This work presents iron isotope data in the western equatorial Pacific. Marine aerosols and top core margin sediments display a slightly heavy Fe isotopic composition (δ56Fe) of 0.33 ± 0.11‰ (2SD) ...and 0.14 ± 0.07‰, respectively. Samples reflecting the influence of Papua New Guinea runoff (Sepik River and Rabaul volcano water) are characterized by crustal values. In seawater, Fe is mainly supplied in the particulate form and is found with a δ56Fe between −0.49 and 0.34 ± 0.07‰. The particulate Fe seems to be brought mainly by runoff and transported across continental shelves and slopes. Aerosols are suspected to enrich the surface Vitiaz Strait waters, while hydrothermal activity likely enriched New Ireland waters. Dissolved Fe isotopic ratios are found between −0.03 and 0.53 ± 0.07‰. They are almost systematically heavier than the corresponding particulate Fe, and the difference between the signature of both phases is similar for most samples with Δ56FeDFe – PFe = +0.27 ± 0.25‰ (2SD). This is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases. The dissolved phase being heavier than the particles suggests that the exchanges result in a net nonreductive release of dissolved Fe. This process seems to be locally significantly more intense than Fe reductive dissolution documented along reducing margins. It may therefore constitute a very significant iron source to the ocean, thereby influencing the actual estimation of the iron residence time and sinks. The underlying processes could also apply to other elements.
Key Points
Isotopic composition of dissolved and particulate Fe in seawaterIsotopic composition of Fe in marine aerosol, Sepik, and margin sedimentsNonreductive release would be an important source of dissolved Fe
The iron isotope composition was used to investigate dissimilatory iron reduction (DIR) processes in an iron-rich waterlogged paddy soil, the iron uptake strategies of plants and its translocation in ...the different parts of the rice plant along its growth. Fe concentration and isotope composition (δ56Fe) in irrigation water, precipitates from irrigation water, soil, pore water solution at different depths under the surface water, iron plaque on rice roots, rice roots, stems, leaves and grains were measured. Over the 8.5–10cm of the vertical profiles investigated, the iron pore water concentration (0.01 to 24.3mg·l−1) and δ56Fe (−0.80 to −3.40‰) varied over a large range. The significant linear co-variation between LnFe and δ56Fe suggests an apparent Rayleigh-type behavior of the DIR processes. An average net fractionation factor between the pore water and the soil substrate of Δ56Fe≈−1.15‰ was obtained, taking the average of all the δ56Fe values weighted by the amount of Fe for each sample. These results provide a robust field study confirmation of the conceptual model of Crosby et al. (2005, 2007) for interpreting the iron isotope fractionation observed during DIR, established from a series of laboratories experiments. In addition, the strong enrichment of heavy Fe isotope measured in the root relative to the soil solution suggest that the iron uptake by roots is more likely supplied by iron from plaque and not from the plant-available iron in the pore water. Opposite to what was previously observed for plants following strategy II for iron uptake from soils, an iron isotope fractionation factor of −0.9‰ was found from the roots to the rice grains, pointing to isotope fractionation during rice plant growth. All these features highlight the insights iron isotope composition provides into the biogeochemical Fe cycling in the soil-water-rice plant systems studied in nature.
