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19. Solid-state solubilities of grain -growth inhibitors in WC-Co and WC-MC-Co hardmetals

Author

Lauter, Lukas, Hochenauer, Roman, Buchegger, Christoph, Bohn, Marcel, Lengauer, Walter, Lauter, Lukas, Hochenauer, Roman, Buchegger, Christoph, Bohn, Marcel, and Lengauer, Walter

Abstract

The solubilities of the most common grain -growth inhibitors (GGIs) Cr, V, Ta, Mo, Nb, Zr and certain combinations there of in the binder phase of WC-Co hardmetals as well as hardmetals of the type WCTiC-TiN-TaC-NbC-Co containing an fcc MC carbide (or carbonitride) phase "MC"("WC-MC-Co" hardmetals) were analysed by means of WDS-EPMA. The carbon potential of the binder phase was fixed at its highest and lowest maxima by the presence of either graphite or eta (M6C) phases. In case of low-carbon hardmetals the composition of M6C, and in the MC-containing hardmetals the composition of the MC phase, was analysed. Two types of solubilities were determined. The first type was measured after controlled cool-down from liquid-phase sintering temperature and represented the binder phase state of an industrially prepared hardmetal. The second type representing the equilibrium solubility was measured in alloys annealed at 1000 degrees C for 168 h after liquid -phase sintering. The measured solubilities were subsequently used to prepare hardmetals with an optimised GGI concentration. The samples were analysed with respect to hardness, toughness, magnetic properties, transverse rupture strength and Weibull parameter. A complex doping with Cr, V and Ta with optimised concentrations near the solubility limit of these metals in the binder phase turned out to yield best combination of mechanical properties and reliability. This was successfully adopted in industrial production.

Published
2016
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21. Possible fluid rock interactions on differentiated asteroids recorded in eucritic meteorites

Author

Barrat, Jean-Alix, Yamaguchi, A., Bunch, T.E., Bohn, Marcel, Bollinger, Claire, Ceuleneer, Georges, Domaines Océaniques (LDO), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS), Antarctic Meteorite Research Center, National Institute of Polar Research [Tokyo] (NiPR), Department of Polar Science, Graduate University for Advanced Sciences, Department of Geology, Northern Arizona University [Flagstaff], Dynamique terrestre et planétaire (DTP), Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), and Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Université Fédérale Toulouse Midi-Pyrénées-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS)

Subjects
[SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, [SDE.MCG]Environmental Sciences/Global Changes
Abstract

International audience; The eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4-Vesta. A few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy. Three distinct alteration stages are identified: 1/ Fe-enrichment along the cracks that cross cut the pyroxene crystals ("Fe-metasomatism"); secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage; 2/ deposits of Fe-rich olivine (Fa64-86) and minor amounts of troilite are frequent inside the cracks; sporadic secondary Ca-rich plagioclase (An97-98) is associated with the fayalitic olivine; 3/ at this stage, the Fe-enrichment of the pyroxene is accompanied by a marked Al-depletion; moreover, secondary Ca-rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals. The composition of the secondary phases on one hand, the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand, rule out a silicate melt as the metasomatic agent. Although no hydrous phase has been yet identified in the studied samples, aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites.

Published
2011

23. Evidence for K-rich terranes on Vesta from impact spherules

Author

Barrat, Jean-Alix, Bohn, Marcel, Gillet, P., Yamaguchi, A., Domaines Océaniques (LDO), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS), Université européenne de Bretagne - European University of Brittany (UEB), Laboratoire de Sciences de la Terre (LST), École normale supérieure - Lyon (ENS Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Antarctic Meteorite Research Center, National Institute of Polar Research [Tokyo] (NiPR), and École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL)

Subjects
Melts, Chemistry, Geochemistry, Diogenites, [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, [SDE.MCG]Environmental Sciences/Global Changes, Solar-System, Glass, Dawn Mission, Mineralogy, Meteorites, Petrology
Abstract

International audience; The howardite-eucrite-diogenite (HED) clan is a group of meteorites that probably originate from the asteroid Vesta. Some of them are complex breccias that contain impact glasses whose compositions mirror that of their source regions. Some K-rich impact glasses (up to 2 wt% K2O) suggest that in addition to basalts and ultramafic cumulates, K-rich rocks are exposed on Vesta's surface. One K-rich glass (up to 6 wt% K2O), with a felsic composition, provides the first evidence of highly differentiated K-rich rocks on a large asteroid. They can be compared to the rare lunar granites and suggest that magmas generated in a large asteroid are more diverse than previously thought.

