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Pages scientifiques 2005

19 juillet 2005
H. Bureau, P. Trocellier, C. Shaw, H. Khodja, N. Bolfan-Casanova, S. Demouchy
In Earth Sciences, the global water cycle is of fundamental importance. For this reason, the H2O content of volcanic glass and mantle minerals must be analysed: usually by micro-infrared spectroscopy (FTIR) or Secondary Ion Mass Spectrometry (SIMS). However, both of these methods require calibration using standards of known water content. To avoid matrix effects, the standards and unknowns must be otherwise identical in composition. In this study we have determined the water content of geological samples, in the range 10 ppm- 5 wt.% H2O, using an absolute analytical technique: a combination of ERDA (Elastic Recoil Detection Analysis) and RBS (Rutherford Backscattering Spectrometry). We compared the results obtained by this method to data obtained by FTIR on the same samples. We discuss the limitations of the method and use the results to calibrate IR extinction coefficients for FTIR spectroscopy. Related paper: NIM B210 (2003) 434-440
19 juillet 2005
H. Bureau and N. Métrich
In order to assess the effect of the melt composition on bromine concentrations in magmas, we have investigated bromide solubility for water-saturated iron-free silicic melts with variable Na+K/Al and Si/Al molar ratios (albite, haplogranite, rhyolite and pantellerite). The experiments were performed in rapid quench cold-seal autoclaves over a range of pressure: 1, 1.5, 2 kbar, and temperature: 900, 1000, and 1080°C with run durations from 5 to 7 days. A series of natural volcanic glasses and melt inclusions hosted in magmatic minerals were analysed together with the synthetic glasses by PIXE (Proton Induced X-rays Emission, Nuclear Microprobe, Pierre Süe Laboratory). The Br concentrations range from 5360 to 7850 ppm for albite, from 2800 to 3900 ppm for haplogranite, from 4300 to 5900 ppm for rhyolite, and from 9745 to 11250 ppm for pantellerite. Bromine concentrations are negatively correlated with pressure in H2O-saturated silicic melts, and vary with (Na+K)/Al molar ratio with a minimum value at the ratio close to unity. Bromine behaves similarly to chlorine for all of these melt compositions. The bromide solubility is similar in albitic and rhyolitic melts, which implies that Df/m is nearly the same for both compositions and is applicable for natural rhyolites as suggested in a previous study. This means that the volcanic Br contribution to the atmosphere may be significant. In natural obsidian samples and MI hosted in quartz, olivine and leucite, the Br concentration varies from < 3 to 28 ppm, with the highest concentrations in pantelleritic melts. We attribute the low bromine concentrations of natural melts to a low initial abundance of this halogen in the Earth mantle. However, because bromine behaves as an incompatible element before water exsolution, our results imply that magmas could contain much more dissolved Br prior to eruption and water degassing than the few ppm usually measured in volcanic rocks. Bromine behaviour during magma crystallisation is controlled by its partitioning into the H2O-rich fluid phase when occurs. On the contrary, its potential high solubility in silicate melts makes it a very sensitive chemical tracer of magma contamination by sea water and Br-rich material. Therefore, the systematic study of Br in subduction-related magmatic samples may help for a better understanding of volatiles cycling between the Earth reservoirs. Our experimental approach is now focused on bromine and iodine degassing processes during magma decompression for subduction-zone related magmas. Related publication: GCA 67, 1689-1697, 2003 Collaboration : Bayerisches Geoinstitut Bayreuth, Germany.
25 mars 2005
H. Bureau, C. Gondé, B. Ménez, M. Semet
Melt and fluid inclusions in olivines from the last 197 days long eruption of Piton de la Fournaise (Réunion Island) that began March 9, 1998, have recorded a range of volatile partial pressures. The highest pressures (400-540 MPa) so far for this volcano are found in unusually Fo-rich olivines from a vent remote from the main eruptive activity. Such pressures indicate that fractionation of olivine (and other crystal phases) occurs below the crust-mantle boundary in a CO2-rich volatile saturated environment. Together with previously published data, the model that emerges for Piton de la Fournaise is one where olivines which have crystallized from multiple past magma injection events, may be picked up by newly intruding magma, anywhere from upper-mantle depths to the surface, providing a record of the composition of a rather permanent and independent gas phase. The proposed model (H. Bureau, N. Métrich, M. Semet, T. Staudacher (1999) GRL, 26, 3501-3504) is likely to apply to other shield volcanoes. Magma production and transfers to the surface is accompanied by continuous open-system degassing through the permeable volcanic pile. The calculated H2O content of primary basalts (MgO ≈ 12-14 wt.%) from Piton de la Fournaise may reach 0.7-1 wt.% implying a rather H2O-rich hot spot mantle source. We are developing analytical protocols for Particle Induced X-ray Emission (PIXE, Laboratoire Pierre Süe, CEA Saclay) and Synchrotron X-Ray Fluorescence (SXRF, ESRF Grenoble) for the determination of major and trace element concentrations in these melt inclusions that had already been studied for major and volatile elements. Calibration of each of these two methods are done using International standards (NIST glasses) and analytically well documented glasses. Consistent results are obtained for elements ranging for K to U. The doubly polished basaltic melt inclusions (50-150 µm diameter) are investigated by the two methods, allowing comparison for the following elements: Ni, Cu, Zn, Rb, Sr, Zr, Pb etc... Compared to other techniques usually used for melt inclusions study, such as SIMS, SXRF and PIXE yield promising complementary results.

