Géochimie de l'arc du Vanuatu (évolution spatio-temporelle des édifices volcaniques et des sources mantelliques.)

L archipel volcanique actif du Vanuatu s édifie au coeur du pacifique sud-ouest au niveau de la frontière convergente des plaques australienne et pacifique. Je présente ici une nouvelle étude géochimique des laves du Vanuatu à partir de la détermination des compositions en éléments majeurs et traces, et des compositions isotopiques (Sr-Nd-Hf-Pb) d une centaine d échantillons de laves (< 2 Ma).L étude des magmas les plus primitifs a permis de mettre en évidence la variation de composition des sources mantelliques le long de l arc et d individualiser 3 segments : "central", dans la zone de collision de la ride D Entrecasteaux, "sud" en face du bassin Nord Fidjien, et "extrême sud" en face du bassin Sud Fidjien. La composition des roches des différentes structures subductées influence celles des laves des volcans adjacents via le composant de subduction, sous forme de fluides et de produits de fusion. Les laves des îles situées en face de la ride D Entrecasteaux sont issues d un manteau enrichi ("type-MORB indien"), différent de celui échantillonné par les autres laves ("type-MORB pacifique"). Cette ride apporte probablement en subduction un composant ancien, pouvant être assimilé à un fragment de croûte inférieure.L étude locale de certaines îles a permis de caractériser la différenciation des laves par cristallisation fractionnée, d identifier des processus d assimilation crustale, et de révéler la présence de magma provenant de portions de manteau distinctes, ayant subi un métasomatisme différent.Ces travaux révèlent une extrême hétérogénéité du manteau sous l arc du Vanuatu, témoignant de la complexité des processus géologiques impliqués au niveau de cette zone de subduction.The Vanuatu island arc is located in the SW Pacific at the convergent boundary between the Australian and Pacific plates. I present here a new geochemical study of the Vanuatu lavas based on major and trace element as well as isotopic (Sr-Nd-Hf-Pb) analyses of approximately one hundred lava samples (< 2 My).Examination of the most primitive magmas reveals the variation of the mantle source composition along the arc and demarcates three segments: a "central" segment in front the d Entrecasteaux ridge, a "south" segment in front of the North Fiji Basin, and a "far south" segment in front of the South Fiji Basin. The composition of the different subducted basins highly influenced those of the adjacent volcanoes via mass transfer of aqueous fluid and melt subduction components. Lavas from islands facing the D'Entrecasteaux Ridge come from an enriched mantle (Indian MORB-like), distinct from that sampled at other islands (Pacific MORB-like). Additionally, this ridge likely carries with it an ancient component that may reflect entrainment of lower crust material.Localized study of some volcanic islands reveals processes related to the differentiation of lavas by fractional crystallization, processes related to crustal assimilation, and indicates the presence of magma coming from different portions of mantle with slightly different metasomatic histories.This body of work highlights extreme mantle heterogeneity under Vanuatu islands arc and testifies to the complex nature of its associated geological processes.BREST-SCD-Bib. electronique (290199901) / SudocMARSEILLE-MAP (130552318) / SudocSudocFranceF

The response of Li and Mg isotopes to rain events in a highly-weathered catchment

