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Concentrically laminated pyrite is a relatively common, although volumetrically minor, component of auriferous conglomerates in the Archaean (ca. 3.0–2.7 Ga) Witwatersrand Basin of South Africa. This type of pyrite contains high amounts (several tens of ppm) of Au, but the origin of the pyrite is debated, and the timing of Au deposition in these grains is not known. In order to constrain the formation of pyrite, we have studied concentrically laminated pyrite and other coexisting types of pyrite (inclusion-rich, massive pyrite) by analysing the contents and distribution of Au and other trace elements by laser ablation ICP-MS, the S and Fe isotope composition by SIMS, and the mineral inclusions by scanning electron microscope and laser Raman spectroscopy. Trace element maps indicate that concentrically laminated pyrite is enriched in Sb, Mn, Au, Ag, Tl, Cu, Mo, Mn, and contains two types of gold: finely dispersed Au (“invisible gold”, with Au/Ag ~ 0.1 and likely of primary origin) and Au inclusions with Au/Ag ~ 10 of secondary origin. The study of mineral inclusions revealed the presence of muscovite, chlorite, fine-grained carbonaceous matter, monazite, Ti-oxides, and quartz. Iron and multiple S isotopes suggest that concentrically laminated pyrite and inclusion-rich pyrite were formed from two separate pools of S and Fe with different isotope characteristics. Sulfur was derived from atmospheric S that had undergone mass-independent isotope fractionation to form SO 4 2− with negative Δ 33 S that constituted concentrically laminated pyrite, and elemental S with positive Δ 33 S that formed inclusion-rich pyrite. Iron pools were derived from partial oxidation of Fe 2 + , so that concentrically laminated pyrite formed from a low-δ 56 Fe residual Fe 2 + (average + 0.2‰) and inclusion-rich pyrite formed from a high-δ 56 Fe Fe 3 + pool (average + 2.7‰). Biological activity may have been involved in the reduction of SO 4 2− , causing a wide spread of δ 34 S values (~ 25‰, S reducing microorganisms), as well as in the partial oxidation of Fe 2 + (anaerobic photosynthetic Fe reducers or photosynthetic O 2 producers), and in the formation of pyrite from Fe 3 + (dissimilatory Fe reducers). We propose that concurrent biogenically-mediated pyrite formation and Au trapping suggest that microbial activity was responsible for the accumulation of Au and other trace elements (e.g. Sb, Mn, Ag, Tl, Cu, Mo, Mn) which are commonly enriched in organic matter-rich sediments. [ABSTRACT FROM AUTHOR]

The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compositions are dominated by the igneous/sedimentary protolith signature, by low-temperature seawater alteration, or by later fluid-rock interactions during the subduction-exhumation cycle. Although the eclogites are thought to be derived from a continental rather than oceanic setting (possibly a rift-drift transition stage), they have mid-ocean ridge basalt (MORB)-like major and trace element compositions. They are often moderately oxidized compared to MORB (Fe3+/ΣFe = 0.06 to 0.30). Their δ56Fe values (+0.00 to +0.17‰; mean + 0.08 ± 0.01‰) mostly resemble those of MORB (+0.07 to +0.17‰). The metagabbros, which are derived from a more enriched mantle source than the eclogites, yielded heavier δ56Fe values (+0.09 to +0.22‰) similar to ocean island basalts, whereas those of the paragneisses (+0.03 to +0.10‰) are typical for pelitic sediments. It appears that the Fe isotope compositions of the igneous protoliths are largely preserved and little if any Fe was mobilized during the diverse fluid-rock interaction stages. The parental magma of the eclogites was probably somewhat isotopically lighter than similarly differentiated MORB magmas, perhaps due to the presence of metasomatized, isotopically light peridotites in the subcontinental lithospheric mantle (SCLM) source. Although it is possible that δ56Fe values were slightly modified during seawater alteration and/or metamorphic fluid-rock interactions in some of the eclogites, the impact of fluid-rock interactions on the major element compositions of the eclogites appears to be small. Furthermore, the scarcity of metamorphic veins in the Münchberg Massif argues against significant Fe mobilization in the slab. We suggest that continental eclogites tend to retain their magmatic δ56Fe values throughout the subduction-exhumation cycle, whereas δ56Fe values of oceanic eclogites may often be dominated by seafloor alteration with potential local modifications in the slab close to fluid channels. The remarkable robustness of the Fe isotope compositions of continental eclogites suggests that they may be used to reconstruct protolith mantle source properties despite the complex post-magmatic history. • Preservation of igneous protolith Fe isotope compositions in the Münchberg eclogites. • This may be usual for continental eclogites. • In oceanic eclogites, seafloor alteration may often dominate Fe isotope compositions. • No evidence for significant Fe mobility during the subduction-exhumation cycle. [ABSTRACT FROM AUTHOR]

This study presents the stable isotopic variations of both Si and Fe recorded in a single well-characterized magmatic suite from Cedar Butte volcano (ID, USA), as well as a sill with progressive compositional change within Finland granophyre (Duluth Complex, MN, USA). Both isotopic systems show a significant enrichment in heavy isotopes in the more differentiated materials, in agreement with previous studies. In addition, the Finland granophyre sill shows a strong dependence between the isotopic composition and the sampling depth, suggesting the isotopic compositions follow a temperature gradient in which the cold part systematically enriches in heavy isotopes. From these results it appears that at Cedar Butte, neither crystal fractionation, nor crustal contamination, nor late stage fluid exsolution can likely explain the isotopic variations we observe for both Fe and Si. We rather attribute these isotopic fractionations to a thermal migration process involving a top–down sill injection during which the isotopic distribution mostly follows a vertical temperature gradient. [ABSTRACT FROM AUTHOR]

Abstract: Iron isotopic compositions of olivine, clinopyroxene and titanomagnetite of the Baima Fe–Ti oxide-bearing layered mafic intrusion, SW China, are used to investigate Fe isotopic fractionation during the formation of the Fe–Ti oxide ore bodies and to constrain the origin of silicate minerals and Fe–Ti oxides. The Baima intrusion comprises the Lower Zone of interlayered troctolite, clinopyroxenite and oxide ores and the Upper Zone of gabbros. Significant differences of Fe isotope values between olivine ( ), clinopyroxene ( ), and titanomagnetite ( ) were observed in rocks and ores of the Lower Zone. Iron isotopic fractionation between olivine and clinopyroxene, olivine and titanomagnetite and clinopyroxene and titanomagnetite exhibits large variations of 0.06‰ to 0.22‰, 0.12‰ to 0.27‰, and 0.00‰ to 0.20‰, respectively, suggesting disequilibrium fractionation. This disequilibrium cannot be explained by thermal or chemical gradient-induced kinetic fractionation or by possible subsolidus exsolution of granular ilmenite from titanomagnetite. Instead, it could be attributed to crystallization of silicates and titanomagnetite from two immiscible Si-rich and Fe-rich liquids. Continuous segregation of the Fe-rich liquid changed the Fe isotopic composition of the Si-rich liquid and thus the Fe isotopic compositions of olivine and clinopyroxene crystallized from it, resulting in disequilibrium Fe isotopic fractionation between them. The effect of oxygen fugacity on the crystallization order of titanomagnetite and ilmenite from the Fe-rich melts, on the other hand, gave rise to the disequilibrium Fe isotope fractionation between titanomagnetite and olivine/clinopyroxene. [Copyright &y& Elsevier]

Copper and iron isotope fractionation by plant uptake and translocation is a matter of current research. As a way to apply the use of Cu and Fe stable isotopes in the phytoremediation of contaminated sites, the effects of organic amendment and microbial addition in a mine-spoiled soil seeded with Helianthus annuus in pot experiments and field trials were studied. Results show that the addition of a microbial consortium of ten bacterial strains has an influence on Cu and Fe isotope fractionation by the uptake and translocation in pot experiments, with an increase in average of 0.99 ‰ for the δCu values from soil to roots. In the field trial, the amendment with the addition of bacteria and mycorrhiza as single and double inoculation enriches the leaves in Cu compared to the soil. As a result of the same trial, the δFe values in the leaves are lower than those from the bulk soil, although some differences are seen according to the amendment used. Siderophores, possibly released by the bacterial consortium, can be responsible for this change in the Cu and Fe fractionation. The overall isotopic fractionation trend for Cu and Fe does not vary for pot and field experiments with or without bacteria. However, variations in specific metabolic pathways related to metal-organic complexation and weathering can modify particular isotopic signatures. [ABSTRACT FROM AUTHOR]