28 novembre 2005
DSM/DRECAM/SPCSI - Oxides group
Spin electronics is a new field of research which associates two domains of physics: magnetism and electronics. In the last decade, effects related to spin dependent transport like giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) in magnetic heterostructures have stimulated a considerable interest. These physical properties can be exploited in a variety of advanced devices such as highly sensitive magnetic sensors (e.g. read heads for magnetic recording) and non volatile magnetic memories (MRAM). Some ferromagnetic oxides have a 100 % spin polarization (there are half metallic) and thus are promising candidates for applications in spin electronics. The use of half metallic ferromagnets electrodes in magnetic tunnel junctions (MTJs) can lead to TMR effects much higher than those obtained with usual ferromagnetic electrodes. One of the predicted half metallic oxide is magnetite Fe3O4 which exhibits a high Curie temperature (Tc= 858 K) so that one can expect the half metallic character to remain significant at room temperature.
26 septembre 2005

La magnétite (Fe3O4) est un des seuls matériaux demi-métalliques (conducteur pour un état de spin et isolant pour l'autre) à présenter une température de Curie (Tc= 860 K) très au-dessus de la température ambiante. A ce titre, cet oxyde ferrimagnétique constitue un matériau de choix pour les composants de l’électronique de spin. Dans ce contexte, le SPCSI a mis au point une méthode d'épitaxie par jets moléculaires (MBE) assistée par plasma d'oxygène atomique permettant l’élaboration de couches minces nanométriques de Fe3O4 sur des substrats monocristallins de saphir. Les films sont caractérisés in situ pendant et après la croissance par différentes techniques d'analyse de surface: diffraction d'électrons rapides (RHEED) et lents (LEED), spectroscopie de photons X (XPS) et d'électrons (AES).

28 novembre 2005
DSM/DRECAM/SPCSI - Oxides group
Spin electronics is a new field of research which associates two domains of physics: magnetism and electronics. In the last decade, effects related to spin dependent transport like giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) in magnetic heterostructures have stimulated a considerable interest. These physical properties can be exploited in a variety of advanced devices such as highly sensitive magnetic sensors (e.g. read heads for magnetic recording) and non volatile magnetic memories (MRAM). Some ferromagnetic oxides have a 100 % spin polarization (there are half metallic) and thus are promising candidates for applications in spin electronics. The use of half metallic ferromagnets electrodes in magnetic tunnel junctions (MTJs) can lead to TMR effects much higher than those obtained with usual ferromagnetic electrodes. One of the predicted half metallic oxide is magnetite Fe3O4 which exhibits a high Curie temperature (Tc= 858 K) so that one can expect the half metallic character to remain significant at room temperature.

15 juin 2005
La résistance électrique générée par les parois de domaines magnétiques a pu être étudiée et utilisée pour le suivi du retournement de l’aimantation d’une nanostructure de FePd. L’échantillon a été fabriqué par le Département de Recherche Fondamentale sur la Matière Condensée (DSM/DRFMC à Grenoble) puis lithographié pour permettre au courant de circuler parallèlement et perpendiculairement aux parois. La figure correspond à une image avec contraste magnétique (MFM) où chaque domaine correspond à une couleur (noir ou blanc selon la direction de l ’aimantation). Entre deux domaines, il existe une paroi magnétique qui génère un petit excès de résistance qu’il est possible de mesurer lors de la disparition successive des domaines sous l’application d’un champ. La mesure de cette résistance à l’échelle nanométrique permet de mieux comprendre la physique du transport électrique dans les ferromagnétiques, nécessaire au développement de capteurs magnéto-sensibles.

