Du partenariat à la controverse. Intervention des autorités judiciaires et remise en cause d'une 'auto-gestion' des risques : le cas des activités sportives de montagne

International audienceUne réflexion sur les transformations du système d’acteurs associés à la question des risques liés aux activités sportives de montagne, et notamment l’apparition de nouveaux « entrants » (acteurs judiciaires, associations de victimes)

The stability and structure of primordial reservoirs in the lower mantle: insights from models of thermochemical convection in three-dimensional spherical geometry

Large-scale chemical lateral heterogeneities are inferred in the Earth's lowermost mantle by seismological studies. We explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time, by using similar analysis in 3-D spherical geometry. In this study, we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle, including the chemical density contrast between the primordial dense material and the regular mantle material (buoyancy ratio), thermal and chemical viscosity contrasts, volume fraction of primordial dense material and the Clapeyron slope of the phase transition at 660 km depth. We find that most of the findings from the 3-D Cartesian study still apply to 3-D spherical cases after slight modifications. Varying buoyancy ratio leads to different flow patterns, from rapid upwelling to stable layering; and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle. Chemical viscosity contrasts in a reasonable range have a second-order role in modifying the stability of the dense anomalies. The volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts, but has significant effects on calculations with intermediate and small thermal viscosity contrasts. The volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast. An endothermic phase transition at 660 km depth acts as a ‘filter' allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantl

Temperature and heat flux scalings for isoviscous thermal convection in spherical geometry

Parametrized convection, which has long been used to reconstruct the thermal history of planetary mantles, is based on scaling relationships between observables (including heat flux) and controlling parameters (the most important being the Rayleigh number, Ra). To explore the influence of spherical geometry on heat transfer, we have conducted two series of numerical experiments of thermal convection (one with bottom heating and the other with mixed heating) in an isoviscous spherical shell with various curvatures. Using these calculations and a generalized non-linear inversion, we then derive scaling laws for the average temperature and for the surface heat flux. In the case of bottom heating, we found that the non-dimensional average temperature is given by θm=f2/(1 +f2), where f is the ratio between the core and total radii. The non-dimensional surface heat flux is fitted well by Nutop= 0.36f0.32 Ra(0.273+0.05f)θ0.6m. This scaling indicates that the available heating power decreases with increasing curvature (decreasing f). There exist strong trade-offs between the inverted parameters, that is, different sets of parameters explain our calculations well within error bars. For mixed heating, the non-dimensional average temperature and surface heat flux are well explained by θH=θm+ (1.68 − 0.8f)[(1 +f+f2)/3]0.79 h0.79/Ra0.234, where h is the non-dimensional rate of internal heating, and Nutop= 0.59f0.05 Ra(0.300−0.003f)θ1.23H. Due to a competition between the radiogenic and convective powers, and for given values of h and Ra, there is a curvature for which the Urey ratio reaches a minimum. Applied to the Earth's mantle, the mixed heating scaling predicts a Urey ratio between 0.4 and 0.6, depending on the Rayleigh number. Additional parameters, including the thermal viscosity ratio, phase transitions, the presence of dense material in the deep mantle, and variability of the flow pattern in time, may enter an appropriate modelling of the Earth's mantle thermal histor

Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state

The heterogeneous precipitate microstructure of a AA2050 Al–Li–Cu in the T8 state after friction stir welding has been mapped by small-angle X-ray scattering (SAXS). 2D resolved maps of the fraction and size of both T1 platelets precipitates and clusters/GP zones formed at room temperature are provided. TEM micrographs of selected zone confirm the interpretation of SAXS intensities. This microstructure mapping is compared to microhardness mapping and a direct correlation is shown. Short duration heat treatments made in a salt bath help understanding precipitate stability and suggest that the temperature exploration alone explains to a large extent the distribution of the precipitates microstructure across the welded structure

Clustering kinetics during natural ageing of Al-Cu based alloys with (Mg, Li) additions

Room temperature solute clustering in aluminium alloys, or natural ageing, despite its industrial relevance, is still subject to debate, mostly due to its experimentally challenging nature. To better understand the complex multi-constituents' interactions at play, we have studied ternary and quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and Al-Cu-Li-Mg. We used a recently introduced correlative technique using small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the chemically resolved kinetics of room temperature clustering in these alloys, which we completed with differential scanning calorimetry (DSC) and micro-hardness measurements. The comparison of the clustering behaviours of each subsystem allowed us to highlight the paramount role of Mg as a trigger for diffusion and clustering. Indeed, while a strong natural ageing was observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very slight addition of Mg (0.4%) to this system, however, drastically changed the situation to a rapid formation of essentially Cu-rich hardening clusters, Mg only joining them later in the reaction. This diffusion enabling effect of Mg is discussed in terms of diffusion mechanism and complex interactions with the quenched-in vacancies