Abstract: Magnetite is ubiquitous in the Earth's crust and its presence in modern marine sediments has been taken as an indicator of biogeochemical Fe cycling. Magnetite is also the most abundant Fe oxide in banded iron formations (BIFs) that have not been subjected to ore-forming alteration. Magnetite is therefore an important target of stable Fe isotope studies, and yet interpretations are currently difficult because of large uncertainties in the equilibrium stable Fe isotope fractionation factors for magnetite relative to fluids and other minerals. In this study, we utilized the three-isotope method (57Fe–56Fe–54Fe) to explore isotopic exchange via an enriched-57Fe tracer, and natural mass-dependent fractionation using 56Fe/54Fe variations, during reaction of aqueous Fe(II) (Fe(II)aq) with magnetite. Importantly, we employed a multi-direction approach to equilibrium by reacting four 57Fe-enriched Fe(II) solutions that had distinct 56Fe/54Fe ratios, which identifies changes in the instantaneous Fe isotope fractionation factor and hence identifies kinetic isotope effects. We find that isotopic exchange can be described by two 56Fe/54Fe fractionations, where an initial rapid exchange (∼66% isotopic mixing within 1 day) involved a relatively small Fe(II)aq–magnetite 56Fe/54Fe fractionation, followed by slower exchange (∼25% isotopic mixing over 50 days) that was associated with a larger Fe(II)aq–magnetite 56Fe/54Fe fractionation; this later fractionation is interpreted to approach isotopic equilibrium between Fe(II)aq and the total magnetite. All four Fe(II) solutions extrapolate to the same final equilibrium 56Fe/54Fe fractionation for Fe(II)aq–magnetite of at 22 °C. Additional experiments that synthesized magnetite via conversion of ferrihydrite by reaction with aqueous Fe(II) yield final 56Fe/54Fe fractionations that are identical to those of the exchange experiments. Our experimental results agree well with calculated fractionation factors using the reduced partition function ratios for from Rustad et al. (2010) and stoichiometric magnetite from Mineev et al. (2007), and these relations may be combined with the experimental constraints to determine the temperature dependence of the Fe(II)aq–magnetite fractionation factor: where T is in K. Part of the reason for large discrepancies in calculated Fe isotope fractionation factors for magnetite likely lies in the stoichiometry of the mineral in specific studies, given the significant effect of octahedral versus tetrahedral Fe isotope fractionation that has been calculated. When our results are applied to BIF genesis, our experimentally determined Fe(II)aq–magnetite fractionation factor indicates that magnetite–siderite mineral pairs in ∼2.5 Ga BIFs did not form in Fe isotope equilibrium with each other, or with ancient seawater. Iron oxides in such BIFs are therefore more likely to have formed through processes that were isolated from equilibrium with the oceans, indicating that such BIF minerals may not be suitable proxies for ancient paleoenvironments. [Copyright &y& Elsevier]

Abstract: In situ O and Fe isotope measurements of magnetite and hematite in banded iron formations (BIFs) from the 2.5 Ga Dales Gorge member of the Brockman Iron Formation, Hamersley Group, Western Australia, document distinct fine-scale isotopic zonation. For hematite, values (VSMOW) range from −7.1 to , and values range from −0.50 to . Magnetite has a range of −5.6 to and a range of −0.76 to . Notably, magnetite shows significant O isotope variability at a <100 μm scale in individual magnetite grains or layers, where up to 6‰ difference in values exists between low-Si core and Si-rich overgrowth. Iron isotope compositions are homogeneous to in values within individual grains or layers. Hematite is always isotopically heavier than co-existing magnetite by in within individual samples, and there is a large variation ( ) in values between BIF samples at the > m-scale (e.g., between meso- or macro-bands of the Dales Gorge Member). In situ isotopic results highlight the distinct behavior of O and Fe isotopes during interaction with post-deposition diagenetic or metamorphic fluids. The large variations in values of iron oxides likely reflect exchange with fluids during post-depositional events at temperatures up to ca. 280 °C, and the lower values of Fe oxides are interpreted to reflect less isotopic modification and lower temperatures. variations support a model where initial Fe(OH)3 precipitates that formed in the photic zone were converted to hematite during very early diagenesis, followed by production of magnetite over a range of temperatures during later diagenesis/metamorphism. In contrast, magnetite and hematite seem to have preserved the initial values of early Fe(OH)3 precipitates, despite significant changes in O isotope compositions in magnetite. The consistency in apparent Fe isotope fractionation between co-existing hematite and magnetite suggests that they share a common precursor, where magnetite was formed in soft sediment through in situ reduction by microbial processes, and possibly including reaction with aqueous Fe(II). Combined in situ Fe–O isotope analyses allow distinctions to be made between near-primary and post-depositional signatures in BIFs, which bears on the use of BIFs as paleo-environmental proxies and recorders of microbial processes. [Copyright &y& Elsevier]

Iron (Fe) is an essential nutrient for living organisms and Fe deficiency is a worldwide problem for the health of both rice and humans. Zinc (Zn) contamination in agricultural soils is frequently observed. Here, we studied Fe isotope compositions and transcript levels of Fe transporter genes in rice growing in nutrient solutions having a range of Zn concentrations. Our results show Zn stress reduces Fe uptake by rice and drives its δ56Fe value to that of the nutrient solution. These observations can be explained by the weakened Fe(II) uptake through Strategy I but enhanced Fe(III) uptake through Strategy II due to the competition between Zn and Fe(II) combining with OsIRT1 (Fe(II) transporter) in root, which is supported by the downregulated expression of OsIRT1 and upregulated expression of OsYSL15 (Fe(III) transporter). Using a mass balance box model, we also show excess Zn reduces Fe(II) translocation in phloem and its remobilization from senescent leaf, indicating a competition of binding sites on nicotianamine between Zn and Fe(II). This study provides direct evidence that how Zn regulates Fe uptake and translocation in rice and is of practical significance to design strategies to treat Fe deficiency in rice grown in Zn-contaminated soils. [Display omitted] • Zn stress weakens Fe(II) uptake but enhances Fe(III) uptake. • Zn stress restrains upward-translocation and remobilization of Fe(II)-NA. • Zn stress alters translocation and isotope fractionation of Fe in rice. [ABSTRACT FROM AUTHOR]

This study investigated the influences of plasma condition, concentration effect, ablation position and on-line addition of an aqueous Ni standard for non-matrix-matched Fe isotopic analysis by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs LA–MC–ICP–MS). Deviations up to ~0.7‰ in δ56Fe values caused by non-matrix-matched analyses were difficult to eliminate under dry plasma condition even with fs-LA. However, the nebulized water after the ablation cell efficiently reduces or eliminates the matrix-dependent fractionation of Fe isotopes without reducing signal intensity. Furthermore, concentration- and position-dependent fractionation of Fe isotopes was not observed under wet plasma condition. For instrumental mass bias correction, the standard–sample bracketing (SSB) method and SSB with Ni doping (SSB + Ni) were performed in the presence of nebulized water with fs LA–MC–ICP–MS. Results indicate that both the SSB and SSB + Ni methods are sufficient to correct for instrumental mass bias under wet plasma condition, with the SSB + Ni method being more effective in calibrating short-term fluctuations in mass bias with improved external reproducibility of ~0.15‰ (2SD) for δ56Fe values. Compared to SN–MC–ICP–MS, deteriorated precision offered by fs LA–MC–ICP–MS may represent a major limitation for some applications relying upon precise Fe isotope ratio measurements. This approach for non-matrix-matched analysis of Fe isotopes by fs LA–MC–ICP–MS under wet plasma condition combined with the SSB + Ni method allows for more flexibility in tracing detailed geological processes with large Fe isotope fractionation of a wide range of solids. Relatively precise and accurate Fe isotopic compositions of potential reference materials of Balmat pyrite (δ56Fe = −1.32 ± 0.12‰, δ57Fe = −1.91 ± 0.20‰; 2SD, n = 166) and PZH12–24 ilmenite (δ56Fe = −0.38 ± 0.15‰, δ57Fe = −0.57 ± 0.23‰; 2SD, n = 134) were obtained using this analytical protocol, consistent with reference values obtained by solution nebulization (SN)–MC–ICP–MS within analytical uncertainties (Balmat pyrite: δ56Fe = −1.30 ± 0.04‰, δ57Fe = −1.94 ± 0.07‰, 2SD, n = 3; PZH12–24 ilmenite: δ56Fe = −0.38 ± 0.06‰, δ57Fe = −0.55 ± 0.07‰, 2SD, n = 6). [Display omitted] • A strategy of non-matrix-matched Fe isotopic analysis by fs LA–MC–ICP–MS is provided. • Evaluation of plasma condition, concentration and position effect are discussed. • A comparison of SSB and SSB + Ni for mass bias correction is discussed. • Fe isotopic compositions of potential standards of pyrite and ilmenite are presented. [ABSTRACT FROM AUTHOR]

[Display omitted] • No resolvable Fe isotope fractionation in metabasites during prograde dehydration. • Eclogites have Fe3+/∑Fe systematically lower than the other metabasites. • Fe reduction likely occurred during eclogitization with simultaneous S oxidization. In order to better understand the mobility and charge balance of Fe in subducted slabs, here we report precise Fe isotopic data, supplemented by Fe3+/∑Fe and C-N-S elemental contents, in a well-characterized suite of metamorphic rocks from the Dabie-Sulu orogen, China, including mafic to felsic greenschists, amphibolites and eclogites. There is no systematic Fe isotopic difference among the mafic schists (δ56Fe = 0.08 ± 0.08‰, 2SD, N = 13), amphibolites (δ56Fe = 0.09 ± 0.04‰, 2SD, N = 19) and eclogites (δ56Fe = 0.07 ± 0.06‰, 2SD, N = 27), implying limited Fe mobility during prograde metamorphic dehydration. The overall δ56Fe variation of the metabasites and felsic schists ranging from 0.02 ± 0.03‰ to 0.20 ± 0.03‰ most likely mirrors heterogeneity in their Neoproterozoic magmatic protoliths, and can be interpreted by derivation from diverse sources and isotope fractionation during magma differentiation. In a contrary to limited Fe mobility, a decrease in Fe3+/∑Fe ratios has been identified for metabasites during eclogitization (on average 0.43 for greenschists, 0.42 for amphibolites and 0.24 for eclogites respectively). Reduction of Fe could have been compensated by oxidation of S where a concurrent expulsion of sulfate seems to be necessary. This study thus supports the hypothesis that Fe-S redox coupling in subducted slabs promotes transfer of oxidizing agents into the mantle wedge in subduction zones. [ABSTRACT FROM AUTHOR]