δ56FeIRMM-14 and two standard error uncertainties (2S.E.) of all samples of soil-water-plant system of the investigated paddy field (empty and full symbols refer to 2012 and 2014 sampling events, respectively). The continental crust baseline (δ56FeIRMM-14=0.07±0.05‰; Poitrasson, 2006) is shown for reference. The blue and green arrows show respectively dissimilatory iron reduction (DIR) and iron uptake from soils processes. Display omitted
•Distinctive Fe isotope signatures associated within soil Fe redox cycling.•Entire Fe isotope distributions from soil pore water to rice grain in paddy field.•Significant linear co-variation between LnFe and δ56Fe in soil pore water•Field study confirmation of Fe isotope fractionation conceptual•model during DIR•Fe isotope fractionation factor of -0.9‰ in strategy II plant growth
This work demonstrates for the first time the feasibility of the measurement of the isotopic composition of dissolved iron in seawater for a typical open ocean Fe concentration range (0.1–1 nM). It ...also presents the first data of this kind. Iron is preconcentrated using a Nitriloacetic Acid Superflow resin and purified using an AG1x4 anion exchange resin. The isotopic ratios are measured with a MC‐ICPMS Neptune, coupled with a desolvator (Aridus II), using a 57Fe‐58Fe double spike mass bias correction. Measurement precision (0.13‰, 2SD) allows resolving small iron isotopic composition variations within the water column, in the Atlantic sector of the Southern Ocean (from δ57Fe = −0.19 to +0.32‰). Isotopically light iron found in the Upper Circumpolar Deep Water is hypothesized to result from organic matter remineralization. Shallow samples suggest that, if occurring, an iron isotopic fractionation during iron uptake by phytoplankton is characterized by a fractionation factor, such as: ∣Δ57Fe(plankton‐seawater)∣ < 0.48‰.
The uptake, transport, and toxicity mechanisms of zinc oxide (ZnO) engineered nanomaterials (ZnO-ENMs) in aquatic plants remain obscure. We investigated ZnO-ENM uptake and phytotoxicity in Phragmites ...australis by combining Zn stable isotopes and microanalysis. Plants were exposed to four ZnO materials: micron-size ZnO, nanoparticles (NPs) of <100 nm or <50 nm, and nanowires of 50 nm diameter at concentrations of 0–1000 mg l −1 . All ZnO materials reduced growth, chlorophyll content, photosynthetic efficiency, and transpiration and led to Zn precipitation outside the plasma membranes of root cells. Nanoparticles <50 nm released more Zn 2+ and were more toxic, thus causing greater Zn precipitation and accumulation in the roots and reducing Zn isotopic fractionation during Zn uptake. However, fractionation by the shoots was similar for all treatments and was consistent with Zn 2+ being the main form transported to the shoots. Stable Zn isotopes are useful to trace ZnO-ENM uptake and toxicity in plants.
The capabilities of an infrared (IR) Ti:sapphire femtosecond laser (approximate800 nm) to ablate and analyze geomaterials such as monazite, zircon and synthetic glass reference materials is ...evaluated, with emphasis on U/Pb ratio determinations useful for dating accessory minerals in rocks. We particularly discuss the influence of pulse duration (respectively 60, 200, 350, 500, 670, 830, 2000 and 3000 fs) on the internal precision (2 min ablation), reproducibility over two weeks and accuracy of quadrupole ICP-MS measurements. The best results for all these criteria are obtained when using the shortest pulse duration (60 fs). It was found that internal precision and reproducibility were improved by a factor of 3 and 4, respectively, from picosecond to 60 fs pulsewidths. Reproducibility at this pulse duration for U/Pb ratio determinations is of 2% RSD or better, depending on the material analyzed, and this ratio is accurate within this uncertainty. Lead isotopic ratios also benefit from the shortest pulsewidth. They are measured at 60 fs with a precision (0.5% RSD) approaching the limitations of quadrupole ICP-MS. Preliminary data were also obtained using the 3rd harmonic (approximate266 nm) of the Ti:sapphire fundamental wavelength and they are compared with the infrared mode. There seems to be no obvious analytical benefit to switch from IR to UV in the femtosecond laser ablation regime. Analyses of zircon 91500 with IR pulses led to better repeatability, around 0.9% (10 values, 1sigma), compared to 3% for the UV pulses. The accuracy appears to be comparable for the two wavelengths.
•This DFT-based study provides new and self-consistent Fe and Si isotope fractionation factors for the main magmatic minerals present in the crust.•Iron isotope fractionation factors between ...Fe2+-bearing minerals are not negligible even at magmatic temperatures.•For a given temperature and oxidation state, the local cationic environment of Fe or Si is the main factor influencing the isotopic properties of silicate minerals.•Fractional crystallization is a viable way to explain the heavy iron isotope signature of the most evolved lavas.
