Effets de style au Moyen Âge

International audienceLa notion de style ne va pas de soi. La question se complexifie encore lorsqu’on analyse les productions artistiques du Moyen Âge. Peut-on parler du style médiéval ? La diversité des qualificatifs qui le rattachent étroitement à une époque, à un espace, à une tradition, montre à quel point on reste encore démuni tant pour le définir que pour « inventer » des méthodes d’analyse. La stylistique, discipline universitaire reconnue et célébrée pour sa richesse, ne peut être convoquée telle quelle pour approcher les œuvres médiévales. Il convient donc de s’interroger à nouveau, de se demander comment cerner dans le domaine des études de médiévistique cette notion apparemment familière ? Étant donné d’une part la « mouvance » des textes, les variations multiples d’une œuvre à l’autre, les interactions entre les arts, et d’autre part le caractère traditionnel de la culture médiévale, quelles approches ménager ? C’est à ces questions et à celles qu’elles suscitent que se sont proposés de répondre les auteurs des 30 contributions réunies dans ce volume, à la suite d’un colloque international et pluridisciplinaire (littérature, peinture, sculpture, architecture, musique), fournissant par là même un aperçu vivifiant de l’état actuel de la recherche

Avant-propos

L’équipe du CUER MA a eu plaisir à organiser les 29, 30 et 31 mai 2008 à l’université d’Aix-Marseille un colloque international et pluridisciplinaire intitulé « Effets de style au Moyen Âge ». Les pages qui suivent témoigneront de la richesse des communications qui furent proposées sur un sujet qui n’allait pas de soi malgré la familiarité de la notion que nous soumettions à la sagacité de nos collègues littéraires, historiens de l’art et musicologues. L’intitulé « effets de style » invitait ..

Delta 66Zn ratios and age of carbonates from ODP Site 138-849 (Table 1)

The carbonate fraction of sediment core ODP 849, leg 138, located in the eastern equatorial Pacific, mostly consisting of coccoliths, was separated and analyzed for its Zn isotopic composition. The overall variation in Zn isotopic composition, as determined by multiple-collector, magnetic-sector, inductively coupled plasma mass spectrometry, was found to be on the order of 1‰ (expressed in delta66Zn, where deltaxZn=[(xZn/64Zn)sample/(xZn/64Zn)standard -1]*10**3 and x=66, 67 or 68) over the last 175 ka. The analytical precision was 0.04 per mil and the overall reproducibility was usually better than 0.07 per mil. The Zn isotopic composition signal exhibits several marked peaks and a high-frequency variability. A periodogram of the delta66Zn signal showed two periodicities of 35.2 and 21.2 ka. We suggest that the latter is caused by the precession of the Earth's axis of rotation. The periodogram exhibits a minimum at 41.1 ka, thus showing that the Zn isotopic composition is independent of the obliquity in the eastern equatorial Pacific. The range of delta66Zn values observed for the carbonate fraction of ODP 849 overlaps with the range observed for Fe–Mn nodules in the world's oceans, which suggests that seawater/carbonate Zn isotope fractionation is weak. We therefore assume that most of the Zn isotope variability is a result of the selective entrainment of the light isotopes by organic matter in the surface ocean. The ODP 849 delta66Zn record seems to follow the changes in the insolation cycles. Changes in the late summer/fall equatorial insolation modulate the intensity of the equatorial upwelling, hence the mixing between deep and surface waters. We propose that during decreased summer/fall equatorial insolation, when a steep thermocline can develop (El Niño-like conditions), the surface waters cannot be replenished by deep waters and become depleted in the lighter Zn isotopes by biological activity, thus resulting in the progressive increase of the delta66Zn values of the carbonate shells presumably in equilibrium with surface seawater

Caractérisation du standard d’eau derivière SLRS-5 (NRC-CNRC) compilationinterlaboratoire du silicium, des terresrares et de 21 autres éléments en trace