Published
2009

24. Geochemistry of diogenites: Still more diversity in their parental melts

Author

Barrat, Jean-Alix, Yamaguchi, A., Greenwood, R.C., Benoit, M., Cotten, Joseph, Bohn, Marcel, Franchi, I.A., Université européenne de Bretagne - European University of Brittany (UEB), Domaines Océaniques (LDO), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS), Antarctic Meteorite Research Center, National Institute of Polar Research [Tokyo] (NiPR), PSSRI, The Open University [Milton Keynes] (OU), Dynamique terrestre et planétaire (DTP), Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), and Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Université Fédérale Toulouse Midi-Pyrénées-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS)

Subjects
[SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, [SDE.MCG]Environmental Sciences/Global Changes
Abstract

International audience; We report on the major and trace element abundances of 18 diogenites, and O-isotopes for 3 of them. Our analyses extend significantly the diogenite compositional range, both in respect of Mg-rich (e.g., Meteorite Hills [MET] 00425, MgO = 31.5 wt%) and Mg-poor varieties (e.g., Dhofar 700, MgO = 23 wt%). The wide ranges of siderophile and chalcophile element abundances are well explained by the presence of inhomogeneously distributed sulfide or metal grains within the analyzed chips. The behavior of incompatible elements in diogenites is more complex, as exemplified by the diversity of their REE patterns. Apart from a few diogenite samples that contain minute amounts of phosphate, and whose incompatible element abundances are unlike the orthopyroxene ones, the range of incompatible element abundances, and particularly the range of Dy/Yb ratios in diogenites is best explained by the diversity of their parental melts. We estimate that the FeO/MgO ratios of the diogenite parental melts range from about 1.4 to 3.5 and therefore largely overlap the values obtained for non-cumulate eucrites. Our results rule out the often accepted view that all the diogenites formed from parental melts more primitive than eucrites during the crystallization of a magma ocean. Instead, they point to a more complex history, and suggest that diogenites were derived from liquids produced by the remelting of cumulates formed from the magma ocean.

Published
2008

25. Origin of the adakite–high-Nb basalt association and its implications for postsubduction magmatism in Baja California, Mexico

Author

Maury, René, Calmus, Thierry, Pallares, Carlos, Benoit, Mathieu, Gregoire, Michel, Aguillón-Robles, Alfredo, Bellon, Hervé, Bohn, Marcel, Domaines Océaniques (LDO), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS), Université européenne de Bretagne - European University of Brittany (UEB), Estacion Regional del Noroeste, Universidad Nacional Autónoma de México (UNAM), Dynamique terrestre et planétaire (DTP), Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Université Fédérale Toulouse Midi-Pyrénées-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Departamento de Geologia, and Universidad Autonoma de San Luis Potosi [México] (UASLP)

Subjects
Basalt, Fractional crystallization (geology), 010504 meteorology & atmospheric sciences, Mantle wedge, Subduction, High-Nb basalts, bajaite, Geochemistry, subduction zone, [SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography, Geology, near-ridge seamount, 010502 geochemistry & geophysics, 01 natural sciences, 13. Climate action, Asthenosphere, Magmatism, Adakite, adakite, Mafic, Baja California, 0105 earth and related environmental sciences
Abstract

International audience; Constraining the origin of the adakite–high-Nb basalt (HNB) association in Baja California, Mexico, is critical to a better understanding of global arc magmatism. Currently the preferred explanation for the close spatial and temporal association of the two rock suites is through melting of the basaltic portion of the subducted Farallon-Cocos plate, thus providing support for the slab-melting origin of adakites elsewhere. Moreover, a tectono-magmatic model involving the production of both adakite and HNB from slab melts offers a comprehensive explanation for the origin of the atypical, arc-related, postsubduction magmatism in Baja California. This paper proposes alternative models for the origin of HNB and postsubduction magmatism in Baja California, wherein the unusual geologic setting of western Mexico and westward movement of North America permitted the influx of Pacific asthenosphere beneath the adjacent Gulf of California after the cessation of subduction. Unlike the previous tectono-magmatic model, the new models propose that the asthenosphere provided a direct source for postsubduction tholeiitic and rare alkali magmas that were erupted in Baja California as tholeiites and HNB, respectively. Fractional crystallization of some of the HNB magmas plus assimilation of tholeiitic materials produced Nb-enriched basalts (NEB). The influx of Pacific asthenosphere after the cessation of subduction also provided thermal energy to melt the mafic lower Baja California crust, producing adakite rocks, and the preexist-ing metasomatized mantle wedge, producing bajaites and calc-alkaline magmas.

Published
2008
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26. Reconstructing P-T paths from multi-stage garnets (Gran Paradiso nappe, Western Alps)

Author

Le Bayon, Benjamin, Pitra, Pavel, Ballevre, Michel, Bohn, Marcel, Laboratoire de Géodynamique des Chaines Alpines (LGCA), Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Géosciences Rennes (GR), Université de Rennes 1 (UR1), Université de Rennes (UNIV-RENNES)-Université de Rennes (UNIV-RENNES)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre Armoricain de Recherches en Environnement-Centre National de la Recherche Scientifique (CNRS), Microsonde Ouest, Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Université de Rennes (UR)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre Armoricain de Recherches en Environnement-Centre National de la Recherche Scientifique (CNRS), Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Institut des Sciences de la Terre (ISTerre), Université Grenoble Alpes (UGA)-Centre National de la Recherche Scientifique (CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-PRES Université de Grenoble-Institut de recherche pour le développement [IRD] : UR219-Institut national des sciences de l'Univers (INSU - CNRS)-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Université Joseph Fourier - Grenoble 1 (UJF)-Centre National de la Recherche Scientifique (CNRS)-PRES Université de Grenoble-Institut de recherche pour le développement [IRD] : UR219-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR), and Dubigeon, Isabelle