In order to elucidate the processes involved in iron and silicon isotopes partitioning during magmatic differentiation, it is essential to know the precise value of equilibrium fractionation factors between the main minerals present in the evolving silicic melts. In this study, we performed first-principles calculations based on the density functional theory to determine the equilibrium iron and silicon isotopes fractionation factors between eleven relevant silicate or oxide minerals in the context of magmatic differentiation, namely: aegirine, hedenbergite, augite, diopside, enstatite, fayalite, hortonolite, Fe-rich and Fe-free forsterites, magnetite and ulvospinel. Results show that Fe2+-bearing silicate minerals display significant differences in iron isotope fractionation factors that cannot be neglected, even at high temperature (1000 °C). Various physical and chemical parameters control the iron isotopic fractionation of silicate minerals. However, the main parameter, after temperature and the iron oxidation state, is the nature and number of iron second neighbors (i.e. the local chemical composition around Fe atoms). This conclusion is also valid for silicon isotopes. In the investigated nesosilicates and inosilicates, silicon isotope reduced partition function ratios (also called β-factors) show no correlation with the average Si-O bond length, which remains almost constant, but Si β-factors are correlated with the local chemical composition of the minerals. Fractional crystallization is one of the mechanisms, which could explain the evolution of iron isotopic compositions during magmatic differentiation. Using the present theoretical set of equilibrium fractionation factors allows us to assess the impact of inter-mineral isotopic fractionations, and shows that pyroxene appears to be the main mineral phase driving the isotopic evolution to a heavier signature in the most evolved lavas.
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 ln
57Fe
/
54Fe and ln
61Ni
/
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 SiO
2 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 SiO
2 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.
The difference in the mean Fe isotope composition of samples from the Earth, Moon, Mars and Vesta has been recently interpreted as tracking contrasted planetary accretion mechanisms F. Poitrasson, ...A.N. Halliday, D.C. Lee, S. Levasseur, N. Teutsch, Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms, Earth Planet. Sci. Lett. 223 (2004) 253–266. Using newly produced Fe isotopic data on terrestrial and lunar samples, pallasites, eucrites and Martian meteorites, Weyer et al. S. Weyer, A.D. Anbar, G.P. Brey, C. Munker, K. Mezger, A.B. Woodland, Iron isotope fractionation during planetary differentiation, Earth Planet. Sci. Lett. 240 (2005) 251–264 reinterpreted these data as fingerprinting planetary differentiation. In particular, these authors suggested that partial melting in the terrestrial and lunar mantles produced melts isotopically heavy. It is shown here that the inference of Weyer et al. S. Weyer, A.D. Anbar, G.P. Brey, C. Munker, K. Mezger, A.B. Woodland, Iron isotope fractionation during planetary differentiation, Earth Planet. Sci. Lett. 240 (2005) 251–264 is strongly biased by the sampling approach taken. Notably, these authors used olivine in place of the host bulk peridotites
δ
57Fe signatures despite this mineral has been shown to be frequently isotopically lighter than coexisting phases, and they analyzed lunar samples heavily affected chemically by the meteoritic bombardment, a process known to alter Fe isotope signatures. Their pallasite metal–silicate fractionation data are also likely biased by the approach adopted to estimate the iron isotope composition of the different mineral phases. In fact, their conclusion of Fe isotopic fractionation during basalt extraction from planetary mantles is invalidated by the observation that basaltic shergottites and eucrites have
δ
57Fe indistinguishable from those of chondrites. Therefore, the heavier Fe isotopic composition of the Moon relative to the Earth, itself heavier than most chondrites and achondrites remains best explained by loss of light iron isotopes during the high temperature event accompanying the interplanetary impact that led to the formation of the Moon F. Poitrasson, A.N. Halliday, D.C. Lee, S. Levasseur, N. Teutsch, Iron isotope differences between Earth, Moon, Mars and Vesta as possible records of contrasted accretion mechanisms, Earth Planet. Sci. Lett. 223 (2004) 253–266., F. Poitrasson, S. Levasseur, N. Teutsch, Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core–mantle differentiation of terrestrial planets, Earth Planet. Sci. Lett. 234 (2005) 151–164.