Storms are responsible for up to ~50% of total annual rainfall on tropical islands and result in rapid increases in discharge from rivers. Storm events are, however, notoriously under-sampled and their effects on weathering rates and processes are poorly constrained. To address this, we have undertaken high-frequency sampling of Quiock Creek catchment, a Critical Zone Observatory located in Guadeloupe, over a period of 21 days, encompassing several storm events. Chemical and isotopic (Li and Mg) analyses of different critical zone reservoirs (throughfall, soil pore water, groundwater and river water) were used to assess the interactions between rock, water and secondary minerals. The Li concentrations and δ7Li values of these different reservoirs range from 14 to 95 nmol/kg and 1.8 to 16.8‰, respectively. After several rain events, the average δ7Li value (13.3‰) of soil solutions from the lower part of the soil profile (>~150 cm below the surface) was unchanged, whereas in the upper part of the profile δ7Li values increased by ~2–4‰ due to increased contribution from throughfall. By contrast, the δ26Mg value of soil waters in the upper part of the soil profile were not significantly affected by the rain events with an average value of −0.90‰. The δ26Mg values of the different fluid reservoirs were generally close to the value of throughfall (~−0.90‰), but higher δ26Mg values (up to −0.58‰) were measured in the deeper parts of the soil profile, whereas groundwaters that have a long residence time had lower δ26Mg values (down to −1.48‰). These higher and lower values are attributed to, respectively, adsorption/desorption of light Mg isotopes on/from the surface of clay minerals. The δ7Li value of the river waters was ~9.3‰, with a Li concentration of 60 μmol/kg, but during a storm these values decreased to, respectively, 7.8‰ and 40 μmol/kg. This change in δ7Li is consistent with an increased contribution of Li from the soil solution. Thus, even in highly weathered catchments, changes in hydrological conditions can have a significant impact on weathering processes and therefore the composition of river waters delivered to the ocean

Geochemical study of the Vanuatu lavas

L’archipel volcanique actif du Vanuatu s’édifie au coeur du pacifique sud-ouest au niveau de la frontière convergente des plaques australienne et pacifique. Je présente ici une nouvelle étude géochimique des laves du Vanuatu à partir de la détermination des compositions en éléments majeurs et traces, et des compositions isotopiques (Sr-Nd-Hf-Pb) d’une centaine d’échantillons de laves (< 2 Ma).L’étude des magmas les plus primitifs a permis de mettre en évidence la variation de composition des sources mantelliques le long de l’arc et d’individualiser 3 segments : "central", dans la zone de collision de la ride D’Entrecasteaux, "sud" en face du bassin Nord Fidjien, et "extrême sud" en face du bassin Sud Fidjien. La composition des roches des différentes structures subductées influence celles des laves des volcans adjacents via le composant de subduction, sous forme de fluides et de produits de fusion. Les laves des îles situées en face de la ride D’Entrecasteaux sont issues d’un manteau enrichi ("type-MORB indien"), différent de celui échantillonné par les autres laves ("type-MORB pacifique"). Cette ride apporte probablement en subduction un composant ancien, pouvant être assimilé à un fragment de croûte inférieure.L’étude locale de certaines îles a permis de caractériser la différenciation des laves par cristallisation fractionnée, d’identifier des processus d’assimilation crustale, et de révéler la présence de magma provenant de portions de manteau distinctes, ayant subi un métasomatisme différent.Ces travaux révèlent une extrême hétérogénéité du manteau sous l’arc du Vanuatu, témoignant de la complexité des processus géologiques impliqués au niveau de cette zone de subduction.The Vanuatu island arc is located in the SW Pacific at the convergent boundary between the Australian and Pacific plates. I present here a new geochemical study of the Vanuatu lavas based on major and trace element as well as isotopic (Sr-Nd-Hf-Pb) analyses of approximately one hundred lava samples (< 2 My).Examination of the most primitive magmas reveals the variation of the mantle source composition along the arc and demarcates three segments: a "central" segment in front the d’Entrecasteaux ridge, a "south" segment in front of the North Fiji Basin, and a "far south" segment in front of the South Fiji Basin. The composition of the different subducted basins highly influenced those of the adjacent volcanoes via mass transfer of aqueous fluid and melt subduction components. Lavas from islands facing the D'Entrecasteaux Ridge come from an enriched mantle (Indian MORB-like), distinct from that sampled at other islands (Pacific MORB-like). Additionally, this ridge likely carries with it an ancient component that may reflect entrainment of lower crust material.Localized study of some volcanic islands reveals processes related to the differentiation of lavas by fractional crystallization, processes related to crustal assimilation, and indicates the presence of magma coming from different portions of mantle with slightly different metasomatic histories.This body of work highlights extreme mantle heterogeneity under Vanuatu islands arc and testifies to the complex nature of its associated geological processes

Géochimie de l'arc du Vanuatu : évolution spatio-temporelle des édifices volcaniques et des sources mantelliques.