Subjects
[SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography, [SDU.STU.PE] Sciences of the Universe [physics]/Earth Sciences/Petrography, [SDU.STU.MI] Sciences of the Universe [physics]/Earth Sciences/Mineralogy, [SDU.STU.MI]Sciences of the Universe [physics]/Earth Sciences/Mineralogy
Abstract

National audience; Most collision zones involve slices of continental crust that have been reworked during several, possibly unrelated, episodes. During the later metamorphic episodes, relics of the early history are progressively erased. However, traces of earlier metamorphic stages can be preserved either in low-strain domains, or within relic prekinematic phases in high-strain domains (e.g. garnets). Garnet-chloritoid-bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations and detailed X-ray element maps reveal that two stages of garnet growth are present in the micaschists, interpreted as (i) relics of an early metamorphism of pre-Alpine age and (ii) newly-grown Alpine garnets, respectively. X-ray element maps for garnet 1 have revealed the occurrence of former fractures that cut across garnet 1, and were sealed during the growth of garnet 2. Furthermore, both generations of garnet preserve growth zoning. From THERMOCALC-based numerical modelling of mineral assemblages in P-T pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P-T conditions are about 620°C, 6 kbar for the peak of the pre-Alpine event, and of 490°C, 18-20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14-15 kbar at 550°C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive reequilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units.

Published
2006

27. Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low-temperature to hydrothermal alteration

Author

Laverne, Christine, Grauby, O., Alt, Jeffrey C., Bohn, Marcel, Laboratoire de pétrologie Magmatique, Université Paul Cézanne - Aix-Marseille 3, Centre de recherche de la matière condensée et des nanosciences (CRMCN), Université de la Méditerranée - Aix-Marseille 2-Université Paul Cézanne - Aix-Marseille 3-Centre National de la Recherche Scientifique (CNRS), Department of Geological Sciences [Ann Arbor], University of Michigan [Ann Arbor], University of Michigan System-University of Michigan System, Domaines Océaniques (LDO), and Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS)

Subjects
[SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, hydrogarnet, [SDE.MCG]Environmental Sciences/Global Changes, titanium, low-temperature alteration, alteration halo, oceanic basalt
Abstract

International audience; Hydroschorlomite, a Ti-, Ca-, Fe-rich andraditic arnet present in the deepest cores of basalts (661?749 bsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro-Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5?50 ?m) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100?C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5?26.5 wt%), TiO2 (22.0?28.6 wt%), and FeOt (6.2?12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2?97.3 wt%) is confirmed by the OH vibration at 3557 cm?1 in the micro-Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny (

Published
2006
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28. Hydroschorlomite in altered basalts from Hole 1256D, ODP Leg 206: The transition from low-temperature to hydrothermal alteration - art. no. Q10O03

Author

Laverne, Christine, Grauby, Olivier, Alt, Jeffrey C., and Bohn, Marcel

Subjects
Titanium, hydrothermal systems [Mineralogy and petrology], Alteration halo, Oceanic basalt, alteration and weathering processes [Mineralogy and petrology], Low temperature alteration, Hydrogarnet
Abstract

Hydroschorlomite, a Ti-, Ca-, Fe-rich andraditic garnet present in the deepest cores of basalts (661-749 mbsf) drilled in Hole 1256D during Ocean Drilling Program (ODP) Leg 206 (equatorial east Pacific), is reported here for the first time in oceanic crust. Detailed petrological and mineralogical studies by optical microscope, electron microprobe, scanning and transmission electron microscope, and micro-Raman spectroscopy are used to characterize this hydrogarnet and its relationships with other minerals. Hydroschorlomite occurs in Hole 1256D as small (5-50 mu m) anhedral or euhedral crystals associated either with celadonite in black halos adjacent to celadonite veins or with brown saponitic phyllosilicate in brown alteration halos adjacent to veins of saponite and iron oxyhydroxides. Both types of halos are formed at low temperature (less than about 100 degrees C). Textural observations suggest that hydroschorlomite formation is contemporaneous with the phyllosilicates. Hydroschorlomite is rich in CaO (22.5-26.5 wt%), TiO2 (22.0-28.6 wt%), and FeOt (6.2-12.9 wt%) and contains significant F (up to 0.85 wt%) and Zr2O3 (up to 0.34 wt%). The presence of OH suggested by the low total percentages of oxides (95.2-97.3 wt%) is confirmed by the OH vibration at 3557 cm(-1) in the micro-Raman spectrum. Chemical mapping indicates that hydroschorlomite is not zoned and is always associated with either celadonitic or saponitic phyllosilicates. Some hydroschorlomite crystals partly include tiny(