Abstract: Iron is one of the most abundant transition metal in higher plants and variations in its isotopic compositions can be used to trace its utilization. In order to better understand the effect of plant-induced isotopic fractionation on the global Fe cycling, we have estimated by quantum chemical calculations the magnitude of the isotopic fractionation between different Fe species relevant to the transport and storage of Fe in higher plants: Fe(II)-citrate, Fe(III)-citrate, Fe(II)-nicotianamine, and Fe(III)-phytosiderophore. The ab initio calculations show firstly, that Fe(II)-nicotianamine is ∼3‰ (56Fe/54Fe) isotopically lighter than Fe(III)-phytosiderophore; secondly, even in the absence of redox changes of Fe, change in the speciation alone can create up to ∼1.5‰ isotopic fractionation. For example, Fe(III)-phytosiderophore is up to 1.5‰ heavier than Fe(III)-citrate2 and Fe(II)-nicotianamine is up to 1‰ heavier than Fe(II)-citrate. In addition, in order to better understand the Fe isotopic fractionation between different plant components, we have analyzed the iron isotopic composition of different organs (roots, seeds, germinated seeds, leaves and stems) from six species of higher plants: the dicot lentil (Lens culinaris), and the graminaceous monocots Virginia wild rye (Elymus virginicus), Johnsongrass (Sorghum halepense), Kentucky bluegrass (Poa pratensis), river oat (Uniola latifolia), and Indian goosegrass (Eleusine indica). The calculations may explain that the roots of strategy-II plants (Fe(III)-phytosiderophore) are isotopically heavier (by about 1‰ for the δ 56Fe) than the upper parts of the plants (Fe transported as Fe(III)-citrate in the xylem or Fe(II)-nicotianamine in the phloem). In addition, we suggest that the isotopic variations observed between younger and older leaves could be explained by mixing of Fe received from the xylem and the phloem. [Copyright &y& Elsevier]

Abstract: The redox balance of the Archean atmosphere–ocean system is among the most significant uncertainties in our understanding of the earliest history of Earth''s surface zone. Most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca. 2.45Ga, after the Great Oxidation Event, but there is less agreement on when O2 production began, and how this may have been consumed by reduced species such as Fe(II) in the oceans. The Fe redox cycle through time has been traced using banded iron formations (BIFs), and Fe isotopes are increasingly used to constrain the conditions of Earth''s paleoenvironments, including the pathways of formation of BIFs. Iron isotope analyses of BIFs from the 3.7 to 3.8Ga Isua Supracrustal Belt (ISB), obtained by micro-sampling of magnetite-rich layers and conventional analysis, as well as by in situ femtosecond laser ablation (fs-LA-ICP-MS), indicate a consistently narrow range of non-zero δ 56Fe values. Analysis of magnetite by fs-LA-ICP-MS allows for precise and accurate micron-scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry (SIMS) analyses. Magnetite δ 56Fe values range from +0.4‰ to +1.1‰ among different bands, but within individual layers magnetite grains are mostly homogeneous. Although these BIFs have been metamorphosed to amphibolite-facies, the metamorphism can neither explain the range in Fe isotope compositions across bands, nor that between hand samples. The isotopic compositions therefore reflect “primary”, low-temperature sedimentary values. The positive δ 56Fe values measured from the ISB magnetites are best explained by deposition of Fe(III)-oxides produced by partial oxidation of Fe(II)-rich ocean water. A dispersion/reaction model, which accounts for rates of hydrothermal Fe(II)aq input, rates of oxidation, and rates of Fe(OH)3 settling suggests exceptionally low O2 contents, <0.001% of modern O2 contents in the photic zone. Such low levels suggest an anoxygenic pathway is more likely, and the data can be well modeled by anoxygenic photosynthetic Fe(II) oxidation. Comparison of the Fe isotope data from the Isua BIFs with those from the 2.5Ga BIFs from the Hamersley and Transvaal basins (Australia and South Africa, respectively) suggests a striking difference in Fe sources and pathways. The 2.5Ga magnetite facies BIFs of Australia and South Africa have δ 56Fe values that range from −1.2‰ to +1.2‰ over small scales, and are on average close to 0‰, which is significantly lower than those reported here from the Isua BIFs. The wide range in Fe isotope compositions for the Hamersley and Transvaal BIFs, in concert with C and O isotope data, have been interpreted to reflect bacterial dissimilatory Fe(III) reduction (DIR). The absence of low δ 56Fe values in the Isua BIFs, as well as the lack of fine-scale isotopic heterogeneity, may indicate formation prior to widespread DIR. [Copyright &y& Elsevier]

Abstract: The Earth''s mantle is currently oxidised and out of chemical equilibrium with the core. The reasons for this and for the relatively oxidised state of Earth''s mantle relative to the mantles of other terrestrial planets are unclear. It has been proposed that the oxidised nature and high ferric iron (Fe3+) content of Earth''s mantle was produced internally by disproportionation of ferrous iron (Fe2+) into Fe3+ and metallic iron by perovskite crystallisation during accretion. Here we show that there is substantial Fe isotope fractionation between experimentally equilibrated metal and Fe3+-bearing perovskite (≥0.45‰/amu), which can account for the heavy Fe isotope compositions of terrestrial basalts relative to equivalent samples derived from Mars and Vesta as the latter bodies are too small to stabilise significant perovskite. Mass balance calculations indicate that all of the mantle''s Fe3+ could readily have been generated from a single disproportionation event, consistent with dissolution of perovskite in the lower mantle during a process such as the Moon-forming giant impact. The similar Fe isotope compositions of primitive terrestrial and low-titanium lunar basalts is consistent with models of equilibration between the mantles of the Earth and Moon in the aftermath of the giant impact and suggests that the heavy Fe isotope composition of the Earth''s mantle was established prior to, or during the giant impact. The oxidation state and ferric iron content of the Earth''s mantle was therefore plausibly set by the end of accretion, and may be decoupled from later volatile additions and the rise of oxygen in the Earth''s atmosphere at 2.45Ga. [Copyright &y& Elsevier]

Background: The determination of the plant-induced Fe-isotopic fractionation is a promising tool to better quantify their role in the geochemical Fe cycle and possibly to identify the physiological mechanisms of Fe uptake and translocation in plants. Here we explore the isotope fractionation caused by translocation of Fe during growth of bean and oat as representatives of strategy I and II plants. Methods: Plants were grown on a nutrient solution supplemented with Fe(III)-EDTA and harvested at three different ages. We used the technique of multi-collector ICP-MS to resolve the small differences in the stable iron isotope compositions of plants. Results: Total bean plants, regardless of their age, were found to be enriched in the light iron isotopes by −1.2‰ relative to the growth solution throughout. During growth plants internally redistributed isotopes where young leaves increasingly accumulated the lighter isotopes whereas older leaves and the total roots were simultaneously depleted in light iron isotopes. Oat plants were also enriched in the light iron isotopes but during growth the initial isotope ratio maintained in all organs at all growth stages. Conclusions: We conclude that isotope fractionation in bean as a representative of strategy I plants is a result of translocation or re-translocation processes. Furthermore we assume that both uptake and translocation of Fe in oat maintains the irons' ferric state, or that Fe is always bound to high-mass ligands, so that isotope fractionation is virtually absent in these plants. However, in contrast to our previous study in which strategy II plants were grown on soil substrate, oat plants grown on Fe(III)-EDTA contain iron that enriches Fe by 0.5 permil over Fe. A possible explanation for the enrichment is the prevalence of a constitutive reductive uptake mechanism of iron in the nutrient solution used which is non-deficient in iron. [ABSTRACT FROM AUTHOR]

Abstract: Plots such as ε54Cr vs. ε50Ti and ε54Cr vs. Δ17O reveal a fundamental dichotomy among planetary materials. The “carbonaceous” chondrites, by virtue of high ε50Ti and high ε62Ni, as well as, especially for any given Δ17O, high ε54Cr, are separated by a wide margin from all other materials. The significance of the bimodality is further manifested by several types of meteorites with petrological-geochemical characteristics that suggest membership in the opposite category from the true pedigree as revealed by the stable isotopes. Ureilites, for example, despite having diversely low Δ17O and about the same average carbon content as the most C-rich carbonaceous chondrite, have clear stable-isotopic signatures of noncarbonaceous pedigree. The striking bimodality on the ε54Cr vs. ε50Ti and ε54Cr vs. Δ17O diagrams suggests that the highest taxonomic division in meteorite/planetary classification should be between carbonaceous and noncarbonaceous materials. The bimodality may be an extreme manifestation of the effects of episodic accretion of early solids in the protoplanetary nebula. However, an alternative, admittedly speculative, explanation is that the bimodality corresponds to a division between materials that originally accreted in the outer solar system (carbonaceous) and materials that accreted in the inner solar system (noncarbonaceous). In any event, both the Earth and Mars plot squarely within the noncarbonaceous composition-space. Applying the lever rule to putative mixing lines on the ε50Ti vs. ε54Cr and Δ17O vs. ε54Cr diagrams, the carbonaceous/(carbonaceous+noncarbonaceous) mixing ratio C/(C + NC) is most likely close to (very roughly) 24% for Earth and 9% for Mars. Estimated upper limits for C/(C + NC) are 32% for Earth and 18% for Mars. However, the uncertainties are such that isotopic data do not require or even significantly suggest that Earth has higher C/(C + NC) than Mars. Among known chondrite groups, EH yields a relatively close fit to the stable-isotopic composition of Earth. [Copyright &y& Elsevier]