The Vanuatu island arc is located in the SW Pacific at the convergent boundary between the Australian and Pacific plates. I present here a new geochemical study of the Vanuatu lavas based on major and trace element as well as isotopic (Sr-Nd-Hf-Pb) analyses of approximately one hundred lava samples (< 2 My).Examination of the most primitive magmas reveals the variation of the mantle source composition along the arc and demarcates three segments: a "central" segment in front the d’Entrecasteaux ridge, a "south" segment in front of the North Fiji Basin, and a "far south" segment in front of the South Fiji Basin. The composition of the different subducted basins highly influenced those of the adjacent volcanoes via mass transfer of aqueous fluid and melt subduction components. Lavas from islands facing the D'Entrecasteaux Ridge come from an enriched mantle (Indian MORB-like), distinct from that sampled at other islands (Pacific MORB-like). Additionally, this ridge likely carries with it an ancient component that may reflect entrainment of lower crust material.Localized study of some volcanic islands reveals processes related to the differentiation of lavas by fractional crystallization, processes related to crustal assimilation, and indicates the presence of magma coming from different portions of mantle with slightly different metasomatic histories.This body of work highlights extreme mantle heterogeneity under Vanuatu islands arc and testifies to the complex nature of its associated geological processes.L’archipel volcanique actif du Vanuatu s’édifie au coeur du pacifique sud-ouest au niveau de la frontière convergente des plaques australienne et pacifique. Je présente ici une nouvelle étude géochimique des laves du Vanuatu à partir de la détermination des compositions en éléments majeurs et traces, et des compositions isotopiques (Sr-Nd-Hf-Pb) d’une centaine d’échantillons de laves (< 2 Ma).L’étude des magmas les plus primitifs a permis de mettre en évidence la variation de composition des sources mantelliques le long de l’arc et d’individualiser 3 segments : "central", dans la zone de collision de la ride D’Entrecasteaux, "sud" en face du bassin Nord Fidjien, et "extrême sud" en face du bassin Sud Fidjien. La composition des roches des différentes structures subductées influence celles des laves des volcans adjacents via le composant de subduction, sous forme de fluides et de produits de fusion. Les laves des îles situées en face de la ride D’Entrecasteaux sont issues d’un manteau enrichi ("type-MORB indien"), différent de celui échantillonné par les autres laves ("type-MORB pacifique"). Cette ride apporte probablement en subduction un composant ancien, pouvant être assimilé à un fragment de croûte inférieure.L’étude locale de certaines îles a permis de caractériser la différenciation des laves par cristallisation fractionnée, d’identifier des processus d’assimilation crustale, et de révéler la présence de magma provenant de portions de manteau distinctes, ayant subi un métasomatisme différent.Ces travaux révèlent une extrême hétérogénéité du manteau sous l’arc du Vanuatu, témoignant de la complexité des processus géologiques impliqués au niveau de cette zone de subduction

Applications de l’imagerie SIMSau sein de la plateforme PATERSON de l’IRSN

International audienceLa microscopie ionique analytique SIMS, permet l’analyse élémentaire et isotopique d’une surface solide par faisceau d’ions couplée à un spectromètre de masse. Le principe consiste à bombarder l’échantillon à étudier à l’aide d’un faisceau de particules chargées (dizaine de kV). Ce faisceau d’ions primaires rentre en collision avec l’échantillon cible provoquant l’éjection de particules provenant des couches superficielles de l’échantillon. Une fraction de ces particules est spontanément ionisée. Ces ions dit secondaires, sont ensuite accélérés et dirigés à l’entrée d’un spectromètre de masse. L’analyse de ces ions secondaires permet ainsi de caractériser et d’imager la composition chimique élémentaire et isotopique de la surface de l’échantillon.Cette technique est d’un apport essentiel dans le domaine du nucléaire notamment pour l’étude de la distribution d’éléments stables ou de radio-isotopes aussi bien dans des matrices biologiques par des études de micro localisation de l’uranium dans des tissus rénaux contaminés que géologiques avec mise en évidence de la signature radiogénique en plomb de phase uranifère issue de résidu de traitement de minerai d’Uranium . Ainsi l’équipement SIMS 4FE7 implanté au sein de la plateforme PATERSON de l’IRSN est utilisé dans le domaine de la radioprotection pour améliorer la compréhension des mécanismes de transport ainsi que d’accumulations des radionucléides. Dans le domaine de la sûreté nucléaire, il permet également d’aider à la compréhension de la tenue des gaines de combustible à l’irradiation en cartographiant les éléments hydrogène et oxygène