Published
2006

29. Hydroschorlomite in basalts cored at hole 1256D, ODP Leg 206

Author

Laverne, C., Grauby, O., Alt, C.J., Bohn, Marcel, Cinam, Hal, Centre de recherche de la matière condensée et des nanosciences (CRMCN), and Université de la Méditerranée - Aix-Marseille 2-Université Paul Cézanne - Aix-Marseille 3-Centre National de la Recherche Scientifique (CNRS)

Published
2006

30. Monte Arci (Sardinia) obsidians: new geochemical data from electron microprobe and ion beam analysis

Author

Le Bourdonnec, François-Xavier, Lugliè, Carlo, Dubernet, Stéphan, Bohn, Marcel, Poupeau, Gérard, IRAMAT-Centre de recherche en physique appliquée à l’archéologie (IRAMAT-CRP2A), Institut de Recherches sur les Archéomatériaux (IRAMAT), Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS)-Université Bordeaux Montaigne-Université de Technologie de Belfort-Montbeliard (UTBM)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS)-Université Bordeaux Montaigne-Université de Technologie de Belfort-Montbeliard (UTBM), Universita degli Studi di Cagliari [Cagliari], Domaines Océaniques (LDO), and Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS)

Subjects
[SHS.ARCHEO]Humanities and Social Sciences/Archaeology and Prehistory, [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry
Abstract

International audience; We bring new geochemical data on Monte Arci (Sardinia) obsidians from elemental analyses by electron microprobe (EMP) and proton-induced X-ray emission (PIXE). Obsidians from the geochemical groups SA, SB1, SB2 and SC can be sorted out from their major element contents by EMP and from their trace element contents by PIXE. While EMP analysis requires for analysis a few milligrams polished fragment, PIXE is strictly non-destructive. Forty samples were part of a program aimed at the geochemical characterization of Monte Arci secondary sources.; Si presentano in questo lavoro nuovi dati geochimici sulle ossidiane del Monte Arci (Sardegna) sulla base di analisi della composizione elementare effettuate alla microsonda (EMP) e per emissione di raggi X indotta da protoni (PIXE). Le ossidiane appartenenti ai gruppi geochimici SA, SB1, SB2 ed SC possono essere discriminate in base al loro contenuto in elementi maggiori mediante EMP ed in base al loro contenuto in tracce mediante PIXE. Mentre le analisi EMP richiedono un frammento lucidato di pochi milligrammi, PIXE è un metodo del tutto non distruttivo. Sono stati analizzati quaranta campioni prelevati nell'ambito di un programma finalizzato alla caratterizzazione dei depositi secondari del Monte Arci.

Published
2004

32. Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): An HED parent body

Author

Barrat, Jean-Alix, Jambon, Albert, Bohn, Marcel, Blichert-Toft, Janne, Sautter, V., Göpel, C., Gillet, Philippe, Boudouma, Omar, Keller, F., Domaines Océaniques (LDO), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS), Institut de Physique du Globe de Paris (IPGP), Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-IPG PARIS-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Centre National de la Recherche Scientifique (CNRS), Microsonde Ouest, Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Laboratoire de Sciences de la Terre (LST), École normale supérieure - Lyon (ENS Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Laboratoire de minéralogie du Muséum National d'Histoire Naturelle (LMMNHN), Muséum national d'Histoire naturelle (MNHN)-Centre National de la Recherche Scientifique (CNRS), Laboratoire de Géodynamique des Chaines Alpines (LGCA), Observatoire des Sciences de l'Univers de Grenoble (OSUG), Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-École normale supérieure - Lyon (ENS Lyon), Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Institut des Sciences de la Terre (ISTerre), and Université Grenoble Alpes (UGA)-Centre National de la Recherche Scientifique (CNRS)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-PRES Université de Grenoble-Institut de recherche pour le développement [IRD] : UR219-Institut national des sciences de l'Univers (INSU - CNRS)-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Université Joseph Fourier - Grenoble 1 (UJF)-Centre National de la Recherche Scientifique (CNRS)-PRES Université de Grenoble-Institut de recherche pour le développement [IRD] : UR219-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)

Subjects
[SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry, [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph], [SDE.MCG]Environmental Sciences/Global Changes, [PHYS.PHYS.PHYS-GEO-PH]Physics [physics]/Physics [physics]/Geophysics [physics.geo-ph], [SDE.MCG.CPE]Environmental Sciences/Global Changes/domain_sde.mcg.cpe
Abstract