Abstract: We detail the results of an experimental study on the kinetics of Fe isotope exchange between aqueous Fe(II)aq and nanoparticulate mackinawite (FeSm) at 25°C and 2°C over a one month period. The rate of isotopic exchange decreases synchronously with the growth of FeSm nanoparticles. 100% isotopic exchange between bulk FeSm and the solution is never reached and the extent of isotope exchange asymptotes to a maximum of ~75%. We demonstrate that particle growth driven by Ostwald ripening would produce much faster isotopic exchange than observed and would be limited by the extent of dissolution–recrystallisation. We show that Fe isotope exchange kinetics are consistent with i) FeSm nanoparticles that have a core–shell structure, in which Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation–growth mechanism. We argue that because of the structure of FeSm nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. FeSm is a reactive component in diagenetic pyrite forming systems since FeSm dissolves and reacts to form pyrite. Isotopic mobility and potential equilibration between FeSm and Fe(II)aq thus have direct implications for the ultimate Fe isotope signature recorded in sedimentary pyrite. [ABSTRACT FROM AUTHOR]

Abstract: Recent sediments from the Gotland Deep display enrichments in reactive Fe, associated with elevated Fe/Al ratios and light Fe isotopic signatures of the bulk sediments that are indicative of euxinic (anoxic and sulfidic) conditions. These enrichments can be explained by the Fe shuttle model where benthic Fe is transported from the shelf to the euxinic basin and transferred to the sediments by pyrite precipitation in the sulfidic water. The data provide evidence that the Fe shuttle at present results in accumulations of Fe that are larger compared to Fe enrichments during the Litorina Sea stage in the Gotland Deep probably caused by an increase of the benthic Fe flux from the shelf to the basin. The derived Fe enrichments are also larger compared to those in recent Black Sea sediments, which likely reflects the larger shelf to basin ratio of the Gotland Deep compare to the Black Sea. The Fe isotope data show no correlation with the organic C content of the samples indicating that the negative Fe isotope signatures are not associated with organic materials, as was suggested as an alternative explanation for the origin of the isotopically light Fe in sediments from the Litorina Sea stage. Conversely, pyrites carry the negative Fe isotopic signature of the sediments, which supports the Fe shuttle model. Variations in the abundance and Fe isotopic signature of reactive Fe and pyrite with depth suggest that syngenetically formed pyrite in the sulfidic water column has a less negative Fe isotopic composition compared to diagenetically produced pyrite. [ABSTRACT FROM AUTHOR]

Abstract: The isotope composition of iron in soils can display the environmental conditions that formed this soil. But plants extract only the mobile iron from soil, which is a small fraction of the soils'' total iron. Yet this fraction is notoriously difficult to extract experimentally. Here we provide evidence that this signature is provided readily in the form of strategy II plants (grasses). We determined the stable Fe isotope signature of iron pools in two agronomic soils with two different sequential extraction methods. The Fe isotopic composition of the following soil mineral pools was measured: exchangeable iron, iron of poorly-crystalline (oxyhydr)oxides, iron in organic matter, iron of crystalline oxides and silicate bound iron. We found variations of about 1 per mil in δ56Fe (δ56Fe/[‰]=[(56/54Fesample/56/54FeIRMM-014)−1]·103) in the iron isotopic composition between the different soil mineral pools. The pools that contribute most to plant nutrition are water-extractable- and exchangeable iron, iron in organic matter and iron of poorly-crystalline (oxyhydr)oxides. These fractions are about 0.3 per mil lighter than the bulk soils. Silicates in our soils have a δ56Fe of up to 0.4‰, suggesting preferential loss of light Fe during weathering. We compared the isotope composition of the plant-available Fe to that of typical strategy I and strategy II plants, grown on the soils. While redox and other transformation processes in the rhizosphere enrich strategy I plants to varying degrees in light Fe isotopes, strategy II plants exhibit a uniform Fe isotopic composition and are only slightly enriched in the heavier iron isotopes by about 0.3‰. Therefore these plants may record the Fe isotope composition of plant-available iron in soils, to which the composition of strategy I plants can be compared to. [Copyright &y& Elsevier]

Abstract: We combined micro computer tomography with Fe and Si isotope measurements of Mokoia, Allende and Grosnaja chondrules. Ten Mokoia chondrules contain 0.9 to 11.8vol.% opaque phases (metal+sulfide), and 6 Allende chondrules contain 0.0 to 6.6vol.% opaque phases. Hence, the Fe isotope composition of many chondrules is dominated by the Fe isotope composition of their opaque phases. We studied Fe isotopes of 35 bulk chondrules. The range is different for each of the three meteorites studied and largest for Allende with δ 56Fe ranging from −0.82 to +0.37‰. Six out of seven chondrules analysed for their Si isotope composition in Mokoia and Grosnaja have similar δ 29Si of around −0.12‰. One anomalous chondrule in Mokoia has a δ 29Si of +0.58‰. We exclude isotopically heterogeneous chondrule precursors and different isotopic chondrule reservoirs as the source of the observed Fe isotope variation among bulk chondrules. We conclude that the observed bulk chondrule Fe isotope variation is the result of evaporation and re-condensation processes in a nebula setting with high dust densities, required to explain the comparatively low isotope fractionations. Subsequent parent body alteration slightly overprinted this pre-accretionary Fe isotope variation. [ABSTRACT FROM AUTHOR]

Abstract: The equilibrium Fe isotope fractionation factor between aqueous Fe(II) and goethite has been experimentally measured to be −1.05±0.08‰ in 56Fe/54Fe (2σ) at 22°C, using the three-isotope method. Experiments were done using two sizes of goethite (81×11nm and 590×42nm), and the experimental products were subjected to serial extraction using acid partial dissolution techniques to determine if surface Fe(III) atoms have different isotopic properties than the bulk goethite. These experiments indicate that the interaction of Fe(II)aq and goethite is dynamic and results in complete or near-complete Fe isotope exchange over 30days, involving at least four components: Fe(II)aq, goethite, sorbed Fe(II), and Fe(III)surface. The equilibrium fractionation factor between Fe(II)aq and Fe(II)sorb is the same for both sizes of goethite, at Δ56FeFe(II)aq–Fe(II)sorb =−1.24±0.14‰; this fractionation factor is significantly different than the results of previous studies on Fe(II) sorption to goethite. The proportion of the Fe(III)surface component is greatest in the experiments that used the smallest goethite, and the Fe(III)surface–Fe(II)aq fractionation is estimated to be at least +2.1‰. The high Fe(III)surface–Fe(II)aq fractionation may exert a significant influence on the Fe isotope compositions of aqueous Fe(II) in natural systems that contain nanoparticulate goethite, including those involving bacterial iron reduction. These results demonstrate that the isotopic properties of nano-scale minerals may be distinct from micron-scale or larger minerals, as is the case for other thermodynamic properties of nanoparticles. [Copyright &y& Elsevier]

Nucleosynthetic Fe isotopic anomalies in meteorites may be used to learn about the early evolution of the solar system and to identify the origin and nature of the material that built the terrestrial planets. Using high-precision iron isotopic data of 23 iron meteorites from nine major chemical groups we show that all iron meteorites define the same dichotomy between non-carbonaceous (NC) and a carbonaceous (CC) meteorites previously observed for other elements. The Fe isotopic anomalies are predominantly produced by variations in 54Fe, where all CC iron meteorites are characterized by an excess in 54Fe relative to NC iron meteorites. This excess in 54Fe is accompanied by an excess in 58Ni observed in the same CC meteorite groups. Together, these overabundances of 54Fe and 58Ni are explained by nuclear statistical equilibrium either in type Ia supernovae or in the Si/S shell of core-collapse supernovae. The Fe isotopic composition of Earth's mantle plots on or close to correlations defined by Fe, Mo, and Ru isotopic anomalies in iron meteorites, indicating that throughout Earth's accretion, the isotopic composition of its building blocks did not drastically change. While Earth's mantle has a similar Fe isotopic composition to CI chondrites, the latter are clearly distinct from Earth's mantle for other elements (e.g., Cr and Ni) whose delivery to Earth coincided with Fe. The fact that CI chondrites exhibit large Cr and Ni isotopic anomalies relative to Earth's mantle, therefore, demonstrates that CI chondrites are unlikely to have contributed significant Fe to Earth and are not its main building blocks. • Fe isotopic anomalies of iron meteorites fall into distinct NC-CC clusters. • Fe isotopic anomalies in bulk meteorites are produced by 54Fe variations. • 54Fe variations originate from nuclear statistical equilibrium in supernovae. • Earth's mantle plots close to the correlations of Fe, Mo, and Ru isotopic anomalies. • No drastic changes in the isotopic composition of Earth's building material. [ABSTRACT FROM AUTHOR]