Applications de l’imagerie SIMSau sein de la plateforme PATERSON de l’IRSN

International audienceLa microscopie ionique analytique SIMS, permet l’analyse élémentaire et isotopique d’une surface solide par faisceau d’ions couplée à un spectromètre de masse. Le principe consiste à bombarder l’échantillon à étudier à l’aide d’un faisceau de particules chargées (dizaine de kV). Ce faisceau d’ions primaires rentre en collision avec l’échantillon cible provoquant l’éjection de particules provenant des couches superficielles de l’échantillon. Une fraction de ces particules est spontanément ionisée. Ces ions dit secondaires, sont ensuite accélérés et dirigés à l’entrée d’un spectromètre de masse. L’analyse de ces ions secondaires permet ainsi de caractériser et d’imager la composition chimique élémentaire et isotopique de la surface de l’échantillon.Cette technique est d’un apport essentiel dans le domaine du nucléaire notamment pour l’étude de la distribution d’éléments stables ou de radio-isotopes aussi bien dans des matrices biologiques par des études de micro localisation de l’uranium dans des tissus rénaux contaminés que géologiques avec mise en évidence de la signature radiogénique en plomb de phase uranifère issue de résidu de traitement de minerai d’Uranium . Ainsi l’équipement SIMS 4FE7 implanté au sein de la plateforme PATERSON de l’IRSN est utilisé dans le domaine de la radioprotection pour améliorer la compréhension des mécanismes de transport ainsi que d’accumulations des radionucléides. Dans le domaine de la sûreté nucléaire, il permet également d’aider à la compréhension de la tenue des gaines de combustible à l’irradiation en cartographiant les éléments hydrogène et oxygène

Combined U-Pb isotopic signatures of U mill tailings from France and Gabon: A new potential tracer to assess their fingerprint on the environment

International audienceUranium milling activities have produced high volumes of long-lived radioactive processed wastes stored worldwide in near surface environment. The aim of this study is to highlight relevant tracers that can be used for environmental impact assessment studies involving U mill tailings. A multi-tracer study involving elemental content, 238 U decay products disequilibria and stable Pb isotopes was performed in different types of U mill tailings (alkaline, acid, neutralized acid) collected from five Tailings Management Facilities in France (Le Bosc, L'Ecarpière, Le Bernardan, and Bellezane) and Gabon (Mounana). Our results showed that U and Pb concentrations range between 30-594 ppm and 66-805 ppm, respectively. These tailings have a strong disequilibrium of (234 U/ 238 U) and (230 Th/ 238 U) activity ratios (1.27-1.87 and 6-65, respectively), as well as higher 206 Pb/ 207 Pb (1.86-7.15) and lower 208 Pb/ 207 Pb (0.22-2.39) compared to geochemical background ((234 U/ 238 U) and (230 Th/ 238 U) equal to unity; 206 Pb/ 207 Pb = 1.20; 208 Pb/ 207 Pb = 2.47). In situ analyses (SEM, SIMS) showed that Pb-bearing phases with high 206 Pb/ 207 Pb are related to remaining U-rich phases, S-rich phases and potentially clay minerals or oxyhydroxides. We suggest that the combination of the 206 Pb/ 207 Pb with the (234 U/ 238 U) ratio is a relevant tool for the fingerprinting of the impact of U milling activities on the environment

Determination of 144^{144}Ce/238^{238}U atom ratio in spent nuclear fuel using double spike isotope dilution mass spectrometry.

