicative of Mo attenuation.

The discovery of chemically produced mass independent isotope effects: The physical chemistry basis and applications to the early solar system, planetary atmospheres, and the origin of life

The discovery of chemically produced mass independent isotope effects: The physical chemistry basis and applications to the early solar system, planetary atmospheres, and the origin of life

Volatile loss following cooling and accretion of the Moon revealed by chromium isotopes

Terrestrial and lunar rocks share chemical and isotopic similarities in refractory elements, suggestive of a common precursor. By contrast, the marked depletion of volatile elements in lunar rocks together with their enrichment in heavy isotopes compared with Earth's mantle suggests that the Moon underwent evaporative loss of volatiles. However, whether equilibrium prevailed during evaporation and, if so, at what conditions (temperature, pressure, and oxygen fugacity) remain unconstrained. Chromium may shed light on this question, as it has several thermodynamically stable, oxidized gas species that can distinguish between kinetic and equilibrium regimes. Here, we present high-precision Cr isotope measurements in terrestrial and lunar rocks that reveal an enrichment in the lighter isotopes of Cr in the Moon compared with Earth's mantle by 100 ± 40 ppm per atomic mass unit. This observation is consistent with Cr partitioning into an oxygen-rich vapor phase in equilibrium with the proto-Moon, thereby stabilizing the CrO2 species that is isotopically heavy compared with CrO in a lunar melt. Temperatures of 1,600-1,800 K and oxygen fugacities near the fayalite-magnetite-quartz buffer are required to explain the elemental and isotopic difference of Cr between Earth's mantle and the Moon. These temperatures are far lower than modeled in the aftermath of a giant impact, implying that volatile loss did not occur contemporaneously with impact but following cooling and accretion of the Moon.

Examination of deep root water uptake using anomalies of soil water stable isotopes, depth-controlled isotopic labeling and mixing models

Knowledge on the water uptake depths of vegetation is crucial for understanding water transport processes of the soil-vegetation atmosphere continuum and relevant for many applications (e.g. the estimation of groundwater recharge, irrigation planning and the parameterization of (eco-) hydrological models). The identification and quantification of water uptake from deep soil layers and groundwater remain challenging. This study uses a combined framework based on natural abundances of stable water isotopes and isotopic labeling experiments with deuterium oxide (2H2O) to study root water uptake and identify uptake from deep soil in a semi-arid environment.Between 2013 and 2016, more than 1000 soil (isotope depth profiles); plant (xylem and transpiration) and water (precipitation and groundwater) samples for the analysis of isotope ratios were collected. Two experiments using isotopic labeling were carried out in order to assess root water uptake depths. Herein, we i) present series of deep soil water isotope depth profiles, interpret water transport dynamics and ecohydrological feedbacks; ii) examine the suitability of natural isotope depth profiles for identifying deep root water uptake; iii) apply the Bayesian mixing model MixSIAR to quantify deep root water uptake and iv) constrain water uptake depths using isotopic labeling experiments and derive an active root water uptake distribution.Our results show that the form of isotope depth profiles of soil in water-limited environments follows characteristic shapes for the end of the rainy and dry seasons, respectively. Isotope ratios in the upper 4 m of the soil are heavily dependent on the character of the respective rainy season. Under certain conditions – e.g. droughts or weak rainy seasons – the isotope depth profile displays an enrichment in heavy isotopes up to 4 m depth. Such pronounced anomalies provide an opportunity for studies on source water partitioning. In the present experiment, the studied individuals of Acacia erioloba were found to obtain 37% [24–52%] of their water from deep soil (>4 m) and groundwater at the end of the dry season of 2015. All other investigated trees (individuals of B. plurijuga, S. luebertii, T. sericea and C. collinum) mainly utilize water originating from 1 m to 2.5 m depth. Under “average” rainy season conditions, the similarity of isotope ratios of potential plant water sources hinders a conclusive identification of water uptake depths.Deep natural isotope depth profiles in dry environments can – under certain conditions – be used to identify and quantify access of vegetation to deep water resources. Isotopic labeling enables to determine active root distributions for the lateral root zone. Combined frameworks contribute to a better understanding of deep water uptake. A differentiated consideration of water uptake from the lateral root zone and deep, potentially groundwater-tapping roots is required in order to fully investigate ecohydrological feedbacks and for a proper parameterization of models.