25 mars 2005
H. Bureau, C. Gondé, B. Ménez, M. Semet
Melt and fluid inclusions in olivines from the last 197 days long eruption of Piton de la Fournaise (Réunion Island) that began March 9, 1998, have recorded a range of volatile partial pressures. The highest pressures (400-540 MPa) so far for this volcano are found in unusually Fo-rich olivines from a vent remote from the main eruptive activity. Such pressures indicate that fractionation of olivine (and other crystal phases) occurs below the crust-mantle boundary in a CO2-rich volatile saturated environment. Together with previously published data, the model that emerges for Piton de la Fournaise is one where olivines which have crystallized from multiple past magma injection events, may be picked up by newly intruding magma, anywhere from upper-mantle depths to the surface, providing a record of the composition of a rather permanent and independent gas phase. The proposed model (H. Bureau, N. Métrich, M. Semet, T. Staudacher (1999) GRL, 26, 3501-3504) is likely to apply to other shield volcanoes. Magma production and transfers to the surface is accompanied by continuous open-system degassing through the permeable volcanic pile. The calculated H2O content of primary basalts (MgO ≈ 12-14 wt.%) from Piton de la Fournaise may reach 0.7-1 wt.% implying a rather H2O-rich hot spot mantle source. We are developing analytical protocols for Particle Induced X-ray Emission (PIXE, Laboratoire Pierre Süe, CEA Saclay) and Synchrotron X-Ray Fluorescence (SXRF, ESRF Grenoble) for the determination of major and trace element concentrations in these melt inclusions that had already been studied for major and volatile elements. Calibration of each of these two methods are done using International standards (NIST glasses) and analytically well documented glasses. Consistent results are obtained for elements ranging for K to U. The doubly polished basaltic melt inclusions (50-150 µm diameter) are investigated by the two methods, allowing comparison for the following elements: Ni, Cu, Zn, Rb, Sr, Zr, Pb etc... Compared to other techniques usually used for melt inclusions study, such as SIMS, SXRF and PIXE yield promising complementary results.
25 mars 2005
H. Bureau, B. Ménez, V. Malavergne, A. Somogyi, M. Munoz, A. Simionovici, D. Massare, M. Burchard, S. Kubsky, C. Shaw
Subduction zones are the locations where the oceanic crust, which experienced intense interaction with seawater, is buried back into the Earth’s mantle. These environments play a critical role in the global geochemical cycle of Earth, being the location at which chemical components are both recycled into the mantle and returned to the atmosphere through volcanism. It is now generally admitted that magma genesis at subduction zones is mainly induced by the presence and movement of fluids (aqueous fluids and/or silicate melts) released continuously in large amounts due to dehydration reactions in the subducted slab. It is also believed that these fluids may be responsible of the atypical trace-element signature of the Island Arc Basalts (IAB), which are enriched in Large Ion Lithophile Element (LILE= Sr, Rb, Ba, Pb) and depleted in High-Field Strength Element (HFSE= Zr, Ti, Hf, Nb, Ta) compared to Mid-Ocean Ridge Basalts (MORB). This program combine diamond anvil cells experiments with the high-brillance third-generation synchrotron light source of the European Synchrotron Radiation Facility (ESRF µFID beam line, Grenoble, France,), to study in situ the partitioning of key elements (Pb, Rb, Sr) between aqueous fluids (pure water, NaCl aqueous solutions) and silicate melts, under subduction zones pressure and temperature conditions. We try to evaluate the effect of chlorine on the partition coefficients of elements such as Pb, Ba, Sr, Rb, Zr… as function of pressure and temperature. Related publication: EOS Trans. AGU 2004, 85(47), Fall Meet. Suppl., Abstract V11C-05.
25 mars 2005
Solenn Réguer, Philippe Dillmann, Pierre Lagarde*

Synchrotron based micro X-ray Absorption Spectroscopy was used to obtain micro scale chemical information such as coordination and oxidation state of phases constituting corrosion products within archaeological iron artefacts buried in soil. The knowledge about iron corrosion process related to the presence of chlorine, is particularly important for restoration and conservation of the metallic artefacts. The samples available for X-ray microprobe analyses are cross sections from iron corroded objects coming from different archaeological excavation sites dating from 12th to 16th century AD. Previously, several analytical techniques such MEB-EDS, µRaman and µXRD have been employed to reveal morphological, compositional and structural information of corrosion products. X-ray Absorption Near Edge Structure (µXANES) was used to determine the spatial variation of the predominant Fe oxidation state and the corresponding crystallographic phase. The micro-XAS and imaging experiments were conducted on the ID21 beamline at ESRF and on the LUCIA beamline at SLS. Analyses performed at Fe and Cl K-edge (µXANES) revealed the correlation between the Fe2+ and Fe3+ distribution in the corrosion products, and the evolution of the chlorine concentration. In addition to the presence of the well-known beta iron hydroxide βFeOOH: akaganeite, in iron corrosion product, we highlight the presence of an other important phase, the βFe2(OH)3Cl hydroxychloride. This result is particularly interesting because, to our knowledge, this phase has never been identified in archaeological artefacts corrosion products. These findings help to gain new insights concerning the influence of such phases in iron corrosion mechanism within their precise characterisation. *LURE and SLS

