Episyénitisation dans le Gisement d'Uranium du Bernardan (Marche): Comparaison avec des Gisements Similaires du Nord-Ouest du Massif Central Français

Résumé Le gisement d'uranium du Bernardan exploité dans le granite à deux micas de la Marche Occidentale appartient au groupe des gisements «d'imprégnation». La minéralisation uranifère (pechblende, coffinite, «produits noirs», autunite, ...) est disseminée dans une roche vacuolaire appelée «épisyénite». Cette dernière résulte d'une altération hydrothermale du granite par des solutions chaudes (370–260°C; 2–5,7% poids équiv. NaCl). Le quartz est dissout; les feldspaths (principalement les plagioclases) et les biotites sont partiellement transformés en phengites. Les muscovites et orthoses granitiques se réequilibrent avec perte de sodium. Cette altération est guidée par un réseau de fractures à différentes échelles. Une altération comparable a été observée dans d'autres gisements du Nord Ouest du Massif Central français, comme Margnac et Fanay dans le massif de St Sylvestre et Hyverneresse dans celui du Millevaches. Les transformations minéralogiques, à l'exception de la quantité de phengites secondaires, les changements chimiques (lessivage de silice et métasomatisme potassique), les conditions P-T et le contrôle tectonique sont les mêmes dans tous ces gisements.

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The Bernardan uranium deposit, mined in the Marche Occidentale two-mica granite, is a disseminated type deposit. Similar deposits are known in hercynian belt. Uraniferous mineralizations (pitchblende, coffinite, black minerals, autunite, ...) are disseminated in a vuggy rock called episyenite. It results from an hydrothermal alteration of the granite by hot aqueous solutions (370 to 260°C, 2–5.7 wt% equiv. NaCl). Quartz is dissolved; feldspars (mainly plagioclases) and biotites are partially transformed into phengites. Granitic muscovites and orthoclases are reequilibrated with loss of sodium. This alteration is controlled by fractures at several scales. Such an alteration can be observed in other deposits of the North West part of the French Massif Central, as Margnac and Fanay in the St Sylvestre massif and Hyverneresse in the Millevaches massif. Mineralogical transformations except the content in secondary phengites, chemical changes (SiO₂ decrease and potassic metasomatism), P-T conditions and tectonic control are the same in all these deposits.

     

Equilibrium state of diopside-bearing harzburgites from ophiolites: Geobarometric and geodynamic implications

The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct magmatic diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioK _f=(X _Di ^opx )/(1 –X _Di ^cpx ), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularX _En ^cpx solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges.     

A Seeing Monitor for Solar and Other Extended Object Observations

In this article I describe a site survey facility, which measures the signals of the solar equivalent of a Differential Image Motion Monitor (S-DIMM) and of a six element linear array of solar scintillometers. Combining the S-DIMM r _o andscintillometer _I observations allows the determination of the fractions of the seeing in the free atmosphere and in the ground/lake layer. From the scintillometer array observations C _n ² (h) is determined for heights corresponding to the first 500 m along the line-of-sight. With minor changes this seeing monitor can also be used for other extended objects like the Moon and planets.     

One-station estimates of seismic moments from the mantle magnitudeM m: The case of the regional field (1.5°≤Δ≤15°)

We extend to the regional field of distances the procedure of one-station estimation of seismic moments using the mantle magnitudeM _m, as introduced earlier in the case of teleseismic events. A theoretical analysis of the validity of the asymptotic expansion of normal modes in terms of surface waves, which was used in the development ofM _m, upholds the validity of the algorithm for distances as short as 1.5°. This is confirmed by the analysis of a dataset of 149 GEOSCOPE records obtained at distances ranging from 1.5 to 15°, from earthquakes with moments between 10²⁴ and 2.5×10²⁷ dyn-cm. The performance ofM _m as measured in terms of average residual with respect to published values ofM ₀, and standard deviation of the residuals, is not degraded in this distance range, with respect to the teleseismic case. This indicates that the mantle magnitudeM _mcan be reliably used at regional distances, notably for tsunami warning applications.     

The product spectra of gravity and barometric pressure in Europe

A total of 111 000 hourly values of gravity and barometric pressure from stations in Europe is analysed. The data consist of two sets of records from Brussels, an early set of 36 000 h length and a more recent set of 21 000 h length, a set of records from Bad Homburg of 24 000 h length, and a set of records from Strasbourg of 30 000 h length. All of the gravity measurements were made with similar superconducting instruments and the pressure data were recorded simultaneously at each superconducting gravimeter site. The four sets of records have different time bases, and to bring out common features and suppress individual station systematic errors, the product spectrum is introduced. Spectral density estimates are first computed for a common spectral window for each record, and the product spectrum is formed by multiplying individual spectral estimates across records. The cumulative distribution function is found for the product spectrum and confidence intervals are calculated from it by iteration. The product spectrum in gravity reveals a triplet of resonances in the subtidal band which are shown by an automated computer search to be uniquely associated with the translational modes of the solid inner core. The product spectrum in barometric pressure clearly reveals the first 10 solar heating tides in the atmosphere, but otherwise does not show common features with the gravity product spectrum. In particular, the triplet of resonances in the subtidal band of the gravity product spectrum do not show up in the product spectrum of barometric pressure, climinating the atmosphere as their source.     

Les tholeiites a affinité alcaline du secteur ouest du Siroua (Anti-Atlas central, Maroc): temoins d'une distension au Néoproterozoïque

The Upper Proterozoic granitoids of the western part of the Siroua Massif (central Anti-Atlas, Morocco) are cut by doleritic dykes injected along submeridian and equatorial directions. These dykes were emplaced before deposition of late Neoproterozoic formations (PIII). According to and incompatible element ratios, these dolerites constitue two discrete chemical groups unrelated to orientation and thickness. They are both enriched in large ion lithophile elements, Zr, Th and light rare earth elements, and correspond to continental tholeiites displaying an alkaline affinity. This alkaline affinity is also confirmed by clinopyroxene and amphibole chemistry. Emplacement of these doleritic dykes occurred under an extensional regime active in the Anti-Atlas range during the Upper Proterozoic.     

Holographic interferometry study of the dissolution and diffusion of gypsum in water

We have performed holographic interferometry measurements of the dissolution of the (0 1 0) plane of a cleaved gypsum single crystal in pure water. These experiments have provided the value of the dissolution rate constant k of gypsum in water and the value of the interdiffusion coefficient D of its aqueous species in water. D is 1.0 × 10⁻⁹ m² s⁻¹, a value close to the theoretical value generally used in dissolution studies. k is 4 × 10⁻⁵ mol m⁻² s⁻¹. It directly characterizes the microscopic transfer rate at the solid-liquid interface, and is not an averaged value deduced from quantities measured far from the surface as in macroscopic dissolution experiments. It is found to be two times lower than the value obtained from macroscopic experiments.     

A Twilight Zone: Identity Crisis in French Geopolitics

The paper initially describes some of the most important historical processes that have conditioned France's geopolitical position in the contemporary world. It discusses the current changes in France's geopolitics as a changing reality of international relations and as a changing discourse of scholars and experts, that reflects those real world changes. The paper focuses on French politics in Africa and on France's policy in Europe after the end of the Cold War. In Africa the French post-colonial empire with its close ties between domestic regimes, French business interests and the French state becomes unsustainable after the bipolar rivalry has been suspended. In Europe French policy can no longer be presented as either an independent view among a series of others (Europe des patries) nor as a successful effort to press French views by way of a larger European entity. The paper presents an overview of geopolitical literature written in France as a reflection of these conditions.