NWA 1240 is an unusual eucrite recently recovered in Morocco as a single stone of 98 g. It is an unbrecciated greenish-brown rock nearly devoid of fusion crust. It displays porphyritic texture consisting of skeletal hollow low-Ca pyroxene phenocrysts set in a variolitic (fan-spherulitic) mesostasis of fine elongate pyroxene and plagioclase crystals. Minor phases are skeletal chromite, iron, silica, troilite, ilmenite and minute amounts of phosphate and fayalite. Pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite, from En76.0Wo1.9Fs22.1 to compositions nearly devoid of Mg (unusual ferrosilite and Fe-augite symplectites and possibly pyroxferroite). Plagioclase crystals contain significant amounts of Fe and Mg, which are possibly controlled by the Ca(Mg,Fe2!)Si3O8 plagioclase component. To discuss the potential effects of hot-desert weathering on NWA 1240, we have analyzed a series of Saharan eucrites (Agoult, Aoufous, Igdi, Smara, NWA 047 and NWA 049) and large aliquots (0.39 to 2.8 g) of eucrite falls (Bereba, Bouvante, Jonzac, Juvinas and Serra de Mage´). These results indicate that among the elements we have determined, Pb, Ba and Sr are the most sensitive indicators of Saharan weathering. The bulk composition of NWA 1240 has been determined for 45 elements by ICP-AES and ICP-MS. The data show that the meteorite is not significantly weathered: its Pb concentration is very low; Ba and Sr concentrations are not anomalously high; the Th/U and Hf/Sm ratios are chondritic (Th/U " 3.65, Hf/Sm " 0.74). NWA 1240 is rich in MgO (10.4 wt%) and Cr2O3 (0.71 wt%), and displays striking similarities with cumulate eucrites, such as having similar incompatible trace element patterns and a significant positive Eu anomaly (Eu/Eu* " 1.37). The combination of fast cooling and cumulate eucrite-dominated composition suggests that NWA 1240 is not an igneous rock but rather an impact melt.

Published
2002
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33. X-ray mapping of microstructures in hardmetals ans cermets

Author

Wollein, B., Bohn, Marcel, Lengauer, W., Domaines Océaniques (LDO), and Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Observatoire des Sciences de l'Univers-Institut d'écologie et environnement-Centre National de la Recherche Scientifique (CNRS)

Subjects
[SDU.STU.MI]Sciences of the Universe [physics]/Earth Sciences/Mineralogy
Published
2002

34. Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56 degrees S and 66 degrees S

Author

Vlastelic, Ivan, Dosso, Laure, Bougault, Henri, Daniel Aslanian, Louis Géli, Etoubleau, Joel, Bohn, Marcel, JORON JL, and Bollinger, C.

Abstract

Axial bathymetry, major/trace elements, and isotopes suggest:that the Pacific-Antarctic Ridge (PAR) between 56 degrees S and 66 degrees S is devoid of any hotspot influence. PAR (56-66 degrees 5) samples have in-average lower Sr-87/Sr-86 and Nd-143/Nd-144 and higher Pb-206/(204)pb than northern Pacific midocean ridge basalts (MORB), and also than MORE from the other oceans. The high variability of Pb isotopic ratios (compared to Sr and Nd) can be due to either a;general high mu (HIMU) (high U/Pb) affinity of the southern Pacific upper mantle or to a mantle event first recorded in time by Pb isotopes. Compiling the results of this study with those from the PAR between 53 degrees S and 57 degrees S gives a continuous view of mantle characteristics from south Pitman Fracture Zone (FZ) to Vacquier FZ, representing about 3000 km of spreading axis. The latitude of Udintsev FZ (56 degrees S) is a limit between, to the north, a domain with large geochemical Variations and, to the south, one with small Variations. The spreading rate has intermediate values (54 mm/yr at 66 degrees S to 74 mm/yr at 56 degrees S) which increase along the PAR, while the axial morphology: changes from valley to dome. The morphological transition is not recorded by the chemical properties of the ridge basalts nor by the inferred mantle temperature which displays few variations (30-40 degrees C) along the PAR. Contrary to what is observed along the South-East Indian Ridge, PAR morphology appears to be controlled more by spreading rate rather than by mantle temperature. Much of the major and trace element variability results from segmentation control on the shallowest thermal structure of the mantle. The cold edge of a fracture zone seems to be more efficient when occurring in an axial dome context.:It is expressed as an increase of the magnitude of the Transform Fault Effect along the valley-dome transition, resulting in a clear increase of trace element ratio variability (such as Nb/Zr). There is no strong evidence for the previously proposed southwestward asthenospheric flow in the area. However, this flow model could explain the intrasegment asymmetric patterns.

Published
2000

35. Cascade of Peritectic Reactions in the B-Fe-U System

Author

Dias, M., Carvalho, P. A., Dias, A. P., Bohn, Marcel, Franco, N., Tougait, O., Noel, H., Goncalves, A. P., Dias, M., Carvalho, P. A., Dias, A. P., Bohn, Marcel, Franco, N., Tougait, O., Noel, H., and Goncalves, A. P.

Abstract

The solidification paths for UFeB4, UFe3B2 and UFe4B, ternary compounds, situated along the U:(Fe,B) = 1:5 line in the B-Fe-U phase diagram, are proposed based on x-ray powder diffraction measurements, differential thermal analysis, heating curves and scanning electron microscopy observations complemented with energy and wavelength dispersive x-ray spectroscopies. The compounds melt incongruently and are formed by peritectic reactions. The present work demonstrates the existence of a cascade of peritectic reactions along the U:(Fe,B) = 1:5 composition line, establishes peritectic temperatures and proposes an isopleth diagram along this line.