The Rabaul volcanic complex, Papua New Guinea, is among the few calderas worldwide with ongoing volcanic activity. Its volcanic lithologies vary from mafic lavas from outer caldera cones to differentiated lavas within. In this study, representative lavas have been analysed for their major and trace element concentrations and radiogenic Sr-Nd-Hf isotope compositions to study the geochemical evolution of the magmatic system. Stable Fe isotopes and concentrations of high field strength elements (HFSE) complement the analyses as novel tools to assess the effect of high-temperature fractional crystallisation during ongoing differentiation. Major element systematics reveal a typical fractional crystallisation sequence of olivine, pyroxene, and plagioclase as the critical process controlling the magmatic evolution. A distinct increase of Zr/Hf from the basaltic (older) outer caldera lavas (~39) to the dacitic (younger) inner caldera lavas (~41–44) can be explained by fractionation of clinopyroxene and amphibole. Ratios of Nb/Ta tend to decrease with an increasing degree of differentiation, consistent with fractional crystallisation of amphibole but not clinopyroxene. Additional fractionation of Ti-magnetite and rather oxidising conditions are further supported by the Fe isotope compositions in the inner caldera lavas (δ57Fe from +0.03 to +0.22‰, ±0.04‰, 2 SD). The high Nb/Ta in more primitive outer caldera samples are coupled with increasing rare earth elements (REE) abundances, and slab melt-like subduction components can explain high Sr/Y and Gd N /Yb N in the magma sources. Complementary enrichments in fluid-mobile trace elements indicate that slab dehydration controlled the sub-arc enrichment of the inner caldera volcanism. Coupled Hf-Nd isotope compositions reveal the presence of the Indian-Australian mantle domain beneath Rabaul and a temporal trend towards sediment melt components overprinting inner caldera lavas. In conclusion, geochemical features show a temporal evolution controlled by (i) variable influence of partial slab melts vs slab fluids and (ii) a change in fractional crystallisation patterns from solely olivine and pyroxene-controlled to increasingly titano-magnetite and amphibole-controlled fractionation. • RVC lavas show island arc magma like fractional crystallisation. • Outer to inner caldera evolve from partial melt to sediment-fluid dominated. • Hf-Nd isotopes reveal presence of Indian-Australian mantle domain beneath RVC. • Inner to outer caldera HFSE variations caused by clinopyroxene and amphibole crystallisation. • Coexistent magnetite and clinopyroxene causes Fe isotope variations in the RVC. [ABSTRACT FROM AUTHOR]

Abstract: The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe3+/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe3+/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ 56/54FeIRMM-014 =0.072±0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ 56/54FeIRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe3+/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed. [Copyright &y& Elsevier]

A study on tropospheric aerosols involving Fe particles with an industrial origin is tackled here. Aerosols were collected at the largest exhausts of a major European steel metallurgy plant and around its near urban environment. A combination of bulk and individual particle analysis performed by SEM–EDX provides the chemical composition of Fe-bearing aerosols emitted within the factory process (hematite, magnetite and agglomerates of these oxides with sylvite (KCl), calcite (CaCO3) and graphite carbon). Fe isotopic compositions of those emissions fall within the range (0.08‰< δ 56Fe<+0.80‰) of enriched ores processed by the manufacturer (−0.16‰< δ 56Fe<+1.19‰). No significant evolution of Fe fractionation during steelworks processes is observed. At the industrial source, Fe is mainly present as oxide particles, to some extent in 3–4μm aggregates. In the close urban area, 5km away from the steel plant, individual particle analysis of collected aerosols presents, in addition to the industrial particle type, aluminosilicates and related natural particles (gypsum, quartz, calcite and reacted sea salt). The Fe isotopic composition (δ 56Fe=0.14±0.11‰) measured in the close urban environment of the steel metallurgy plant appears coherent with an external mixing of industrial and continental Fe-containing tropospheric aerosols, as evidenced by individual particle chemical analysis. Our isotopic data provide a first estimation of an anthropogenic source term as part of the study of photochemically promoted dissolution processes and related Fe fractionations in tropospheric aerosols. [Copyright &y& Elsevier]

Abstract: The isotopic composition of Fe was determined in water, Fe-oxides and sulfides from the Tinto and Odiel Basins (South West Spain). As a consequence of sulfide oxidation in mine tailings both rivers are acidic (1.45

The increase in atmospheric oxygen ca. 2.4 Ga had a significant impact on the geochemical cycling of Fe. The history of environmental oxygenation may be recorded in the Fe isotope composition of Archean and Proterozoic sediments, but this record cannot be interpreted accurately until we understand the mechanisms causing isotope variations. Here we present Fe isotope data and iron/aluminum ratios from the Black Sea oxic shelf and euxinic basin. The isotope data demonstrate that shelf Fe is depleted in the lighter isotope compared to both the detrital weathering input and the sediments of the euxinic basin. We propose that there is net transport of isotopically light Fe from sediments of the shelf to those of the distal, anoxic basin, consistent with enrichments in reactive Fe seen in the deep basin. The low δ56Fe benthic Fe flux is generated during the coupling of microbial Fe(III) reduction or sulfidization with Fe2+aq oxidation. Low δ56Fe values reported previously from Late Archean sedimentary pyrites may be an isotopic fingerprint of analogous Fe redox cycling in the Late Archean oceans. This interpretation implies shallow-water Fe redox recycling in the Late Archean. We predict that the light isotopic compositions of the Late Archean will prove to be distinct from those of the Early Archean, before Fe redox cycling became an important process, and we infer that this difference may be related to the presence of oxygen in the surface ocean. [ABSTRACT FROM AUTHOR]

We present Fe isotope compositions for magnetite crystals from >3.7 Ga banded iron formation (BIF) from the Isua Greenstone Belt and younger, spatially related pyrite obtained in situ by secondary ion mass spectrometry (SIMS). Across a distance of several millimeters, individual magnetite crystals of BIF show δ56Fe values up to 2‰, and within single magnetiterich layers, there is a >2‰ range in δ56Fe. The highest positive δ56Fe values are consistent with those predicted by the equilibrium fractionation factor for oxidation of ferrous to ferric Fe in aqueous solution at a likely Early Archean ocean temperature of ∼70 °C, and further suggest that subsequent precipitation of ferric oxides was sufficiently slow that kinetic fractionation effects were minimal. These high values also imply that the supply of oxidant was limited. Heterogeneity in δ56Fe values is attributed to diagenetic reactions occurring in limited-volume pore waters, isolated from the bulk ocean (δ56Fe = 0‰), with development of isotope reservoir effects. Secondary pyrite in BIF and a conglomerate also show a distinct enrichment in heavy Fe isotopes, interpreted as redistribution and/or fractionation during postformational events. [ABSTRACT FROM AUTHOR]

Abstract: High precision Fe isotope measurements have been performed on various mantle peridotites (fertile lherzolites, harzburgites, metasomatised Fe-enriched peridotites) and volcanic rocks (mainly oceanic basalts) from different localities and tectonic settings. The peridotites yield an average δ56Fe=0.01‰ and are significantly lighter than the basalts (average δ56Fe=0.11‰). Furthermore, the peridotites display a negative correlation of δ56Fe with Mg# indicating a link between δ56Fe and degrees of melt extraction. Taken together, these findings imply that Fe isotopes fractionate during partial melting, with heavy isotopes preferentially entering the melt. The slope of depletion trends (δ56Fe versus Mg#) of the peridotites was used to model Fe isotope fractionation during partial melting, resulting in α mantle-melt ≈1.0001–1.0003 or lnα mantle-melt ≈0.1–0.3‰. In contrast to most other peridotites investigated in this study, spinel lherzolites and harzburgites from three localities (Horoman, Kamchatka and Lherz) are virtually unaffected by metasomatism. These three sites display a particularly good correlation and define an isotope fractionation factor of lnα mantle-melt ≈0.3‰. This modelled value implies Fe isotope fractionation between residual mantle and mantle-derived melts corresponding to Δ 56Femantle–basalt ≈0.2–0.3‰, i.e. significantly higher than the observed difference between averages for all the peridotites and the basalts in this study (corresponding to Δ 56Femantle–basalt ≈0.1‰). Either disequilibrium melting increased the modelled α mantle-melt for these particular sites or the difference between average peridotite and basalt may be reduced by partial re-equilibration between the isotopically heavy basalts and the isotopically light depleted lithospheric mantle during melt ascent. The slope of the weaker δ56Fe–Mg# trend defined by the combined set of all mantle peridotites from this study is more consistent with the generally observed difference between peridotites and basalts; this slope was used here to estimate the Fe isotope composition of the fertile upper mantle (at Mg#=0.894, δ56Fe≈0.02±0.03‰). Besides partial melting, the Fe isotope composition of mantle peridotites can also be significantly modified by metasomatic events, e.g. melt percolation. At two localities (Tok, Siberia and Tariat, Mongolia) δ56Fe correlates with iron contents of the peridotites, which was increased from about 8% to up to 14.5% FeO by post-melting melt percolation. This process produced a range of Fe isotope compositions in the percolation columns, from extremely light (δ56Fe=−0.42‰) to heavy (δ56Fe=+0.17‰). We propose reaction with isotopically heavy melts and diffusion (enrichment of light Fe isotopes) as the most likely processes that produced the large isotope variations at these sites. Thus, Fe isotopes might be used as a sensitive tracer to identify such metasomatic processes in the mantle. [Copyright &y& Elsevier]

