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
Multi-collector inductively coupled plasma mass spectrometers (MC-ICPMS) are widely used for Fe isotope measurements. The latter may be perturbed by interferences (notably from Cr and Ni) and matrix ...effects (notably from major elements), caused by elements remaining in the samples after purification. We quantified some of these perturbations and our ability to correct them whenever possible, using Thermo Neptune and Neptune Plus MC-ICPMS with a
57-58
Fe double-spike mass bias correction.
54
Cr and
58
Ni isobaric interference corrections were found to be extremely efficient up to Cr/Fe=0.12 and Ni/Fe=0.04 (g/g natural Fe). Matrix effects were found negligible up to at least Na/Fe=175, Mg/Fe=10, K/Fe=1.5, and Mo/Fe=75 (g/g natural Fe).
28
Si
2
+
interference was found negligible up to Si/Fe=50. Finally, we found that calcium and aluminum could cause significant interferences (e.g.,
40
Ca
16
O and
27
Al
2
+
), for Ca/Fe ≥ 2.5 and Al/Fe ≥ 2.5. The perturbation intensity relative to the Ca/Fe ratio was found dependent on the measurement conditions (plateau width). While working with samples with potentially high calcium or aluminum contents (such as calcite minerals or tests, bones and teeth, or marine samples and crustal rocks), we recommend to carefully take into account Ca and Al while tuning the instrument and checking the measurement accuracy with isotopic standards (i.e., doping the isotopic standard with Ca and Al levels comparable to those of the samples).
The natural river water certified reference material SLRS‐5 (NRC‐CNRC) was routinely analysed in this study for major and trace elements by ten French laboratories. Most of the measurements were made ...using ICP‐MS. Because no certified values are assigned by NRC‐CNRC for silicon and 35 trace element concentrations (rare earth elements, Ag, B, Bi, Cs, Ga, Ge, Li, Nb, P, Rb, Rh, Re, S, Sc, Sn, Th, Ti, Tl, W, Y and Zr), or for isotopic ratios, we provide a compilation of the concentrations and related uncertainties obtained by the participating laboratories. Strontium isotopic ratios are also given.
Le matériau de référence certifié d'eau de rivière naturelle SLRS‐5 (NRC‐CNRC) est analysé régulièrement comme contrôle qualité par dix laboratoires français étudiant les éléments majeurs et en trace dans les solutions naturelles. La plupart des mesures sont réalisées par ICP‐MS. Le silicium et 35 éléments en trace (terres rares, Ag, B, Bi, Cs, Ga, Ge, Li, Nb, P, Rb, Rh, Re, S, Sc, Sn, Th, Ti, Tl, W, Y et Zr) ne sont pas certifiés par NRC‐CNRC. Aucun rapport isotopique n'est disponible. Nous proposons, pour ces éléments, des valeurs moyennes et leurs incertitudes associées obtenues par les différents laboratoires participants. Le rapport isotopique de Sr est aussi mesuré.
A new method for the in situ measurement of stable Si isotope ratio using UV-femtosecond laser ablation connected to a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has ...been established. The use of medium mass resolution mode (with a resolving power
m/Δ
m
=
8000) permits to resolve spectral interferences on
28Si,
29Si and
30Si allowing for determination of the
29Si/
28Si and
30Si/
28Si ratios with high accuracy and precision on wide, interference-free plateaus. A three-isotope plot demonstrated that interferences, if existing, are negligible for Si isotope ratios. The laser spot size is 35 μm and measurements are made using square rasters of 150 μm side length. Different types of elemental Si, Si-oxide and silicate matrices have been analysed by laser ablation using the international standard NBS28 as the bracketing standard. Thus
δ
29Si and
δ
30Si have been determined for the silicon isotope standard IRMM-017 (
δ
30Si
±
2 S.D.
=
−
1.26
±
0.24‰;
n
=
89) and BigBatch (
δ
30Si
=
−
10.55
±
0.42‰;
n
=
15), as well as San Carlos Olivine (
δ
30Si
=
−
0.81
±
0.19‰;
n
=
14), Caltech Rose Quartz (
δ
30Si
=
0.10
±
0.13‰;
n
=
14) sponge needles (
Stylocordyla borealis;
δ
30Si
=
−
2.19
±
0.32‰;
n
=
14) and JER-diopside glass (
δ
30Si
=
0
±
0.09‰;
n
=
14) samples. The overall repeatability achievable is 0.15‰ (2 S.D.) on
δ
29Si and 0.24‰ (2 S.D.) on
δ
30Si. The silicon isotope standard IRMM-018 has also been measured and was confirmed to be heterogeneous. The two isotope ratios follow an equilibrium mass-dependent fractionation law which can be represented as
δ
30Si
=
1.93
×
δ
29Si. Published extractions methods have been used to wet-chemically purify Si from the JER-diopside glass. The Si extracted was presented as Si-gel and ablated like a solid. The results show excellent agreement with in situ measurements of this glass that confirms that this technique can be employed to a wide variety of matrices, including Si purified from solution. The new technique presents a viable alternative to solution MC-ICP-MS for bulk measurements and the most precise technique so far for in situ measurement of Si isotope ratios.
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.
Improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface. The rate of magma transfer can ...in favorable cases be estimated from radioactive disequilibria between nuclides of the
238U series. Young lavas from southern Chile, in which
238U–
230Th disequilibria have been measured Sigmarsson et al., Nature 346 (1990) 163–165; Sigmarsson et al., Nature 394 (1998) 566–569, were analyzed for
226Ra abundances. The disequilibrium between
226Ra and
230Th in these lavas is found to correlate with
238U–
230Th disequilibria and
10Be/Be Morris et al., Nature 344 (1990) 31–36. These correlations strongly suggest that the excess of
226Ra over
230Th is due to the addition of a slab-derived fluid to the magma source, since Ra and U are fluid-mobile elements and the cosmogenic
10Be is most likely derived from the subducting Nazca plate beneath the Andes. The largest slab signature is observed in the lavas of Villarrica volcano, which is the most active volcano in South America. A model for subduction fluxing is discussed, in which the U series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle, but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge. Repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the
10Be/Be–B/Be correlation for different arcs Morris et al., Nature 344 (1990) 31–36. The fact that excesses of
226Ra and
238U over
230Th are correlated indicates that linear arrays on the (
230Th/
232Th)–(
238U/
232Th) diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts. The
226Ra–
230Th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer, or less than 8000 years. This disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m/year. Finally, the correlations of (
226Ra
/230Th) with (
238U/
232Th) and
10Be/Be in Andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied.
Par le biais d'analyses précises des déséquilibres radioactifs des séries de l'uranium, permises par des améliorations effectuées aussi bien au niveau de l'acquisition des données par spectrométrie ...de masse qu'au niveau de la qualité de la chimie, deux grandes questions ont été abordées et des réponses au sujet des périodes de glaciation en Islande et la pétrogenèse des laves Andines ont été proposées.
Deux méthodes ont été utilisées pour permettre la datation de coulées de laves de la péninsule de Reykjanes et de l'île d'Heimaey, en Islande, directement en rapport avec les périodes de déglaciation ou de réchauffement autour de la dernière période glaciaire. Une méthode originale basée sur l'utilisation des veines de ségrégation présentes dans les coulées de laves a été mise au point. Cet objet géologique particulier permet de s'affranchir des xénocristaux lors de l'application de la méthode de l'isochrone interne dans le système 238U-230Th, du fait même de son mode de formation. Grâce à cette méthode et à son utilisation sur une coulée de lave de la région de Reykjanes et une coulée au nord de l'île d'Heimaey des contraintes sont apportées à la fois sur la géologie quaternaire de la région de Reykjavik et sur l'âge de la fin de la glaciation de Würm sur l'île d'Heimaey. En parallèle, l'homogénéité des laves historiques et holocènes de la péninsule de Reykjanes, en ce qui concerne les rapports (230Th/232Th) et Th/U, a permis de dériver dans le temps le rapport (230Th/232Th) pour déterminer l'âge de coulées datant de la période glaciaire. Ainsi il a été confirmé que l'excursion magnétique enregistrée dans la coulée de Skálamælifell est très probablement la même que celle de Laschamp-Olby et qu'à cette époque (environ 48.000 ans) le glacier, recouvrait la péninsule de Reykjanes. Par cette même méthode l'étendue du glacier a également été contrainte par la datation d'un bouclier de picrite aux environs de 22.000 ans, âge, pourtant en pleine glaciation, où la péninsule devait être libre de toute chape de glace.
Dans un deuxième temps, la pétrogenèse des laves de l'arc Andin a été contrainte dans le temps et l'espace. Des excès quasiment systématiques de 226Ra par rapport à 230Th dans les laves des zones volcaniques australes, sud et nord des Andes (AVZ, SVZ et NVZ) nous renseignent sur le temps de remontée des magmas après leur formation qui est nécessairement rapide (moins de 8.000 ans pour conserver ces déséquilibres). Un lien a également pu être fait entre l'activité éruptive et les excès de 238U (et dans une moindre mesure 226Ra) par rapport à 230Th. Cette observation suggère que les volcans les plus actifs (Villarrica et Llaima pour la SVZ) sont ceux dont la source est la plus enrichie en fluides, provenant de la plaque subduite et des sédiments qui la couvrent, lors de la fusion partielle du coin de manteau et que les déséquilibres nous renseignent sur le contexte géodynamique à l'échelle régionale. Enfin l'étude plus fine de volcans particuliers (Nevado de Longavi (SVZ) et Guagua Pichincha (NVZ)), suggère que les déséquilibres radioactifs des séries de l'uranium enregistrent l'évolution d'un volcan. Ainsi l'histoire complexe du Nevado de Longavi peut être appréhendée par cette approche et les signaux particuliers de ce volcan peuvent être traduits en terme de variation de fusion partielle de la source, cristallisation fractionnée ou contamination crustale. Dans le cas du Guagua Pichincha c'est la source du volcanisme adakitique qui pourra être discutée grâce à l'étude des excès de 238U et 226Ra par rapport à 230Th et un modèle de formation d'adakites par fusion partielle du coin de manteau hydraté puis cristallisation de grenat à haute pression sera proposé.
Reproduction de : Thèse de doctorat : Géochimie : Clermont-Ferrand 2 : 2005.
Thèse avec annexes et publication. Titre provenant de l'écran-titre. Bibliogr. p. 181-195.
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