International audienceThe low abundance cerium-144 radionuclide is one of the significant contributors to the decay heat from spent nuclear fuel for cooling times of less than ten years after nuclear reactor discharge. The accurate quantification of the $^{144}$Ce content (or $^{144}$Ce/$^{238}$U) in irradiated nuclear fuel is necessary to validate and extend the neutronic calculation codes as well as to improve the short-term nuclear waste management strategies. In order to quantify the $^{144}$Ce/$^{238}$U atomic ratios at low uncertainty, we developed a new analytical technique based on double spike isotope dilution associated with mass spectrometry. This includes (1) the chemical elimination of the major neodymium-144 isobaric interference by two steps of liquid chromatography prior to isotope analysis by Thermal Ionization Mass Spectrometry (TIMS) using both total evaporation and sequential methods, and (2) the preparation and use of an in-house double spike solution, using a mixture of a natural Ce solution with a $^{233}$U-enriched solution. This new approach was applied for the first time on two Mixed Oxide (MOx) spent nuclear fuel samples and allowed the determination of $^{144}$Ce/$^{238}$U atomic ratios ranging from 35 × 10$^{-6}$ to 59 × 10$^{-6}$ with a relative expanded uncertainty of measurement of around 1% at a 95% confidence level

Determination of the 144Ce/238U ratio in spent nuclear fuel samples by double spike isotope dilution mass spectrometry

International audiencePrecise knowledge of the energy released by radioactive decay of nuclides (decay heat) after the shut down of a nuclear reactor is of major importance for spent fuel management and nuclear safety. Among those radionuclides, the 144Ce isotope is a fission product that significantly contributes to the decay heat for cooling time less than 10 years. The accurate quantification of this low abundance isotope in nuclear fuels is therefore required (1) to bring new constrains on neutronic calculation codes that predict the evolution of the spent nuclear fuel composition with time and (2) to improve the short-term management of irradiated fuels.An innovative method combining liquid chromatography with double spike isotope dilution and state of the art mass spectrometric techniques (MC-ICP-MS, TIMS) has been developed for this purpose.This new method was applied for the first time on two Mixed Oxide (MOx) spent nuclear fuel samples. It enabled the determination of 144Ce/238U atomic ratios of less than a hundred of ppm with expanded measurement uncertainty at around 1 % at the 95 % confidence level. This corresponds to a reduction of at least three times of the measurement uncertainty compared to classical gamma-ray approaches. In the case of UOX samples or irradiated fuels cooled for longer periods, the 144Ce content is even lower, i.e. 144Ce/238U in the range of a few ppm. Consequently, specific developments were carried out based on either extration techniques or HPLC and will also be discussed

Deciphering sources of U contamination using isotope ratio signatures in the Loire River sediments: exploring the relevance of 233 U/ 236 U and stable Pb isotope ratios

International audienceA broad range of contaminants has been recorded in sediments of the Loire River over the last century. Among a variety of anthropogenic activities of this nuclearized watershed, extraction of uranium and associated activities during more than 50 years as well as operation of several nuclear power plants led to industrial discharges, which could persist for decades in sedimentary archives of the Loire River. Highlighting and identifying the origin of radionuclides that transited during the last decades and were recorded in the sediments is challenging due to i) the low concentrations which are often close or below the detection limits of routine environmental surveys and ii) the mixing of different sources. The determination of the sources of anthropogenic radioactivity was performed using multi-isotopic fingerprints (236 U/ 238 U, 206 Pb/ 207 Pb and 208 Pb/ 207 Pb) and the newly developed 233 U/ 236 U tracer. For the first time 233 U/ 236 U data in a well-dated river sediment core in the French river Loire are reported here. Results highlight potential sources of contamination among which a clear signature of anthropogenic inputs related to two accidents of a former NUGG NPP that occurred in 1969 and 1980. It is important to mention that the 233 U and 236 U levels measured in this work by recent high performance analytical methods are at ultra-trace levels and present a negligible radiological impact on health and on the environment. The determination of the mining activities by the use of stable Pb isotopes is still challenging probably due to the limited dissemination of the Pbbearing material marked by the U-ore signature downstream to the former U mines