Abstract: A novel analytical technique for isotopic analysis of dissolved and particulate iron (Fe) from various marine environments is presented in this paper. It combines coprecipitation of dissolved Fe (DFe) samples with Mg(OH)2, and acid digestion of particulate Fe (PFe) samples with double pass chromatographic separation. Isotopic data were obtained using a Nu Plasma MC-ICP-MS in dry plasma mode, applying a combination of standard-sample bracketing and external normalization by Cu doping. Argon interferences were determined prior to each analysis and automatically subtracted during analysis. Sample size can be varied between 200 and 600ng of Fe per measurement and total procedural blanks are better than 10ng of Fe. Typical external precision of replicate analyses (1S.D.) is ±0.07‰ on δ56Fe and ±0.09‰ on δ57Fe while typical internal precision of a measurement (1S.E.) is ±0.03‰ on δ56Fe and ±0.04‰ on δ57Fe. Accuracy and precision were assured by the analysis of reference material IRMM-014, an in-house pure Fe standard, an in-house rock standard, as well as by inter-laboratory comparison using a hematite standard from ETH (Zürich). The lowest amount of Fe (200ng) at which a reliable isotopic measurement could still be performed corresponds to a DFe or PFe concentration of ∼2nmolL−1 for a 2L sample size. To show the versatility of the method, results are presented from contrasting environments characterized by a wide range of Fe concentrations as well as varying salt content: the Scheldt estuary, the North Sea, and Antarctic pack ice. The range of DFe and PFe concentrations encountered in this investigation falls between 2 and 2000nmolL−1 Fe. The distinct isotopic compositions detected in these environments cover the whole range reported in previous studies of natural Fe isotopic fractionation in the marine environment, i.e. δ56Fe varies between −3.5‰ and +1.5‰. The largest fractionations were observed in environments characterized by redox changes and/or strong Fe cycling. This demonstrates the potential use of Fe isotopes as a tool to trace marine biogeochemical processes involving Fe. [Copyright &y& Elsevier]

Abstract: Despite the large number of successful applications of laser ablation, elemental and isotopic fractionation coupled to inductively coupled plasma mass spectrometry (ICP-MS) remain as the main limitations for many applications of this technique in the fields of analytical chemistry and Earth Sciences. A substantial effort has been made to control such fractionations, which are well-established features of nanosecond laser ablation systems. Technological advancements made over the past decade now allow the ablation of solids by femtosecond laser pulses in the deep ultraviolet (UV) region at wavelengths less than 200 nm. Here the use of femtosecond laser ablation and its effects on elemental and isotopic fractionation is investigated. The Pb/U system is used to illustrate elemental fractionation and stable Fe isotopes are used to illustrate isotopic fractionation. No elemental fractionation is observed beyond the precision of the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) measurements. Without a matrix match between standard and sample, elemental fractionation is absent even when using different laser ablation protocols for standardization and samples (spot versus raster). Furthermore, we found that laser ablation-induced isotope ratio drifts, commonly observed during nanosecond laser ablation, are undetectable during ultraviolet femtosecond laser ablation. So far the precision obtained for Fe isotope ratio determinations is 0.1‰ (2 standard deviation) for the 56Fe/54Fe ratio. This is close to that obtainable by solution multiple-collector inductively coupled plasma mass spectrometry. The accuracy of the results appears to be independent of the matrix used for standardization. The resulting smaller particle sizes reduce fractionation processes. Femtosecond laser ablation carries the potential to solve some of the difficulties encountered during the two prior decades since the introduction of laser ablation. [Copyright &y& Elsevier]

Abstract: We measured iron isotopic composition of surface (10–20 cm) and subsurface (50–70 cm) basaltic soil horizons from the Island of Maui along a climate gradient (MCG) ranging from 2.2 to 4.2 m mean annual precipitation (MAP). All soil forming factors except climate were conserved. The MCG has a documented decrease in Fe with increasing rainfall that is highly correlated with decreasing mean annual Eh values. We found that increasing MAP from 2.8 to 4.2 m resulted in a surface plus subsurface average increase of 0.56‰±0.09‰ δ56Fe with the subsurface consistently 0.33±0.06‰ δ56Fe greater than the surface horizons. Based on loss of Fe relative to Nb, Rayleigh fractionation was observed with 103lnα lost–retained values of −0.37±0.03 and −0.34±0.04 for the surface and subsurface, respectively. Equivalent 103lnα lost–retained values for the surface and subsurface soils suggests Fe loss is driven by similar mechanisms throughout the soil profile. Our calculated fractionation factor is about 1/3 the magnitude of laboratory determined fractionation factors for Fe reduction, suggesting other processes (organic complexation, Fe re-precipitation) modulate the net Fe loss along the MCG. These results offer field-scale confirmation of laboratory experiments on model systems that show anoxic weathering reactions produce materials enriched in heavy Fe isotopes. [Copyright &y& Elsevier]

Abstract: Iron concentrations and isotopic compositions were measured in spherical hematite and goethite concretions, together with associated red (Fe-oxide coated) and white (bleached) sandstones from the Jurassic Navajo formation, Utah (USA). Earlier studies showed that, in the Navajo Sandstone, reducing fluids (presumably rich in hydrocarbons) mobilized Fe present as Fe-oxide coatings on detrital quartz grains. Dissolved Fe then precipitated as spherical concretions by interaction with oxidizing groundwater. Despite being depleted in Fe by ∼50%, the bleached sandstones have Fe isotopic compositions similar to adjacent red sandstones (∼0‰/amu relative to IRMM-014). This shows that dissolution of Fe-oxide did not produce significant isotope fractionation, in agreement with previous experimental studies of abiotic Fe-oxide dissolution. In contrast, the concretions are depleted in the heavy isotopes of iron by −0.07 to −0.68‰/amu. This is opposite to the expected fractionation for partial Fe oxidation, which tends to enrich the precipitate in the heavy isotopes. Several scenarios are considered for explaining the measured Fe isotopic compositions. Although diffusion might be an important process in controlling the growth of spherical concretions, the associated isotopic fractionation is negligible compared to the observed variations. Kinetic isotope fractionation during precipitation can be ruled out as well because no isotopic zonation is seen within indurated concretions and Fe isotope evidence supports the occurrence of dissolution–reprecipitation reactions consistent with equilibrium growth conditions. The Fe isotopic compositions of the concretions are best explained by evolution of the fluid composition through precipitation and/or adsorption of isotopically heavy Fe during fluid flow through the sandstone. This scenario is supported by a regional trend in the isotopic composition of Fe, showing that this element was transported in fluids over several kilometres along major tectonic structures. These results demonstrate for the first time the virtue of Fe isotopes for tracing the directions and scales of paleofluid flows in porous media. [Copyright &y& Elsevier]

Abstract: Iron isotope fractionation between liquid and vapor iron pentacarbonyl was measured in a closed system at ∼0 and ∼21°C to determine if Fe isotope analysis of iron pentacarbonyl vapor is viable using electron-impact, gas-source mass spectrometry. At the 2σ level, there is no significant Fe isotope fractionation between vapor and liquid under conditions thought to reflect equilibrium. Experiments at ∼0°C indicate iron pentacarbonyl vapor is ∼0.05 per mil (‰) greater in 56Fe/54Fe than liquid iron pentacarbonyl, which is just resolvable at the 1σ level. Partial decomposition of iron pentacarbonyl vapor or liquid to an iron oxide or iron metal shows that significant isotopic fractionation occurs, where the decomposed product has a lower 56Fe/54Fe ratio as compared to the starting iron pentacarbonyl. It follows that methods to decompose iron pentacarbonyl must be quantitative to obtain accurate isotope values. [Copyright &y& Elsevier]

Abstract: Assessing the level of homogeneity of different planetary reservoirs is an essential step to establish a new isotopic tracer. In recent studies, we concluded that most igneous rocks from the continental crust were homogeneous to < ±0.05‰ in δ 57Fe [Poitrasson, F., Halliday, A.N., Lee, D.C., Levasseur, S., Teutsch, N., 2004. Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms. Earth Planet. Sci. Lett., 223, 253-266.; Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.]. This conclusion was previously reached by Beard et al. [Beard, B.L., Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003. Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol., 195, 87-117.], though defined by a broader range (±0.08‰). The main purpose of the Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on “Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS” by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] comment is to emphasize again this conclusion reached by the two groups: the degree of the iron isotope variability observed in most bulk rocks from the continental crust is small. However, controversy lies with high silica magmatic rocks (> ∼ 71 wt.% SiO2) that we concluded were significantly heavier than less silicic rocks from the crust on the basis of Fe isotopic analyses of seven bulk rock samples from different geological settings. Beard and Johnson [Beard, B.L., Johnson, C.M., 2006. Comment on “Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS” by F. Poitrasson and R. Freydier, Chemical Geology, volume 222, pages 132-147. Chem. Geol., this issue.] challenge this finding on the basis of previously “published” data. It is shown here that this controversy stems from the confusion between whole-rock and mineral iron isotope analyses, since this isotopic tracer, like many others, is more heterogeneous at the mineral scale than the bulk rock scale. Thus, minerals cannot be used in place of bulk rock Fe isotope compositions. The controversy also arises from different levels of analytical uncertainties. If only the best Fe isotope probes of planetary reservoirs — bulk rocks — are considered, then the conclusion that igneous rocks with SiO2 >71 wt.% are significantly heavier than the mean δ 57Fe of the continental crust [Poitrasson, F., Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol., 222, 132-147.] remains valid. Nonetheless, given that granites represent only a minor fraction of the continental crust, this finding does not change the bulk igneous Fe isotope composition estimate previously defined for the Earth. [Copyright &y& Elsevier]