Published
2010
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36. U-Th stratigraphy of a cold seep carbonate crust

Author

Bayon, Germain, Henderson, G. M., Bohn, Marcel, Bayon, Germain, Henderson, G. M., and Bohn, Marcel

Abstract

On continental margins, oxidation of methane-rich fluids from the sediment often leads to formation of authigenic carbonate pavements on the seafloor. The biogeochemical processes involved during this carbonate precipitation are increasingly understood, but little is known about the duration or mode of carbonate crust formation. Here, we report uranium and thorium concentrations and isotope compositions for a set of 14 samples drilled across an authigenic carbonate pavement, which provide the first stratigraphy for a cold-seep carbonate crust. The 5.5-cm thick crust (NL7-CC2) was collected by Submersible on the Nile deep-sea fan in an area of active fluid venting. U-Th analyses must be corrected for initial Th and measurement of co-existing sediments indicates the presence of both scavenged and detrital initial Th-230, which must be considered during this correction. The calculated Th-230/U age-depth profile for NL7-CC2 provides evidence for continuous downward carbonate precipitation at the studied location over the last similar to 5000 years. Three distinct phases can be distinguished from top to bottom with average growth rates of similar to 0.4, 5 and 0.8 cm/kyr, respectively, corresponding to carbonate precipitation rates ranging from similar to 7 to 92 mu mol m(2) h(-1) (rates consistent with previous estimates), High-resolution delta C-13 profiles [Gontharet, S., Pierre, C., Blanc-Valleron, M.-M., Rouchy,J.M., Fouquet, Y., Bayon, G., Foucher,J.P., Woodside,J., Mascle,J., The Nautinil Scientific Party, 2007. Nature and origin of diagenetic carbonate crusts and concretions from mud volcanoes and pockmarks of the Nile deep-sea fan (eastern Mediterranean Sea). Deep Sea Res. 1154,1292-1311] and major elements across NL7-CC2 show that the variations in carbonate precipitation rates were also accompanied by changes in carbonate mineralogy and fluid composition. We suggest that these changes primarily reflect modification of the diagenetic environment, i.e. a progres

Published
2009
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37. Akimotoite in the Tenham meteorite: Crystal chemistry and high-pressure transformation mechanisms

Author

Ferroir, T, Beck, P, Van De Moortele, B, Bohn, Marcel, Reynard, B, Simionovici, A, El Goresy, A, Gillet, P, Ferroir, T, Beck, P, Van De Moortele, B, Bohn, Marcel, Reynard, B, Simionovici, A, El Goresy, A, and Gillet, P

Abstract

The transformation of pyroxene to its high-pressure polymorph akimotoite (MgSiO3 ilmenite type structure) is documented in the shock-induced melt veins of the L6 Tenham chondrite. Four textural relationships between pyroxene and akimotoite are observed in former pyroxene grains entrained in the shear melt vein and in pyroxene grains attached to the wall of the melt vein. In one of the entrained enstatite grains the transformation to akimotoite is partial. One third of the grain is transformed to a polycrystalline aggregate of akimotoite with a scalloped interface with enstatite. Akimotoite (Fe0.4Mg1.24Ca0.07Na0.12Al0.14)(Si1.963Al0.037)O-6 is slightly enriched in Ca (CaO = 1.74%), Al and Na with respect to enstatite (CaO = 0.71%). Narrow bands of polycrystalline akimotoite with three crystallographic orientations and a chemical composition similar to the Surrounding enstatite intersect the other part of the grain. In a second grain entrained in the shear melt vein, tablets of akimotoite are interwoven with a pyroxene glass with a lower Na and Fe content and a higher Mg content compared to the adjacent akimotoite. This pyroxene glass is probably the product of (Mg,Fe)SiO3-perovskite amorphization. Polycrystalline akimotoite is also formed at the expense of enstatite at the vein wall of the shear melt vein. In that case akimotoite is also enriched in Ca (CaO = 2.85%) Na (NaO = 1.72%) and Al (Al2O3=4.14%). All these observations suggest that akimotoite is mainly formed by solid-state transformation of former pyroxenes with subsequent diffusion of calcium, aluminum and sodium from the chondritic melt of the shear melt vein. Finally, the first complete X-ray diffraction pattern of natural akimotoite is presented. (c) 2008 Elsevier B.V. All rights reserved.