Les plantes fossiles du gisement hettangien de Talmont-Saint-Hilaire (Vendée, France) : intérêts systématique et paléoécologique

The Liassic Talmont-Saint-Hilaire (Vendée, France) outcrop has been studied here according to the abundance of fossil plants of the Cheirolepidiaceous family (Gymnosperms, Coniferales). A new species of Brachyphyllum has been studied in light microscopy, even in scanning electron microscopy and transmission electron microscopy. It is, at that time, one of the most complete study realized on a fossil taxon. Epidermal cells, subsidiary cells and guard cells have been described on both adaxial and abaxial surfaces, in transversal and longitudinal sections. Cuticle ultrastructural variations have been observed between epidermal and stomatal cells. To conclude, we propose a discussion about the cuticle structure variations related to environmental conditions and the functional morphology of the different cells

Azimuthal anisotropy of Rayleigh-wave phase velocities in the east-central United States

We explore the Rayleigh-wave phase velocity structure of the east-central US in a broad period range (10-200 s). Using a recent implementation of the two-stations method, we first measure interstation dispersion curves of Rayleigh-wave phase velocities along 60 paths. We then invert our collection of dispersion curves for isotropic and azimuthally anisotropic (2Ψ and 4Ψ) phase-velocity maps. The inversion is performed by a damped, smoothed LSQR, and the output model is parametrized on a triangular grid of knots with a 140 km grid spacing. Using the isotropic component of the phase velocity maps to constrain regional variations in shear velocity and Moho-depth, we observe that over the upper-middle crust depth range (z 1 per cent), and the azimuth of the fast-propagation direction is uniform over the entire region and equal to 54°. Our results suggest that azimuthal anisotropy beneath the east-central US is vertically distributed in three distinct layers, with a different geodynamic origin for each of the

Estimating core-mantle boundary temperature from seismic shear velocity and attenuation

The temperature at Earth’s core-mantle boundary (CMB) is a key parameter to understand the dynamics of our planet’s interior. However, it remains poorly known, with current estimate ranging from about 3000 K to 4500 K and more. Here, we introduce a new approach based on joint measurements of seismic shear-wave velocity, VS, and quality factor, QS, in the lowermost mantle. Lateral changes in both VS and QS above the CMB provide constraints on lateral temperature anomalies with respect to a reference temperature, Tref, defined as the average temperature in the layer immediately above the CMB. The request that, at a given location, temperature anomalies inferred independently from VS and QS should be equal gives a constraint on Tref. Correcting Tref for radial adiabatic and super-adiabatic increases in temperature gives an estimate of the CMB temperature, TCMB. This approach further relies on the fact that VS-anomalies are affected by the distribution of post-perovskite (pPv) phase. As a result, the inferred Tref is linked to the temperature TpPv at which the transition from bridgmanite to pPv occurs close to the CMB. A preliminary application to VS and QS measured beneath Central America and the Northern Pacific suggest that for TpPv = 3500 K, TCMB lies in the range 3,470–3880 K with a 95% likelihood. Additional measurements in various regions, together with a better knowledge of TpPv, are however needed to determine a precise value of TCMB with our method

Development of compositional-gradient metallic alloys for combinatorial investigation of microstructures

International audienc

Use of space-resolved in-situ high energy X-ray diffraction for the characterization of the compositional dependence of the austenite-to-ferrite transformation kinetics in steels

In-situ high energy X-Ray diffraction (HEXRD) was used on compositionally graded steels to study the effect of substitutional elements on ferrite growth kinetics in Fe-C-X and Fe-C-X-Y systems. Two systems have been selected to illustrate the applicability of the combinatorial approach in studying such transformations, Fe-C-Mn and Fe-C-Mn-Mo. Comparison between the measured ferrite growth kinetics using HEXRD and the predicted ones using Para-Equilibrium (PE) and Local Equilibrium with Negligible Partitioning (LENP) models indicates that the fractions reached at the stasis of transformation are lower than the predicted ones. Experiments indicated a deviation of measured kinetics from both PE and LENP models when increasing Mn and decreasing Mo (in Fe-C-Mn-Mo system). The large amount of data that can be obtained using this approach can be used for validating existing models describing ferrite growth kinetics