Abstract: Scrutiny of published Fe isotope data for igneous rocks that have >71 wt.% SiO2 reveals that most samples have the same Fe isotope composition as mafic igneous rocks, contrary to the conclusions of Poitrasson and Freydier [Poitrasson, F. and Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147]. The only silicic igneous rocks that have Fe isotope compositions that are significantly different than mafic igneous rocks are 4 granitic samples that have SiO2 >75 wt.% analyzed by Poitrasson and Freydier [Poitrasson, F. and Freydier, R., 2005. Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS. Chem. Geol. 222, 132–147]; three silicic igneous rocks analyzed by Beard et al. [Beard, B.L, Johnson, C.M., Skulan, J.L., Nealson, K.H., Cox, L., Sun, H., 2003a. Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chem. Geol. 195 87–117] (SiO2 >75 wt.%) have Fe isotope compositions that are identical to mafic igneous rocks, but these samples were ignored. Moreover, the Fe isotope composition of upper continental crust as deduced from the 58 sedimentary clastic rocks (shales, loess, modern marine sediments, and suspended load of rivers), reported by Beard et al. [Beard, B.L., Johnson, C.M., von Damm, K.L. and Poulson, R.L, 2003b. Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans. Geology 31 629–632], is the same as the average of igneous rocks. These data suggest that granitic rocks which deviate from the igneous average may be rare. [Copyright &y& Elsevier]

Measurements of Fe oxyhydroxides [reactive Fe(III)] in two shallow sediment profiles from reducing and oxic environments on the Arabian Sea margin demonstrate Fe isotope fractionation during early marine diagenesis. Reactive Fe(III) has δ56Fe values between -0.77‰ and -0.19‰. Values are lowest at the top of the sediment profile and considerably lower than bulk sediment Fe (δ56Fe ≈ 0‰). Preferential reduction and dissolution of light Fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive Fe(III) residual. Initially, the isotopic composition of reactive Fe(III) evolves down-core according to the fractionation factor typical of microbial dissimilatory Fe reduction. Deeper in the profile, δ56Fe values remain unchanged despite further Fe reduction. Here, another process with a different fractionation factor becomes dominant, probably reduction by dissolved sulfide. The δ56Fe of the residual reactive Fe(III) suggests that ∼25% of the initially present reactive Fe(III) was reduced by microbial Fe reduction. When Fe is diagenetically recycled between reducing sediments at depth and an oxic top layer, the process, depending on recycling efficiency, may result in the accumulation of light Fe in the top layer while the complementary heavier residual is buried. Fe diffusing from the seafloor back into the ocean should reflect the low-δ56Fe diagenetic source of dissolved Fe. [ABSTRACT FROM AUTHOR]

Abstract: Temporal variations in the iron isotopic composition, δ 56Fe between − 0.13‰ and 0.31‰, have been measured in the suspended fraction in a Boreal river. The major mechanism behind these variations is temporal mixing between two types of particles–colloids, Fe-oxyhydroxides and Fe–C colloids. Data in this study indicate that these two types of colloids have different Fe-isotope composition. The Fe–C colloid has a negative δ 56Fe value whereas the Fe-oxyhydroxide colloid is enriched in 56Fe. These two types of colloidal matter have different hydrogeochemical origin. The Fe–C colloid reaches the river during storm events when the upper sections of the soil profile (O and E horizons) are flooded by a rising water table. Colloidal Fe-oxyhydroxides reach the river via inflow and subsequent oxidation of groundwater enriched in dissolved Fe(II). [Copyright &y& Elsevier]

Abstract: Temporal variations in Fe isotope compositions at three locations in the Pacific Ocean over the last 10 Ma are inferred from high-resolution analyses of three hydrogenetic ferromanganese crusts. Iron pathways to the central deep Pacific Ocean appear to have remained constant over the past 10 Ma, reflected by a remarkably constant Fe isotope composition, despite large changes in the Fe delivery rates to the surface ocean via dust. These results suggest that the Fe cycle in the deep ocean is decoupled from that in surface waters. By contrast, one ferromanganese crust from the Izu-Bonin (IB) back-arc/marginal basin of the W. Pacific exhibits large δ56Fe variations. In that crust, decreases in δ56Fe values correlate with increases in Mn, Mg, Ni, Cu, Zn, Mo, and V contents, and consistent with periods of intense hydrothermal input and increased growth rates. A second crust located within 100 km of the first IB sample does not record any of these periods of enhanced hydrothermal input. This probably reflects the isolated pathways by which hydrothermally sourced Fe may have migrated in the back arc, highlighting the high degree of provinciality that Fe isotopes may have in the modern (oxic) oceans. Our results demonstrate that despite efficient removal at the source, hydrothermal Fe injected into the deep ocean could account for a significant fraction of the dissolved Fe pool in the deep ocean, and that hydrothermally sourced Fe fluxes to the open ocean may have lower δ56Fe values than those measured so far in situ at hydrothermal vents. Correlation between δ56Fe values and elements enriched in hydrothermal fluids may provide a means for distinguishing hydrothermal Fe from other low-δ56Fe sources to the oceans such as dissolved riverine Fe or porewaters in continental shelf sediments. [Copyright &y& Elsevier]

Direct fossil evidence for early microbial life on Earth is rare. Microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration, and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments. Fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in Earth's history. We present Fe isotopic data for sedimentary pyrite from the 2.7 Ga Belingwe sedimentary basin in Zimbabwe. Isotopically light Fe and a remarkable covariation between Fe and S isotopes provide strong evidence for coexisting Fe and S reduction. Our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial Fe reduction. The covariation of Fe and S isotopes may provide a useful new tracer of microbial evolution early in Earth history. [ABSTRACT FROM AUTHOR]

Abstract: High mass resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) was assessed for iron isotope measurement of natural samples after matrix separation by anion exchange chromatography. No remaining interferences were observed on the plateaus used for the mass spectrometric measurements. The approach developed and the instrument used permitted analyses in the static mode. Various mass bias corrections using Ni doping were tested, and even the assumption of similar fractionation factors for Fe and Ni did not produce significantly inaccurate data. However, the daily regression method between ln57Fe/ 54Fe and ln61Ni/ 60Ni on the standard reference material IRMM-14 to characterize the instrumental mass bias appeared to give the best precision. The reproducibility observed over four months is about 0.013‰/amu, 2 SD, on both δ 57Fe/ 54Fe and δ 56Fe/ 54Fe values, provided that each sample is analyzed at least six times. Accuracy, as estimated on interlaboratory comparison of natural samples that included geostandards, lies within this uncertainty. Among the bulk granitic rocks analysed, those with MgO below 0.6 wt.% and SiO2 above 71 wt.% have δ 57Fe/ 54Fe values significantly heavier than the bulk mafic Earth. This shows that the iron isotope composition of terrestrial igneous rocks is more scattered than previously thought. There are good correlations between the Fe isotope composition and the MgO and SiO2 contents of the granitoids. These correlations are interpreted as reflecting the exsolution of late magmatic aqueous fluids from the granitic melt that preferentially removed the lighter isotopes of iron and enriched the residual magma in the heavier isotopes. [Copyright &y& Elsevier]

Abstract: Iron sulfide phases are the ultimate repository of iron and reduced sulfur in sediments. The sulfur isotope geochemistry of pyrite has had much to tell us about modern and ancient Earth environments and it is likely that Fe isotopes will too, once fractionations for key processes are known. We report the results of an experimental study of Fe isotope fractionation on precipitation of FeS, synthetic mackinawite, from excess aqueous Fe(II) solutions by addition of sodium sulfide solution at 2–40 °C. The results show a significant kinetic isotope effect in the absence of a redox process. No detectable effect of temperature on the fractionation factor was observed. The Fe isotope fractionation for zero-age FeS is Δ Fe(II)–FeS =0.85±0.30‰ across the temperature range studied, giving a kinetic isotope fractionation factor of α Fe(II)–FeS =1.0009±0.0003. On ageing, the FeS in contact with aqueous Fe(II) becomes progressively isotopically heavier, indicating that the initial fractionations are kinetic rather than equilibrium. From published reaction mechanisms, the opportunity for Fe isotope fractionation appears to occur during inner sphere ligand exchange between hexaqua Fe(II) and aqueous sulfide complexes. Fe isotope fractionation on mackinawite formation is expected to be most significant under early diagenetic situations where a readily available reactive Fe source is available. Since FeS(aq) is a key reactive component in natural pyrite formation, kinetic Fe isotope fractionations will contribute to the Fe isotope signatures sequestered by pyrite, subject to the relative rate of FeS2 formation versus FeS–Fe(II)(aq) isotopic equilibration. [Copyright &y& Elsevier]

Abstract: Seven bulk chondrites, with δ 57Fe/54Fe values between −0.1‰ and 0‰ relative to IRMM-14, tend to be slightly lighter than 11 bulk iron meteorites, which have δ 57Fe/54Fe values ranging from 0.04‰ to 0.2‰. At the mineral scale, taenite from two iron meteorites, Cranbourne and Toluca, shows δ 57Fe/54Fe values heavier by up to 0.3‰ than their kamacite counterpart, thus calling into question the significance of bulk iron meteorite data. On three pallasites (Esquel, Marjalahti and Springwater) we measured a heavier iron isotope composition for the metal fractions compared to the coexisting olivines as previously observed on two other pallasites (Eagle Station and Imilac), but the range of δ 57Fe/54Fe differences (from 0.32‰ to 0.07‰) is larger than that originally found. Troilite from two pallasites appears to be even heavier than the metal fraction, whereas schreibersite is lighter than its olivine counterpart. There is thus a general tendency for minerals within a given rock to show a heavier Fe isotope composition as the coordination number of Fe increases, although troilite is an exception to this rule. Iron meteorites are classically considered as remnants of asteroid cores and pallasites as core–mantle interfaces. The simultaneous finding that the metal fractions of pallasites have a higher δ 57Fe/54Fe signature than the coexisting olivines, and that the iron meteorites are slightly heavier than chondrites could be taken as an indication that planetary core–mantle differentiation is accompanied by sizeable iron isotope fractionation. In this hypothesis, mass balance constraints imply that resultant planetary mantles should be isotopically lighter than the chondritic starting material. That is not observed, however, since all planetary mantles analyzed so far have δ 57Fe/54Fe values equivalent to or heavier than those of chondrites. It thus appears that the moderate temperature and pressure metal–silicate fractionation that occurred in pallasite and iron parent bodies is not readily transposable to planets as far as Fe isotopes are concerned. Instead, these mantle signatures could reflect equilibrium fractionation at a higher temperature, or the lack of a global core–mantle equilibration at the planetary scale. Overall, these new results show that the mass-dependent isotopic scatter observed among inner solar system bodies from the bulk-rock to the planetary scale (∼0.3‰ δ 57Fe/54Fe) is more restricted than previously thought. This likely confirms a homogenization process that occurred in the protoplanetary accretion disk, between refractory inclusion condensation and chondrule formation. [Copyright &y& Elsevier]