Stable isotope fractionation of zinc and cadmium in soil-plant system: A review

Stable isotope fractionation provides a useful tool for tracing the transfer of zinc (Zn) and cadmium (Cd) in the soil-plant system. Isotope fractionation processes include equilibrium fractionation, e.g. dissolution, complexation, precipitation, and kinetic fractionation, e.g. diffusion and evaporation. Therefore, isotope fractionation of metallic element can be used to study the mechanisms of metal translocation in biogeochemical cycling. With the development of multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS), Zn and Cd isotopic composition in soil and plant samples can be accurately measured. The determination of Zn and Cd stable isotope ratios usually includes sample digestion, purification and mass discrimination correction. In the soil-plant system, plants are generally enriched with heavy Zn isotopes in comparison to soils, while roots are enriched with heavy Zn isotopes in comparison to shoots. For Cd, shoots are likely to be isotopically heavier than roots. The transport processes of Zn/Cd in the soil-plant system mainly involve transport in rhizosphere, root uptake and transport within plants. The adsorption and desorption of Zn/Cd by soil minerals can affect the metal mobility and the isotope fractionation between soil solid phases and soil solution. Heavy Zn isotopes are likely to be complexed, while light Zn isotopes are more mobile in soil solution. Conversely, soil minerals tend to adsorb light Cd isotopes, thereby resulting in an enrichment with heavy isotopes in the soil solution. Among the rhizospheric processes, the secretion of root exudates and the changes in pH also have an impact on the speciation and translocation of heavy metals, which lead to the release of heavy Zn isotopes into soil solution. Root uptake process is of pivotal importance for the determination of the isotopic composition of the whole plant. The diffusion of Zn2+ from the soil solution to the root surface leads to a light isotopes enrichment in plant. Heavy Zn isotopes are preferentially chelated or precipitated in the root apoplast. During the transport across the root cell membranes, the low-affinity transport leads to an enrichment with light isotopes in plant, while the high-affinity transport generates little or no isotope fractionation. Specific transporter of Cd in roots has never been found yet. Therefore, Cd may be took up mainly through low-affinity transport systems and thus be enriched in light isotopes in plants. The isotope fractionation between roots and shoots is related to the sequestration of Zn/Cd in roots and the xylem transport processes. The functional groups containing oxygen and nitrogen of organic acids in plant roots tend to chelate heavy Zn isotopes, with subsequent compartmentation in root vacuoles. The light Zn isotopes could be then loaded by the xylem and transferred to the aboveground parts. Interestingly, the intensity of isotope fractionation is mainly related to the transport distance in the xylem. Contrary to Zn, light Cd isotopes mainly bind to the thiol groups in roots, leading to the enrichment in heavy isotopes in shoots. The different behaviors between Zn and Cd in isotope fractionation in the soil-plant system could reflect the distinct accumulation and tolerance mechanisms of plants in response to Zn or Cd stress.

Redox state of the Baogutu reduced porphyry Cu deposit in the Central Asian Orogenic belt

Reduced porphyry Cu deposits are scarce and their origins remain enigmatic. The Late Carboniferous Baogutu reduced porphyry Cu deposit in the western Junggar Terrane, NW China, provides an opportunity to address this issue. In this study, we conducted systematic analyses in terms of petrography, mineral chemistry, major elements, trace elements and Fe isotopes to investigate the redox state of its primary magma and further constrain the origin of Baogutu copper deposit. Based on the petrological observation and mineral chemistry, two magmatic stages have been recognized during the formation of ore-forming diorite. The early stage is characterized by the crystallization of magnetite and high Mg# [Mg/(Mg + Fe2+)] hornblende under oxidized condition (ΔNNO > 2.36, calculated by the equation given by Scaillet and Evans (1999). The late stage is characterized by the crystallization of ilmenite and low Mg# biotite under reduced condition (ΔNNO <−0.6). The large decrease of oxygen fugacity (log fO2, more than 3 units) between the two stages may result from the contamination of organic carbon during the emplacement of magma. In addition, major and trace element compositions of the Baogutu ore-forming diorites show the affinity of arc magmas which are like those that host oxidized porphyry Cu deposits.In this study, we first report the Fe isotopic compositions of reduced porphyry copper deposit and discuss its application for the origin of ore deposit. The δ56Fe values of less altered diorites and granodiorites (+0.083‰ and +0.054‰) fall into the field of arc magmas, consistent with their major and trace elements results. The δ56Fe values of sulfides (chalcopyrite and pyrite) show a large span (−0.228 to +0.614‰) and the early formed disseminated sulfides are enriched in light Fe isotopes relative to the late formed vein-hosted sulfides, which indicates that hydrothermal fluid evolves toward heavy Fe isotope enrichment. The average δ56Fe of all hydrothermal sulfides (chalcopyrite and pyrite) is much higher than that of Baogutu silicate rocks, which demonstrates that the hydrothermal fluid are Fe isotopically heavy and it well explains the positive correlations between δ56Fe and indices of hydrothermal alteration for altered plutonic rocks and wall rocks.In summary, major elements, trace elements and Fe isotopes of the Baogutu porphyries show the affinity of arc magmas, same as the typical oxidized porphyry Cu deposits. Both petrological observations and Fe isotope features of the Baogutu porphyry Cu deposit indicate that the primary magma of Baogutu porphyries is oxidized and its reduced features are secondary.