Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10−8.2 to 10−9.9 atm and for run durations from 0.5 to 1 h. Both ...kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO2 dependent and decreased from 17 to 1 hour, as conditions became more reducing.
The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (δNi down to −0.98‰.amu−1 relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with ΔNiMetal-Silicate = 0.02 ± 0.04‰.amu−1.
Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically.
Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a ΔNiMetal-Silicate close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion.
The iron isotope compositions of Shergotty–Nakhla–Chassigny (SNC) meteorites thought to come from Mars, eucrites and diogenites assumed to sample asteroid 4 Vesta, and rocks from the Moon and Earth ...have been measured using high precision plasma source mass spectrometry. The means of eight samples from Mars and nine samples from Vesta are within error identical despite a range of rock types. They are lighter by ∼0.1‰ in
δ
57Fe/
54Fe compared to the average of 13 terrestrial mantle-derived rocks. The latter value is identical within uncertainty with a previously published mean of 46 igneous rocks from the Earth. The average for 14 lunar basalts and highland plutonic rocks covering a broad spectrum of major element composition is heavier by ∼0.1‰ in
δ
57Fe/
54Fe relative to our estimate for the Earth's mantle, and therefore ∼0.2‰ heavier than the eucrites, diogenites and SNC meteorites. However, the data scatter somewhat and the Apollo 15 green glass and Apollo 17 orange glass are identical to samples from Mars and Vesta. There is no clear relationship between petrological characteristics and Fe isotope composition despite a wide spectrum of samples. Instead, contrasted planetary isotopic signatures are clearly resolved statistically. After evaluating alternative scenario, it appears that the most plausible explanation for the heavier Fe in the Earth and Moon is that both objects grew via processes that involved partial vaporisation leading to kinetic iron isotope fractionation followed by minor loss. This is consistent with the theory in which the Moon is thought to have originated from a giant impact between the proto-Earth and another planet. Combined with numerical simulations, Fe isotope data can offer the potential to provide constraints on the processes that occurred in planetary accretion.
Allanite is, with monazite, the main repository for light rare earth elements (REE) in the continental crust and can be used in U-Th-Pb geochronology. This mineral has been shown to be prone to ...alteration. The geochemical exchanges occurring between allanite and hydrothermal fluids were explored using backscattered scanning electron microscopy, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry (LAICP-MS).
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 Zn stable isotope composition of plants demonstrates that ZnO engineered nanomaterials dissolve before their uptake and accumulation by the roots (brightest inclusions in root cortex).
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•Geochemistry and C, O, Fe, Cr isotopes of the Cauê BIF, Quadrilátero Ferrífero.•Negative δ13C and positive δ56Fe suggest microbial Dissimilatory Iron Reduction (DIR).•Slightly ...positive δ53Cr and high U/Th suggest locally oxygenated conditions.•Lack of Ce cycling might suggest Cr(VI) and U(VI) were mobilized in microenvironments.•DIR was a common and widespread process in the Archean/Paleoproterozoic transition.
The Banded Iron Formation (BIFs) of the Cauê Formation, Quadrilátero Ferrífero, Brazil, are part of the great volume of BIFs deposited worldwide around 2.45Ga. Samples of carbonate (ankerite/dolomite) BIF from this unit were collected from a drill core ca. 600m deep in the Alegria region. Geochemical data suggest low clastic contamination (Y/Ho=37.7) and seawater-like signatures, with positive La and Y anomalies, no true Ce anomalies and mean Eu/Eu∗=1.62. U/Th shows a sharp increasing-downwards pattern (up to ca. 15) below the 535m mark. Values of δ13C and δ18O are all negative, reaching a minimum of −10.0‰ and −24.6‰, respectively, in the 540–560m interval. Values of δ56Fe are mostly positive, between 0.5 and 1.3‰; the higher values are attained in the same interval. Values of authigenic δ53Cr range between 0.01‰ and 0.26‰, with an increasing-downwards pattern which mirrors the U/Th variation. A possible explanation for both the low δ13C and the high δ56Fe is the activity of microorganisms in the dissimilatory iron reduction (DIR) of the precursor ferric oxides. Thus, we put forward a model for deposition of the Cauê BIFs that starts with the generation of Fe(II)aq through hydrothermal input to the deeper basin, which upon reaching a chemocline in the platform border, oxidizes to Fe(III) and settles as iron (oxy)hydroxides to the basin floor. When reaching the sediment layer, Fe(III) is then reduced by microbial activity to generate magnetite and iron-rich carbonates with negative δ13C and positive δ56Fe. An important issue is how U(VI) and Cr(VI) were mobilized to the basin to form the positive U/Th and δ53Cr excursions. These could reflect oxidative continental weathering, but an alternative possibility is that those elements became available through local oxygenation in continental microenvironments, which would better explain the lack of Ce redox cycling. In either case, our results, together with a growing body of data for BIF sequences of similar age around the world (e.g. Kuruman, South Africa and Hamersley, Australia), suggest that microbial-mediated DIR was a common and widespread mechanism for the genesis of BIFs at the Archean/Paleoproterozoic transition.