Abstract: Multicollector ICP-MS-based stable isotope procedures provide the capability to determine small variations in metal isotope composition of materials, but they are prone to substantial bias introduced by inadequate sample preparation. Such a “cryptic” bias is not necessarily identifiable from the measured isotope ratios. The analytical protocol for Fe isotope analyses of organic and inorganic materials described here identifies and avoids such pitfalls. In medium-mass resolution mode of the ThermoFinnigan Neptune MC-ICP-MS, a 1-ppm Fe solution with an uptake rate of 50–70μLmin−1 yielded 3×10−11 A on 56Fe for the ThermoFinnigan stable introduction system and 1.2–1.8×10−10 A for the ESI Apex-Q uptake system. Sensitivity was increased again 3–5-fold when using Finnigan X-cones instead of the standard H-cones. The combination of the ESI Apex-Q apparatus and X-cones allowed the determination of the isotope composition on as little as 50ng of Fe. Fe isotope compositions were corrected for mass bias with both the standard-sample bracketing (SSB) method, and by using the 65Cu/63Cu ratio of added synthetic copper (Cu-doping) as internal monitor of mass discrimination. Both methods provide identical results on high-purity Fe solutions of either synthetic or natural samples. We prefer the SSB method because of its shorter analysis time and more straightforward correction of instrumental mass bias compared to Cu-doping. Strong error correlations of the data are observed in three isotope diagrams. Thus, we suggest that the quality assessment in such diagrams should be performed with error ellipses rather than error bars. Reproducibility of δ56Fe, δ57Fe and δ58Fe values of natural samples alone is not a sufficient criterion for accuracy. A set of tests is lined out that identify cryptic matrix effects and ensure a reproducible level of quality control. Using these criteria and the SSB correction method, we determined the external reproducibilities for δ56Fe, δ57Fe and δ58Fe at the 95% confidence interval from 318 measurements of 95 natural samples to be 0.049, 0.071 and 0.28‰, respectively. [Copyright &y& Elsevier]

The Rainbow hydrothermal vent site, which is the largest known point source for dissolved Fe delivered to the deep North Atlantic ocean, has remained invariant in its Fe isotope composition over at least the past 16,000 years, based on analysis of metalliferous sediments beneath the plume. Because of the conservative behavior of Fe in the Rainbow plume, δ56Fe values of particles in the neutrally buoyant plume (-0.18±0.05‰) and underlying sediments (-0.19±0.05‰) are indistinguishable from the δ56Fe values of the high-temperature fluid sources (-0.23±0.04‰). Particles from the near-vent, buoyant stage of the plume, however, have higher δ56Fe values (+0.15‰ to +1.20‰) relative to the original vent fluid, consistent with fractionation during oxidation of Fe(II)aq to Fe(III)aq. Isotope compositions become invariant in the plume once all Fe(II)aq is fully oxidized, preserving the original composition of the vent fluid. The constant Fe isotope compositions of the vent fluids over time implies that changes in seawater Fe isotope composition of the North Atlantic ocean, as they are recorded in Fe–Mn crusts, requires changes in the relative fluxes of Fe to the ocean. [Copyright &y& Elsevier]

#x03B5;65Cu ([65Cu/63Cusample/65Cu/63CuNIST-976 -1]*10,000) in chalcopyrite from the 3 three igneous intrusions that make up the Grasberg Igneous Complex (GIC) and associated skarn deposits range from 0.2 to 13.4. Chalcopyrite grains from each intrusion have a specific range of #x03B5;65Cu that is isotopically heavier in each successive intrusion. These variations may be interpreted in two ways: (1) isotope fractionation during distillation of Cu from the underlying source and establishment of hydrothermal cells associated with each intrusion; (2) isotope fractionation as the ore ore-bearing fluid moved outward from a central core. Within each sample, the smaller disseminated chalcopyrite grains yield consistently lower #x03B5;65Cu values than larger disseminated grains, suggesting multiple mineralisation events.Chalcopyrite #x03B5;65Cu from the Pyrite Shell, a sulfide-rich zone that surrounds the GIC, completely overlaps those from the three intrusions: suggesting that the source of Cu was the same. In contrast, while some chalcopyrite #x03B5;65Cu values from the skarn deposits also overlap those from the GIC, a significant number are enriched in 65Cu. These data may indicate that Cu within the skarns was derived from multiple sources or from multiple mineralisation events.#x03B5;57Fe ([57Fe/54Fesample/57Fe/54FeIRMM-14 -1]*10,000) for pyrite and chalcopyrite range from -30.2 to 16.2. The #x03B5;57Fe ranges of these two minerals do not overlap, suggesting that pyrite preferentially incorporated heavy Fe isotopes. #x03B5;57Fe for GIC and skarn pyrite overlap, as do GIC and skarn chalcopyrite, suggesting genetic relationships. A large proportion of pyrite and chalcopyrite grains in the skarns also have low #x03B5;57Fe values that do not overlap the igneous sulfides. This, coupled with the presence of pyrite grains with extremely low #x03B5;57Fe values (#x03B5;57Fe=-25), suggests that mixing between sedimentary and igneous Fe occurred during skarn replacement mineralisation. The chalcopyrite and pyrite Fe isotope data from the Pyrite Shell do not overlap those from the GIC, which indicates that Cu and Fe in the chalcopyrite were derived from different sources: the Cu is igneous, whereas the Fe is probably a mixture between sedimentary and igneous sources.The Cu and Fe isotope data from the Grasberg porphyry and skarn sulfides show that transition metal isotope variations occur within magmatic–hydrothermal systems, and that transition metal isotopes can become an important tool for interpretation of hydrothermal processes. [Copyright &y& Elsevier]

Nanophase Fe metal grains (np-Fe°) are a product of space weathering, formed by processes related to meteorite impacts, and solar-wind sputtering on airless planetary bodies, such as the Moon. Iron isotopes of lunar soils are fractionated during these processes, and the np-Fe° in the finest (<10 μm), mature, size fractions of the soil become enriched in heavier isotopes by ∼0.3‰ in 56Fe/54Fe in comparison to the bulk rocks (0.03±0.05‰), from which the soil was formed. A positive correlation of δ56Fe values with the soil maturity index, IS/FeO, suggests that the high δ56Fe values reflect production of nanophase Fe metal that is produced by space weathering that occurs on airless planetary bodies. Furthermore, the enrichment of δ56Fe in the smallest size fraction of lunar soils supports a model for creation of np-Fe° through vapor deposition induced by micrometeorites, as well as that by solar-wind sputtering. [Copyright &y& Elsevier]

Over 100 high-precision Fe isotope analyses of rocks and minerals are now available, which constrain the range in δ56Fe values (per mil deviations in 56Fe/54Fe ratios) in nature from −2.50‰ to +1.5‰. Re-assessment of the range of δ56Fe values for igneous rocks, using new ultra-high-precision analytical methods discussed here, indicate that igneous Fe is isotopically homogeneous to ±0.05‰, which represents an unparalleled baseline with which to interpret Fe isotope variations in nature. All of the isotopic variability in nature lies in fluids, rocks, and minerals that formed at low temperature. Equilibrium (“abiotic”) isotopic fractionations at low temperatures may explain the range in δ56Fe values; experimental measurements indicate that there is a large isotopic fractionation between aqueous Fe(III) and Fe(II) (ΔFe(III)–Fe(II)=2.75‰). However, many of the natural samples that have been analyzed have an unquestionable biologic component to their genesis, and the range in δ56Fe values are also consistent with the experimentally measured isotopic fractionations produced by Fe-reducing bacteria.In this work, we touch on a number of aspects of Fe isotope geochemistry that bear on its application to geochemical problems in general, and biological cycling of metals in particular. We report on new state-of-the-art Fe isotope analytical procedures, which allow precisions of ±0.05‰ (56Fe/54Fe) on samples <300 ng in size. In addition, we discuss the implications of experimental work on Fe isotope fractionations during metabolic processing of Fe by bacteria and the need to take a “mechanistic” approach to understanding the pathways in which Fe isotopes may be uniquely fractionated by biology. Additionally, we discuss experimental methods, such as the use of enriched isotope tracers that are necessary to evaluate if experimental isotope exchange reactions are transient kinetic fractionations, equilibrium isotopic exchange reactions, or a combination of both, which can be caused by the complexities of multiple isotope exchange reactions taking place in an experimental system. [Copyright &y& Elsevier]