Contrôle structural du partage de Rb entre fluide et micas potassiques dans les pegmatites du Brésil

A geochemical study of pegmatitic micas from Minas Gerais State in Brazil was performed with an electron microprobe, in order to examine the variations of Rb, K, Al and F contents. It is observed a linear decreasing relationship between the [Rb/K] ratio of the micas and their contents in Al^VI. The interpretation is based on the hypothesis that the partition coefficient C_Rb/K^mica/fl between fluid and mineral does not vary significantly as a function of temperature and pressure in the narrow conditions of crystallization of pegmatites. It is suggested that the relation: C_Rb/K^mica/fl=0.55×(5?[Al^VI]) is of crystallochemical order. Micas with low contents in Al take higher contents in Rb because the potassic sites where Rb enters are larger. This relation gives another way of calculating the values [Rb/K] of the fluids knowing Rb, K, Al and Si in the micas. This crystallochemistry also allows us to foresee a direct correlation between Rb and F in the pegmatitic micas. To cite this article: J. Quéméneur et al., C. R. Geoscience 336 (2004).     

Phase de Laves dans la première opale CT bidisperse

A Mexican opal from the state of Jalisco has a bidisperse, perfectly ordered structure with framboid silica lepispheres of two different diameters. Its structure has been determined by the observation of a vicinal section, close to a dense plane. Fresh breaks along a vicinal plane emphasized regular steps, which were studied by means of SEM. We have been able to propose unit cell models, theoretical positions of the two sets of spheres, and probable space group, P6₃/mmc. This structure arranges similarly to that of a diatomic compound having an AB₂ stoichiometry and corresponds to a cubic (MgCu₂-type) or hexagonal (MgZn₂-type) Laves phase. This last variety seems to be the most probable structure. To cite this article: J.-P. Gauthier et al., C. R. Geoscience 336 (2004).     

Natural gas hydrates: myths,facts and issues

Gas hydrates are solid-like substances naturally occurring beneath the oceans and in polar regions. They contain vast, and potentially unstable, reserves of methane and other natural gases. Many believe that, if released in the environment, the methane from hydrates would be a considerable hazard to marine ecosystems, coastal populations and infrastructures, or worse, that it would dangerously contribute to global warming. On the other hand, hydrates may contain enough natural gas to provide an energy supply assurance for the 21st century. This paper attempts to separate the myths, the facts and the issues that relate to natural gas hydrates beyond the doomsday environmental scenarios and overly optimistic estimates. To cite this article: B. Beauchamp, C. R. Geoscience 336 (2004).     

The Liuchiu Hsu island offshore SW Taiwan: tectonic versus diapiric anticline development and comparisons with onshore structures

A structural and microtectonic analysis performed in the Liuchiu Hsu island demonstrates that its Plio-Pleistocene tectonic evolution was dominated by alternating NW–SE shortening and local radial extension caused by mud diapirism. Previous models based on seismic data considering both the formation of the Liuchiu Hsu island and the fold development in SW Taiwan as mainly driven by mud diapirism, fail to account for both the asymmetry of the west vergent thrust-related anticlines onshore and the elongated character of the ridges formed by diapir alignments offshore, which rather argue in favour of a tectonic origin. To cite this article: O. Lacombe et al., C. R. Geoscience 336 (2004).     

Caractérisation de zones sources de DNAPL à l'aide de traceurs bisolubles : mise en évidence d'une cinétique de partage

Pollution source characterization by the Partitioning Interwell Tracer Test method requires the assessment of the partitioning coefficient of the tracers with the pollutant. Experiments conducted on batch tests (static) and laboratory columns (dynamic) indicated a partitioning kinetics between the reactive tracer (4-methyl 2-pentanol) and the organic phase (trichlorethene); the partitioning coefficient decreases when the flow velocity through the porous medium increases. This partitioning kinetics has been confirmed by the interpretation of the tracer breakthrough curves with a first-order kinetic transfer model. To cite this article: M. Bohy et al., C. R. Geoscience 336 (2004).     

Les formations burdigaliennes de Corse orientale dans leur cadre géodynamique

The earliest marine sediments of eastern Corsica, linked to the birth of the Corsica Basin, are represented by granitic breccia of the Saint-Antoine Formation and pelagic marls of the Alzelle Formation. They are dated as Mid-Burdigalian by nannoplankton and planktonic foraminifera (relative age and grade datings between 18.7 and 18.3 Ma). The Aghione Formation (Latest Burdigalian–Langhian) lies upon the Alzelle Formation. A component of the major East-Corsican faults, the Saint-Antoine Fault, underwent extensional activity during the Burdigalian and until the Late Miocene, when occurred the uplift of the Castagniccia antiform. To cite this article: M.-D. Loÿe-Pilot et al., C. R. Geoscience 336 (2004).     

UPb emplacement and 40Ar/39Ar cooling ages of the eastern Mont-Louis granite massif (Eastern Pyrenees,France)

Zircon UPb dating by SIMS of the Mont-Louis granite yields an age of 305±5 Ma, intrepreted to reflect the igneous emplacement age of the massif. It is in agreement with the Hercynian syntectonic character of Pyrenees granite. ⁴⁰Ar/³⁹Ar on hornblende, biotite and K-feldspar permit, to estimate the massif cooling. A rapid temperature decrease (≈30 °C/Ma) is revealed from Westphalian to Late Stephanian, coeval with the emplacement of a laccolithe in the upper crust. Then, the cooling rate decreases to ≈1 °C/Ma. This would be consistent with a long time residence for the pluton from the Late Palaeozoic to the Early Cainozoic at 6–8 km depth. To cite this article: O. Maurel et al., C. R. Geoscience 336 (2004).     

Geometry and kinematics of early Alpine nappes in a Briançonnais basement (Ambin Massif,Western Alps)

Petrological and structural observations from the Ambin pre-alpine basement dome and from its Briançonnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+?D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).     

Les surfaces boisées à l'échelle de la planète : usages conjoints pour la séquestration du carbone et la production d'énergie

The role of forest ecosystems in the regulation of greenhouse effect at the global scale is developed here, from two points of view, sometimes considered as opposed: carbon storage and wood production for energy. A nomenclature is proposed to understand all the various mechanisms implied in carbon storage. A comparison is made between the effects on carbon emissions of storage alone and storage with wood fuel production. Use of wood energy is proved to be a ‘bonus’ that could optimise, in the middle and long terms, the use of fossil fuel reserves. To cite this article: A. Prieur et al., C. R. Geoscience 336 (2004).     

Eaux fortement minéralisées et circulations profondes dans le socle. Exemple des Alpes franco-italiennes

The presence in the French–Italian Alps of several springs characterized by a high sodium–chloride content, depleted in oxygen-18, tritium free and with low carbon-14 activity, is well known. The geological, hydrological, and geochemical studies in progress for the new high-speed railway tunnel crossing the French–Italian Alps, show, in the drilled boreholes at a mean elevation of 700 masl, the presence of brines. Brines appear to be the basis of the geological, structural, and hydrodynamic systems, and fill the largest faults and the main tectonic contacts. The brines squeezed by tectonic activity, are mobilised at depth by freshwaters from the surface. To cite this article: G.-M. Zuppi et al., C. R. Geoscience 336 (2004).