27 juillet 2005
N. Métrich, J. Susini*, E. Foy, D. Massare, F. Farges **, L. Sylla, S. Lequien, M. Bonnin-Mosbah+
ΣΞIron and sulfur may be considered as the couple of elements buffering the redox state of magmas during their ascent from Earth’s mantle to the surface. However, the in-situ determination of both the redox state of magmas and the relative variation of the [Fe3+/(Fe3++Fe2+)] atomic ratio is challenging because these silicate melts crystallised in response to the temperature decrease, the decompression and the release of most of their volatile constituents (H2O, CO2, H2S, SO2, HCl) prior to eruption. Previous micro-X-ray absorption spectroscopy (µXANES) experiments performed on ID21 (ESRF) on Fe-bearing silicate reference glasses, at room temperature, indicated that the energy resolution allows an accurate discrimination between the pre-edge features of Fe2+ (7111.5 eV) and Fe3+ (7113.2 eV) and that the area ratios between the Fe3+ and Fe2+ peaks correlate with the Fe3+/ΣFe ratio determined independently by classical chemical analyses. However, the effects of the cooling rate and of the glass transition may induce bias in the determination of the redox state of Fe in glasses, because of rapid electron exchange. This problem may be addressed by direct in-situ measurements of the chemical environment of Fe as a function of temperature. We provide, here, the first in-situ µXANES experiments at the iron K-edge (7130 eV), carried out on ID21 beamline for determining the chemical environment of iron in natural silicate glass/melt, between 20 and 800°C. These experiments were performed on H2O-rich silicate glass inclusions trapped within natural hermetic containers as pure SiO2 quartz crystals. These inclusions are droplets of magma trapped at high temperature (i.e. 750°C) in growing crystals and preserved as glass in volcanic samples, because of rapid cooling. The prepared as double face polished, 150 µm thick lamella in order to preserve the inclusion in the centre of the crystals and thus isolated from the surroundings. The samples were placed in a water-cooled furnace. Different series of µXANES spectra (2x2 µm2) probe were collected on melt/glass inclusions, by decreasing the temperature by step of 100°C, from 800 to 20°C. The pre-edge features significantly and progressively evolved towards reduce state (Fe2+) at high temperature up to 797°C. and towards the oxidised (Fe3+) pole when cooling down to 20°C (Fig). It demonstrates the dominance of Fe3+ in the pre-edge structure, a shift of the centroid position by nearly 0.5 eV and an increase in the energy of the main edge at 20°C compared to high temperature. This results in an apparent and drastic change of the oxidation state of iron, as the temperature decreases. The slower the cooling rate the higher the ferric iron contribution. The [Fe3+/ΣFe] ratio of samples may be overestimated by a factor 1.7 to >2, implying that this ratio cannot be reliably retrieved by probing natural cooled glass inclusions and most likely silicate glasses. Iterative µXANES experiments coupled with in-situ infrared spectroscopy measurements performed on the same samples show that the process is reversible. The possible mechanisms at the origin of apparent oxidation of iron upon cooling are thus discussed. High temperature µXANES experiments led first to an assessment of the ferric-ferrous ratio in the water-rich peralkaline melt in pre-eruptive magmatic conditions and second to the determination of the corresponding oxygen fugacity at 740°C. Publication Chemical Geology (2005 in course of revision) * European Synchrotron Radiation Facility, Grenoble, France ** Laboratoire de Geosciences, Université de Marne La Vallée, France + INSTN, CEA, CE-Saclay, , Gif sur Yvette, France