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.
We performed, over a 598 day sampling period, chemical analyses of commercially bottled spring water (namely Alet water) rising in the vicinity of Saint Paul de Fenouillet (French eastern Pyrenees) ...where a ML = 5.2 earthquake occurred. These data display a chloride ion anomaly starting 5 days prior to the quake and lasting 10–13 days. The anomaly is characterized by Cl− concentrations 36% above background values. This precursory chemical change is attributed to a pre-seismic strain change, which induced mixing of geochemically different aquifers. Chloride-rich groundwaters are known to rise close (10 km) to the Alet spring, and mixing calculations suggest that a limited inflow of similar waters within the Alet hydrologic system is a viable explanation for the chloride ion anomaly. This result confirms that mineral springs are promising sites for the search of earthquake geochemical precursors.
We compared the analytical performance of ultraviolet femtosecond and nanosecond laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The benefit of ultrafast lasers was evaluated ...regarding thermal-induced chemical fractionation, that is otherwise well known to limit LA-ICPMS. Both lasers had a Gaussian beam energy profile and were tested using the same ablation system and ICPMS analyzer. Resulting crater morphologies and analytical signals showed more straightforward femtosecond laser ablation processes, with minimal thermal effects. Despite a less stable energy output, the ultrafast laser yielded elemental (Pb/U, Pb/Th) and Pb isotopic ratios that were more precise, repeatable, and accurate, even when compared to the best analytical conditions for the nanosecond laser. Measurements on NIST glasses, monazites, and zircon also showed that femtosecond LA-ICPMS calibration was less matrix-matched dependent and therefore more versatile.
This work demonstrates the feasibility of the measurement of the isotopic composition of dissolved iron in seawater for an iron concentration range, 0.05−1 nmol L−1, allowing measurements in most ...oceanic waters, including Fe depleted waters of high nutrient low chlorophyll areas. It presents a detailed description of our previously published protocol, with significant improvements on detection limit and blank contribution. Iron is preconcentrated using a nitriloacetic acid superflow resin and purified using an AG 1-×4 anion exchange resin. The isotopic ratios are measured with a multicollector-inductively coupled plasma mass spectrometer (MC-ICPMS) Neptune, coupled with a desolvator (Aridus II or Apex-Q), using a 57Fe−58Fe double spike mass bias correction. A Monte Carlo test shows that optimum precision is obtained for a double spike composed of approximately 50% 57Fe and 50% 58Fe and a sample to double spike quantity ratio of approximately 1. Total procedural yield is 91 ± 25% (2SD, n = 55) for sample sizes from 20 to 2 L. The procedural blank ranges from 1.4 to 1.1 ng, for sample sizes ranging from 20 to 2 L, respectively, which, converted into Fe concentrations, corresponds to blank contributions of 0.001 and 0.010 nmol L−1, respectively. Measurement precision determined from replicate measurements of seawater samples and standard solutions is 0.08‰ (δ56Fe, 2SD). The precision is sufficient to clearly detect and quantify isotopic variations in the oceans, which so far have been observed to span 2.5‰ and thus opens new perspectives to elucidate the oceanic iron cycle.
This paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration, namely sericitization, chloritization, and greisenization. The samples ...were taken from three Paleozoic granites: the St. Nectaire granite (Massif Central, France), the Carnmenellis granite (Cornwall, England), and the Skiddaw granite (Lake District, England). Fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340°C and salinities from 3 to 18 wt% NaCl equivalent. Possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy (BSE-SEM), electron microprobe, laser Raman spectroscopy and laser ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) include cationic substitutions, monoclinic to hexagonal structure transition accompanied by chemical exchanges, selective Th removal, dissolution-reprecipitation, and dissolution with replacement by a different mineral. These results show that, despite the compositional and crystallographic simplicity of monazite, it exhibits varied chemical responses to different mineral-fluid interactions. Elemental and isotopic measurements by LA-ICP-MS for the unaltered monazites yield statistically significant
232Th-
208Pb and
238U-
206Pb magmatic crystallization ages. In some cases it was possible to give a reasonable estimate of the age of mineral-fluid interaction using the altered parts of the monazites or newly precipitated crystals. For other minerals,
232Th-
208Pb and
238U-
206Pb systematics were strongly disturbed by variable inputs and/or depletions of U, Th, and Pb. The data suggest that monazite-like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies, especially if the fluids are oxidizing or tend to dissolve monazite.
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