Published
2008
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39. Evidence for hydrothermal venting in Fe isotope compositions of the deep Pacific Ocean through time

Author

Chu, Nan-chin, Johnson, C, Beard, B, German, C, Nesbitt, R, Frank, M, Bohn, Marcel, Kubik, P, Usui, A, Graham, I, Chu, Nan-chin, Johnson, C, Beard, B, German, C, Nesbitt, R, Frank, M, Bohn, Marcel, Kubik, P, Usui, A, and Graham, I

Abstract

Temporal variations in Fe isotope compositions at three locations in the Pacific Ocean over the last 10 Ma are inferred from high-resolution analyses of three hydrogenetic ferromanganese crusts. Iron pathways to the central deep Pacific Ocean appear to have remained constant over the past 10 Ma, reflected by a remarkably constant Fe isotope composition, despite large changes in the Fe delivery rates to the surface ocean via dust. These results suggest that the Fe cycle in the deep ocean is decoupled from that in surface waters. By contrast, one ferromanganese crust from the Izu-Bonin (IB) back-arc/marginal basin of the W. Pacific exhibits large delta Fe-56 variations. In that crust, decreases in delta Fe-56 values correlate with increases in Mn, Mg, Ni, Cu, Zn, Me, and V contents, and consistent with periods of intense hydrothermal input and increased growth rates. A second crust located within 100 km of the first IB sample does not record any of these periods of enhanced hydrothermal input. This probably reflects the isolated pathways by which hydrothermally sourced Fe may have migrated in the back arc, highlighting the high degree of provinciality that Fe isotopes may have in the modem (oxic) oceans. Our results demonstrate that despite efficient removal at the source, hydrothermal Fe injected into the deep ocean could account for a significant fraction of the dissolved Fe pool in the deep ocean, and that hydrothermally sourced Fe fluxes to the open ocean may have lower delta(56) Fe values than those measured so far in situ at hydrothermal vents. Correlation between 656 Fe values and elements enriched in hydrothermal fluids may provide a means for distinguishing hydrothermal Fe from other low-delta Fe-56 sources to the oceans such as dissolved riverine Fe or porewaters in continental shelf sediments. (c) 2006 Elsevier B.V. All rights reserved.

Published
2006
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40. Diffusion induced Li isotopic fractionation during the cooling of magmatic rocks: The case of pyroxene phenocrysts from nakhlite meteorites

Author

Beck, Pierre, Chaussidon, M, Barrat, J.a., Gillet, Ph., Bohn, Marcel, Beck, Pierre, Chaussidon, M, Barrat, J.a., Gillet, Ph., and Bohn, Marcel

Abstract

Ion-microprobe was used to measure Li abundances and isotopic compositions in pyroxenes from three Martian meteorites belonging to the nakhlite family. The profiles performed across augite crystals from Northwest Africa 817 show a large isotopic zoning from crystal cores (delta Li-7 similar to 0 parts per thousand) to rims (delta Li-7 similar to +20 parts per thousand) while Li abundances are almost constant (similar to 9.2 mu g/g). Unlike NWA 817, the pyroxene studied in the Miller Range 03346 nakhlite shows a zoning in Li abundance, with concentrations increasing from similar to 2.5 mu g/g in the core to similar to 9 mu g/ g in the rim. The augite rim (delta Li-7 = +7 parts per thousand) is slightly enriched in Li-7 with regard to the core (delta Li-7 = +4 parts per thousand), but most of the isotopic variations observed occur at an intermediate position along the profile, where 67 Li falls down to similar to-11 parts per thousand. In the case of Nakhla, Li concentrations in augite increase from cores (similar to 3.5 mu g/g) to rims (similar to 6.5 mu g/g), while the delta Li-7 variation is restricted (i.e., between delta Li-7 = +6.0 and +12.6 parts per thousand). For the three meteorites the Li abundances were also measured in the groundmass, which was found to be enriched in lithium (similar to 10 mu g/g). Conventional magmatic and post-magmatic processes such as alteration and fractional crystallization, fail to explain the dataset obtained on nakhlites. Degassing processes, which were previously proposed to explain the Li distribution in shergottite crystals, cannot result in the strong decoupling between Li abundances and isotopic composition observed in nakhlites. We suggest that the original magmatic Li distributions (concentrations and isotopic compositions) in nakhtites have been modified by diffusion of Li from the Li-rich groundmass towards the pyroxene crystals during sub-solidus cooling. Diffusion appears to have been efficient for NWA 817 and MIL 033

Published
2006
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41. Petrology, geochemistry, and cosmic-ray exposure age of lherzolitic shergottite Northwest Africa 1950

Author

Gillet, P, Barrat, J, Beck, P, Marty, B, Greenwood, R, Franchi, I, Bohn, Marcel, Cotten, J, Gillet, P, Barrat, J, Beck, P, Marty, B, Greenwood, R, Franchi, I, Bohn, Marcel, and Cotten, J

Abstract

Northwest Africa (NWA) 1950 is a new member of the lherzolitic shergottite clan of the Martian meteorites recently found in the Atlas Mountains. The petrological, mineralogical, and geochemical data are very close to those of the other known lherzolitic shergottites. The meteorite has a cumulate gabbroic texture and its mineralogy consists of olivine (FO66 to FO75), low and high-Ca pyroxenes (En(78)Fs(19)Wo(2)-En(60)FS(26)W(14); En(53)Fs(16)Wo(31)-En(45)Fs(14)Wo(41)), and plagioclase (An(57)Ab(41)Or(1) to An(40)Ab(57)Or(3); entirely converted into maskelynite during intense shock metamorphism). Accessory minerals include phosphates (merrillite), chromite and spinels, sulfides, and a glass rich in potassium. The oxygen isotopic values lie on the fractional line defined by the other SNC meteorites (Delta(17)O = 0.312 parts per thousand). The composition of NWA 1950 is very similar to the other lherzolitic shergottites and Suggests an origin from the same magmatic system, or at least crystallization from a close parental melt. Cosmogenic ages indicate an ejection age similar to those of the other lherzolitic shergottites. The intensity of the shock is similar to that observed in other shergottites, as shown by the occurrence of small melt pockets containing glass interwoven with stishovite.

