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Séminaire internes 2018-2019

Contact : Guillaume Paris (gparis@crpg.cnrs-nancy.fr)

- 17 septembre 2018 : Céline Baudouin - Alkaline Volcanism associated with early stage of rifting : origin, differentiation & trace element partitioning

East African Rift exposes different stages of extension, from early stage rifting in Tanzania (Natron & Manyara basin) to oceanic accretion in Afar (Ethiopia). Due to its location and tectonic setting, Tanzanian volcanoes offers the opportunity to study the earliest stage of rift initiation and to constrain the origin of alkaline-CO2-rich magmas and magmatic evolution of the silica-undersaturated melts as well as associated carbonatitic magmatism.

- 24 septembre 2018 : Marine Boulanger - Melt migration and interactions in the lower oceanic crust : insights from Atlantis Bank interlayered series at IODP Hole U1473A (Southwest Indian Ridge)

Oceanic crust accretion processes at slow-spreading ridges are yet insufficiently constrained, especially for the lower crust. One of the key challenges is to understand how melts migrate through the lower crust and in what extent they interact with surrounding lithologies. For that purpose we studied a cumulate portion of lower crust exhumed at the Atlantis Bank Oceanic Core Complex, and sampled by drilling during IODP Exp. 360. The cumulate olivine gabbro presents intense layering which origins gave us new insights on magma migration, emplacement, crystallization and interaction processes. We found that the overall sequence displays melt/rock reactions or homogenization overprints developing at all stages of formation of the cumulate. This suggests that interaction processes, and thus the migration of melts in this heterogeneous environment are two of the prevailing key parameters controlling the crust and eruptive material composition.

- 22 octobre 2018 : Guillaume Florin - Influence of redox processes on germanium isotopic composition of ordinary chondrites

Ordinary chondrites (OCs) are undifferentiated meteorites composed of an assemblage of primitive components that underwent thermal metamorphism on their parent body. The three H, L, LL groups of OCs are distinguished by an increase in oxidizing conditions from the H group to the most oxidized L and LL groups. That has been highlighted by Δ17O-%Fa correlation and decrease in metal content from H ( 8%) to LL ( 2%). They are also characterized by an increase in germanium content, a moderately siderophile and volatile element, which is also sensitive to redox conditions. All these features have led authors to argue that the three OCs groups would at least come from three different parent bodies. On each OC parent body, decay of 26Al and 60Fe triggers thermal metamorphism and results in texture, mineralogy and chemistry modifications. They define petrographic types that range from the unequilibrated type 3 to the highly equilibrated type 6. This has been modeled in an "onion shell" structure, or through impact and reassembly mechanisms. In order to precise thermodynamic conditions of formation, as well as nebular versus parent bodies processes, we have investigated germanium elemental and isotopic variations of metal, silicate and sulfide phases in H, L and LL ordinary chondrites.

- 5 novembre 2018 : Camille Litty - Fluvial Sediment Dynamics and Erosional Mechanisms in the Western Peruvian Andes

Rivers have been recognized as one of the primary agents of erosion and controls on landscape evolution. During mountain denudation, rivers incise into bedrock, therbeby controlling the hillslope erosion processes. They also convey the erosion products supplied from upstream to sedimentary basins. In mountainous streams, coarse and fine sediment charateristics bear crucial information about the transport dynamics, the flow strengths of these streams, the sources of sediment, the mechanisms of erosion and transport and the controlling conditions such as uplift and precipitation. I will present some case studies allowing to understand the sediment routing through time and through various climatic conditions in Western Peru.

- 12 novembre 2018 : Apolline Mariotti - Impact of glacial-interglacial cycles on denudation rates in the Western Mediterranean

The interaction between climatic variations and surfaces processes (namely denudation) is still poorly understood (Herman and Champagnac, 2016 vs Willenbring and Jerolmack, 2016) and up to debate. As a way to constrain this interaction, we use high-resolution data of denudation rates, applied to archives that precisely document the effect of glacial-interglacial cycles, independently from the tectonic forcing. We use 10Be to quantify present denudation in the Var watershed (southern French Alps) as well as paleo-denudation rates for this same watershed preserved in two well documented sediments cores in the Mediterranean Sea. We also adresse methodology questions such as what is the seasonal effect on the cosmogenic 10Be concentrations and does the sediment granulometry have a predictable effect on the cosmogenic nuclides concentrations in quartz.

- 26 novembre 2018 : Antoine Crémades

- 10 décembre 2018 : Carlotta Ferrando - Melt transport and assimilation-precipitation processes through the heterogeneous lower oceanic crust : microstructural and petro-geochemical constraints from drill cores

About 55% of the mid-ocean ridge system spread at rates lower than 40 mm/y. There, the slow-spread oceanic crust presents an extremely heterogeneous structure, which is partly composed of serpentinized mantle rocks intruded by multiple gabbroic bodies. It is locally exposed at Oceanic Core Complexes (OCC), where in situ oceanic crust can be sampled. During my PhD thesis I studied two gabbroic sequences drilled at the Atlantis Massif OCC (30°N, Mid-Atlantic Ridge) and the Atlantis Bank OCC (32°S, 57°E, Southwest Indian Ridge). The combined macro- and microscopic observations I performed, together with geochemical analyses and numerical modelling, allowed me to investigate the magmatic and cooling processes that shape the oceanic crust formed at slow-spreading ridges.

- 17 décembre 2018 : Simon Couzinié - Decoding the zircon age-hafnium record of crust evolution : a case study from the French Massif Central

The continental crust has been extracted from the Earth mantle for at least 4 Ga but the mode and pace of new crust production through time both remain highly debated. In the past 10 years, extensive zircon U–Pb–Hf(–O) isotope databases have been interrogated to unravel the extent of crust production vs. reworking at a given time period. We examine the limitations of this approach by reconstructing the evolution of the crust segment today exposed in the eastern part of the French Massif Central. Our results identify two major inconsistencies between the geological record and the signal derived from statistical analysis of zircon U–Pb–Hf(–O) isotope data considered out of any petrological context, as generally performed in studies investigating crust evolution. Consequently, we posit that the global zircon record bears equivocal information which can only be resolved when additional constrains on the zircon host rocks are available.

publié mercredi 14 août 2013