With the aim to better understand the cause of the iron isotope heterogeneity of mantle-derived bulk peridotites, we compared the petrological, geochemical and iron isotope composition of four ...xenolith suites from different geodynamic settings; sub-arc mantle (Patagonia); subcontinental lithospheric mantle (Cameroon), oceanic mantle (Kerguelen) and cratonic mantle (South Africa). Although correlations were not easy to obtain and remain scattered because these rocks record successive geological events, those found between δ
57
Fe, Mg#, some major and trace element contents of rocks and minerals highlight the processes responsible for the Fe isotope heterogeneity. While partial melting processes only account for moderate Fe isotope variations in the mantle (<0.2 ‰, with bulk rock values yielding a range of δ
57
Fe ± 0.1 ‰ relative to IRMM-14), the main cause of Fe isotope heterogeneity is metasomatism (>0.9 ‰). The kinetic nature of rapid metasomatic exchanges between low viscosity melts/fluids and their wall-rocks peridotite in the mantle is the likely explanation for this large range. There are a variety of responses of Fe isotope signatures depending on the nature of the metasomatic processes, allowing for a more detailed study of metasomatism in the mantle with Fe isotopes. The current database on the iron isotope composition of peridotite xenoliths and mafic eruptive rocks highlights that most basalts have their main source deeper than the lithospheric mantle. Finally, it is concluded that due to a complex geological history, Fe isotope compositions of mantle xenoliths are too scattered to define a mean isotopic composition with enough accuracy to assess whether the bulk silicate Earth has a mean δ
57
Fe that is chondritic, or if it is ~0.1 ‰ above chondrites as initially proposed.
The direct determination of silicate melts iron and silicon isotopes signature remains a major challenge of high-temperature isotope geochemistry. For this reason, melts are often approximated by ...silicate glasses. Calculation of precise equilibrium Si and Fe isotopes fractionation factors between minerals and melt would indeed allow us to distinguish equilibrium fractionation from diffusion-driven kinetic fractionation involved in the iron and silicon isotopes signatures of Earth and other planets. In this study, we use for the first time, first-principles molecular dynamics based on density functional theory to determine iron and silicon isotope compositions of different silicate melts, namely: iron-rich basalt, iron-depleted basalt, basanite, trachyte and phonolite. The 57Fe/54Fe reduced partition function ratios (β-factors) of the different melts span over a 1.1 ‰ range at 1000 Kelvin (K) while 30Si/28Si β-factors are much less influenced by the melt composition with a 0.5 ‰ fractionation range at the same temperature. The main parameter controlling iron isotope fractionation in silicate melts having similar iron oxidation state is, after temperature, the average Fe-O bond length. The chemical environment around iron (e.g. Fe-Fe distances) is suggested to contribute to Fe isotope fractionation as well. Silicon isotopes fractionation seems also affected, but to a lesser extent, by its local chemical composition with decreasing Si-Fe distances leading to slightly higher Si β-factor in the melt. From these melts Fe and Si β-factors, a new set of equilibrium fractionation factors between a variety of minerals and melts has been calculated. These new Δ57Femin-melt and Δ30Simin-melt sets allow us to discuss whether processes such as fractional crystallization, partial melting and diffusion could be responsible for the documented Fe and Si isotopes variations in igneous rocks. Our results suggest that: 1) fractional crystallization may explain at least part of the Fe and Si isotopic evolution during magmatic differentiation, for values up to δ57Fe = 0.65 ‰ and δ30Si = -0.1 ‰, respectively, 2) partial melting of the upper mantle can produce the Mid-Ocean Ridge Basalts (MORB) iron isotopes signature. Finally, we calculated that olivine at equilibrium with a basaltic melt could display an iron isotope composition down to −0.1 ‰ for δ57Fe. Hence, the lower isotopic compositions (δ57Fe < -0.1 ‰) observed in natural olivines are most likely due to diffusion-driven kinetic fractionation.
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