Published
2005

42. Characterization of a laser-nitrided titanium alloy by electron backscattered diffraction and electron probe microanalysis

Author

Gordin, D, Gloriant, T, Bohn, Marcel, Guillou, A, Thibon, I, Ansel, D, Gordin, D, Gloriant, T, Bohn, Marcel, Guillou, A, Thibon, I, and Ansel, D

Abstract

After a laser gas nitriding treatment of the Ti-7.5Al (atom %) titanium-based alloy, we used a combination of electron backscattered diffraction (EBSD) in scanning electron microscope and electron probe microanalysis (EPMA) techniques in order to efficiently characterize the different phases in the resolidified layer. Representative measurements of chemical composition and reliable determination of crystal structure were possible for each phase of the complex microstructure. The reaction zone is formed by a mixture of isostructural TiN phases with dendritic and/or 'coarse' needles morphology, fixed into a alpha'-Ti matrix (martensite) with a thin needle aspect. The nitrogen solubility was found to remain very low in the alpha'-Ti matrix (up to 2-3 atom %), while in the TiN phase, an aluminum solubility as high as 4 atom % was measured, reducing drastically the nitrogen content into a Ti79N17Al4 chemical composition. Copyright (C) 2005 John Wiley and Sons, Ltd.

Published
2005
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44. Li isotopic variations in single pyroxenes from the Northwest Africa 480 shergottite (NWA 480): a record of degassing of Martian magmas?

Author

Beck, Pierre, Barrat, J, Chaussidon, M, Gillet, P, Bohn, Marcel, Beck, Pierre, Barrat, J, Chaussidon, M, Gillet, P, and Bohn, Marcel

Abstract

Lithium abundances and isotopic compositions were measured by ion microprobe in individual grains of pyroxene, and in a few maskelynites and Ca-phosphates grains, from the Martian meteorite Northwest Africa 480 (NWA 480). In pyroxenes Li abundances are nearly constant from core to rim with concentrations ranging between 3 and 4 mug/g. In contrast, a significant isotopic zoning is observed with delta(7)Li values increasing within single crystals from similar to -17parts per thousand in the core to similar to +10parts per thousand in the rim, most of the variability being observed in the core. Plagioclase (now maskelynite) and phosphate crystals, which co-crystallized with the pyroxene rims, display similar delta(7) Li values. Because of the incompatible behavior of Li, the present constancy of Li concentrations within zoned pyroxenes rules out any simple crystallization model in a closed system for Li. The large Li isotopic variations observed within pyroxenes support this conclusion. There is no evidence in support of secondary alteration of NWA 480 to explain the Li isotopic variations, which thus most likely reflect magmatic processes on Mars. Degassing might explain the Li systematics observed in NWA 480 pyroxenes. Because Li has a strong affinity with water-rich fluids, a significant loss of Li from NWA 480 parental melt can happen upon melt emplacement and cooling. Such a Li loss could compensate the effect of crystal fractionation and thus help to maintain constant the Li content of the melt. Li isotopic fractionation is anticipated to accompany this process, Li-7 being depleted relative to Li-6 in the volatile phase. The magnitude of the isotopic change of the fractionating melts is difficult to predict because it depends on the value of the Li isotopic fractionation and on the amount of Li loss, but at first glance it seems consistent with the increase of delta(7)Li values observed in NWA 480 pyroxenes with increasing fractionation. The present data suggest

Published
2004
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48. Apport des techniques par rayons X à la caractérisation des encroûtements cobaitifères océaniques

Author

Etoubleau, Joel, Bohn, Marcel, Fouquet, Yves, Cambon, Pierre, Le Suave, Raymond, Etoubleau, Joel, Bohn, Marcel, Fouquet, Yves, Cambon, Pierre, and Le Suave, Raymond

Abstract

A range of X-ray analytical techniques contributes particularly well to the mineralogical and geochemical characterisations of cobalt-rich iron and manganese oxides crusts. X-ray diffraction is used to identify the weak crystallinity of the material. Bulk geochemical characterisation of the crusts is done by X-ray fluorescence for the precise determination of ca. thirty major and trace elements. Fine scale analysis is carried out by electron microprobe to quantitatively map the distribution of elements in oxide phases within the crusts. Microprobe work is essential to obtain the chemical composition of the embedded metallic micro-meteorites.

Published
2000
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