18 février 2005
M. Treuil, J. L. Joron, M. El Maghraoui*, F. Le Borgne
De nombreux travaux sont consacrés actuellement au transport des éléments dans l'hydrosphère continentale. Des bilans ont été dressés qui soulignent tant les modalités et les flux de ces transports (particulaires et en solution) que les processus d'altération et d'érosion de la croûte (physico-chimie des sols, équilibres eau-roche, …). Demeurent néanmoins de nombreuses incertitudes qui tiennent, pour une part aux difficultés d'analyse qualitative et quantitative des différentes espèces véhiculées par une eau naturelle et, pour une autre part, aux difficultés d'observation et d'expérimentation en grandeur réelle dans les milieux naturels représentatifs. Nous nous efforçons de surmonter ces difficultés en recherchant en premier lieu des systèmes naturels modèles dont la dynamique favorise l'observation, en combinant en second lieu les outils analytiques du laboratoire Pierre Süe (activation neutronique et spectrométrie de masse), finalement en concentrant principalement notre attention sur les éléments, codeurs dans le temps de l'information, plutôt que sur les objets supports éphémères de cette information. Cette démarche engagée dans différents contextes où interviennent puissamment l'altération et les grandes transformations de la croûte, conduit à souligner le caractère systématiquement inerte d'un grand nombre d'éléments de transition internes réputés par ailleurs d'éléments "incompatibles" tels que Ta-Nb, Zr-Hf-Th-lanthanides lourds chimiquement. Si l'on considère que ces éléments se caractérisent par leur forte aptitude à donner des espèces discrètes complexes en solution avec de nombreux ligands, ce paradoxe évoqué ci-dessus ne trouve d'explication que par le non passage en solution de ces éléments lors des interactions fluides/roches de ces transformations et donc par la non pertinence du concept d'éléments "incompatibles". Ce résultat est illustré aussi bien par la répartition de ces éléments dans les roches solides résiduelles de ces interactions que par leurs distributions dans les eaux des rivières et des fleuves. Dans ce dernier travail, nous avons montré que les distributions des lanthanides dans les eaux de la Loire correspondent au transfert du spectre de la croûte continentale par l'intermédiaire d'espèces colloïdales. La filtration comme la décantation de ces espèces au cours du transport ne modifie pas l'allure du spectre. Par contre ce spectre des distributions des lanthanides dans les eaux est significativement modifié dans le sens d'un appauvrissement en lanthanides légers lorsque les conditions du milieu en période estivale ou lors du transfert dans un réseaux de chenaux karstiques, mettent en compétition les espèces colloïdales avec des ligands complexant forts (carbonates complexes) ce qui favorise le maintien en solution des lanthanides lourds. * Université de Kenitra, Maroc

22 juillet 2005
Etudes en photoémission
La photoémission est une technique puissante pour étudier la structure électronique des matériaux. La photoémission à haute énergie (5-10 keV) permet en particulier de sonder les états électroniques des niveaux de cœur, permettant de connaître l'environnement chimique d'un élément donné. Le rayonnement synchrotron, puissant et accordable, autorise aujourd'hui des résolutions énergétiques très élevées (de l'ordre du meV) ainsi que le développement de nouvelles imageries en photoémission (tel que le développement en cours d’un instrument NanoESCA en collaboration avec le LETI-CEA). Couplés à d'autres techniques telles que la diffraction de rayons X (ESRF), la microscopie électronique et les méthodes de calcul ab-initio permettant de simuler les spectres, la photoémission est une technique incontournable pour l'étude des nouveaux dispositifs semi-conducteurs tels que les dispositifs CMOS. Leurs miniaturisation implique la réduction de la longueur du canal à des valeurs inférieures à 100 nm à l’horizon 2010. Pour ceci des épaisseurs d’oxyde de grille de l'ordre du nanomètre devront être atteints. Le remplacement, par exemple, de la silice par des oxydes à haute permittivité permet d'atteindre des épaisseurs physiques d’environ 4 nm, des courants de fuite acceptables et des performances capacitives et électroniques intéressantes. Les silicates d’hafnium sont parmi les meilleurs candidats parmi ces oxydes de grille à haute permittivité.

28 novembre 2005
DSM/DRECAM/SPCSI - Oxides group
Spin electronics is a new field of research which associates two domains of physics: magnetism and electronics. In the last decade, effects related to spin dependent transport like giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) in magnetic heterostructures have stimulated a considerable interest. These physical properties can be exploited in a variety of advanced devices such as highly sensitive magnetic sensors (e.g. read heads for magnetic recording) and non volatile magnetic memories (MRAM). Some ferromagnetic oxides have a 100 % spin polarization (there are half metallic) and thus are promising candidates for applications in spin electronics. The use of half metallic ferromagnets electrodes in magnetic tunnel junctions (MTJs) can lead to TMR effects much higher than those obtained with usual ferromagnetic electrodes. One of the predicted half metallic oxide is magnetite Fe3O4 which exhibits a high Curie temperature (Tc= 858 K) so that one can expect the half metallic character to remain significant at room temperature.

 

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