National audienceLe standard d’eau de rivière naturelle SLRS-5 (NRC-CNRC) est uneréférence de contrôle qualité en analyse d’eau de routine pour la communautéinternationale. Dix laboratoires français étudiant les élémentsmajeurs et en trace dans les solutions naturelles (groupe de travail del’atelier du CNRS « Isotrace ») valident leurs mesures avec les 19 élémentscertifiés par le producteur. Or, d’autres éléments non certifiés sontrégulièrement analysés par ces laboratoires qui ont comparé et compilé2 années de leurs mesures sur ce standard d’eau. La plupart desmesures sont réalisées par ICP-MS (Inductively Coupled Plasma Mass-Spectrometry).Dans un premier temps, il a été vérifié que les résultats obtenus étaientcohérents pour les éléments certifiés par le producteur. Ainsi Ba présenteune valeur compilée de 14,2 1,4 g/l (330 mesures issues de 9laboratoires participants) en accord avec la valeur certifiée de 14 0,5g/l.Ensuite, des valeurs moyennes et leurs incertitudes associées sont proposéespour des éléments non certifiés par NRC-CNRC tels que le siliciumet 35 éléments en trace (terres rares, Ag, B, Bi, Cs, Ga, Ge, Li, Nb,P, Rb, Rh, Re, S, Sc, Sn, Th, Ti, Tl,W, Y et Zr). Le rapport isotopique deSr est aussi proposé à partir de mesures par TIMS (Thermo IonisationMass Spectrometry).Il apparaît que les mesures individuelles et compilées varient dequelques ng/l à 2400 g/l avec des incertitudes associées comprisesentre 2 et 240%. Les fortes incertitudes associées ne sont pas proportionnellesaux concentrations puisque certaines mesures s’accordent mêmeà de faibles teneurs tels que Bi (0,9 0,3 ng/l valeur compilée). Danscertains cas, un ou deux laboratoires ont caractérisé l’élément impliquantmoins de mesures et une incertitude associée plus élevée quelleque soit la concentration en l’élément. C’est le cas, par exemple, de Ge(0,015 0,014 g/l valeur compilée) caracterisé par seulement 2 laboratoires(34 mesures) et de P (8,19 3,40 g/l) issu de 8 mesuresd’un seul laboratoire. Les terres rares et onze autres éléments (B, Bi,Cs, Li, Rb, Si, Th, Ti, Tl, Y et Zr) ont été déterminés par au moins 3laboratoires différents et apportent des informations complémentairesessentielles à l’utilisatio

Early Archean serpentine mud volcanoes at Isua, Greenland, as a niche for early life

International audienceThe Isua Supracrustal Belt, Greenland, of Early Archean age (3.81-3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate- rich, high- pH hydrothermal solutions at medium temperature (100-300 degrees C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids

Metamorphic diamonds in a garnet megacryst from the Edough Massif (northeastern Algeria). Recognition and geodynamic consequences

International audienceWe report for the first time the discovery of diamond inclusions (5–30 μm in size), identified by Raman spectroscopy and the characteristic sharp band at 1332 cm− 1 for crystalline diamond, in a garnet megacryst (≥ 5 cm) from the Edough Massif (NE Algeria). The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite–ultramylonite band. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. The host garnet is almandine-dominant and is rich in exsolution of acicular rutile needles. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300 μm in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724–778 °C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥ 3.6 GPa that were reached during subduction of the UHP-rock precursors. We suggest that the mafic protolith originates from the subducted retreating Calabrian branch of the Tethyan slab, that broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by formation of the Edough dome and opening of the Algerian basin

Discovery of metamorphic diamonds in garnets from the Edough massif (Northeastern Algeria): LA-ICP-MS U-Pb constraints

International audienceInclusions in a garnet megacryst (≥ 5 cm) from the Edough Massif (NE Algeria) have been studied and we report for the first time the discovery of diamond inclusions (5-30 micrometer in size), identified by Raman spectroscopy and the characteristic sharp band at 1332 cm-1 for crystalline diamond. The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite-ultramyloniteband. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. This 1 km thick slab of amphibolites and pyrigarnites, derived from layered gabbros of MORB affinity, rest above the granite-gneiss units from the core complex. Garnet-forming reactions and dehydration melting are the oldest metamorphic stages recognized in the Kef Lakhal mafic complex (≥ 800 ◦C, ≥ 17 Kbar). The diamond-bearing garnets display a dense rutile acicular network interpreted as exsolutions. Zircons inclusions have been also observed in this garnet megacryst. Diamond crystals, up to 50 micrometer across, were identified first by optical microscopy and later by Raman spectroscopy. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300 micrometer in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724-778 ◦C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥ 3.6 GPa that were reached during subduction of the UHP-rock precursors. In situ U-Pb dating obtained on rutile inclusions yield a 32.4+/ 3.3 Ma interpreted as the age of the UHP metamorphic event. LA-ICP-MS U-Pb ages on zircon provide a 20.9 +/- 2.2 Ma age attributed to the thrusting onto the North-African margin. We suggest that the mafic protolith of the analyzed garnet megacryst originates from the subducted retreating Tethyan slab, which broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by building up of the Edough dome and opening of the Algerian basin

Permo-Carboniferous and early Miocene geological evolution of the internal zones of the Maghrebides - New insights on the western Mediterranean evolution

International audienceGeochemical and geochronological analyses were performed on metamorphic rocks from the footwall of an oceanic unit thrusted onto the North African margin. The footwall is mainly constituted by metamorphosed sediments and interlayered mafic rocks with scarce ultramafic lenses. Major and trace elements geochemistry of the sediments is consistent with a passive margin setting (16.9 < K2O/Na2O < 20.9; 18.2 < Zr/Th < 24.6). U–Pb analyses of detrital zircons indicate that deposition of the sedimentary sequence is younger than 307 ± 7 Ma (2σ) and concur with a provenance of the detritus from the Eastern Meseta and High Atlas of Morocco. The age of a crosscutting orthogneiss (281 ± 4 Ma) further constrains deposition to the upper Carboniferous/lower Permian times (307–281 Ma), in good agreement with the 295 ± 2 Ma age of an interbedded amphibolite. Geochemistry of the mafic and ultramafic rocks displays subduction related features (LILE and LREE enrichments, HFSE depletion). Pb, Sr, Nd and Hf isotopes further indicate a depleted mantle reservoir modified by a continental crust-derived component brought into the mantle during an ancient subduction event. The range of isotopic compositions and geochemical signatures displayed by the Carboniferous mafic rocks suggest they inherited their geochemical characteristics during melting of a heterogeneous lithospheric mantle containing variable metasomatic additions. Geochronological results indicate that the HT metamorphic evolution of this upper Paleozoic sequence resulted from two distinct events, each followed by fast cooling. The first one, dated at 20.85 ± 0.34 Ma (2σ), followed HP metamorphism and thrusting of the Kef Lakhal oceanic complex onto the Northern margin of Africa. The second event occurred at 17.65 ± 0.46 Ma (2σ) and is coeval with exhumation and anatexis of the lower crustal units of the neighbouring Edough dome. Fast cooling of the footwall rocks, through the closure temperatures of muscovite and biotite, occurred synchronously at 16.94 ± 0.10 Ma (2σ) and 17.02 ± 0.19 Ma (2σ) respectively. This study documents a further example of the complex history recorded by rocks of the Peri-Mediterranean area that contains an imbrication of geological units, witnesses of a protracted history related to Permo-Carboniferous and Miocene events

Abundance of zinc isotopes as a marine biogeochemical tracer

International audienceThe Zn isotopic compositions of ferromanganese nodules, sediment trap samples, sediments, and organic reference samples have been analyzed by multiple collector inductively coupled plasma-mass spectrometry (ICP-MS). The range of isotopic variations (1% for Zn-66/Zn-64) is significant with respect to the analytical precision (0.04% at the 95% confidence level). Marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from Reunion Island. We infer that the depletion of the light Zn isotopes in marine particles and in ferromanganese nodules (presumably equilibrated with seawater) results from biological activity in the upper water column. The delta Zn-66 values of sediment trap samples collected near the upwelling off the coast of Mauritania (central Atlantic) show a seasonal isotopic fluctuation consistent with biological pumping during the high-productivity period. Higher values of delta Zn-66 in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity