Sur trois principes généraux de la Métallogénie et de la recherche minérale

Des publications récentes conduisent à insister sur trois démarches, ou trois principes fondamentaux sans lesquels la Métallogénie (comme la Pétrologie) et la recherche minérale ne peuvent progresser. 1. Sortons des «types» connus et posons en principe que tout élément chimique peut se concentrer à diverses étapes du cycle géologique et géochimique; recherchons donc ces concentrations et traquons les par tous les moyens minéralogiques et géochimiques à notre disposition. 2. Récusons la confusion des concepts chronologiques et des concepts spatiaux, et en particulier la superposition totalement erronée d'«épigénétique» et d'«étranger». 3. Tout est hérité d'un passé plus ou moins lointain; principe qui rejoint le premier et nous conduira, dans certains cas favorables, vers des concentrations antérieures.

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Recent publications lead to three steps without which metallogenic and thus petrologic investigations are impossible: 1. We should depart from known types in ore genesis and assume that any element can be enriched during various stages of the geological and geochemical cicle. These enrichments should thus be investigated with all possible methods of mineralogy and geochemistry. 2. We should reject the confusion of terms referring to time and space of ore genesis, especially the completely confused superposition of the terms epigenetic and introduced from the outside. 3. All deposits show signs of inheritance; this leads us back to point one which involves economic accumulations under certain favorable conditions.

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Neuere Veröffentlichungen zeigen die Notwendigkeit, drei Umstände anzuerkennen, ohne die eine petrologische Forschung nicht mehr möglich ist: 1. Wir müssen davon Abstand nehmen, von den bekannten Lagerstätten-Typen zu reden. Richtig ist vielmehr, daß jedes chemische Element in den verschiedenen Etappen des geologischen und geochemischen Geschehens angereichert werden kann; forschen wir also nach diesen Anreicherungen und verfolgen wir sie mit allen mineralogischen und geochemischen Mitteln, die uns zur Verfügung stehen. 2. Wir müssen die Verwirrung der chronologischen und räumlichen Vorstellungen vermeiden, vor allem die vollkommen falsche Überlagerung von epigenetisch und fremd. 3. Alles was wir beobachten, ist das Erbe einer mehr oder weniger fernen Vergangenheit. Nur wenn wir uns ständig über dieses Faktum klar sind, dann können wir unter günstigen Umständen zu frühen Anreicherungen hingeführt werden.

     

Raman Spectroscopy of Nb,Ta-rich Cassiterite in Beauvoir and Montebras Granites,France

Cassiterites from both the Beauvoir and Montebras geanited of France are typically rich in trace elements such as Nb and Ta, and contain quite a number of inclusions of columbite (dominantly manganocolumbite).Two thin sections of cassiterite crystals have been prepared for Raman microprobe analysis).The spectra obtained from different parts of the cassiterites show that the vibra-tion frequency of the A1g peak decreases with increasing Nb ,Ta,Fe and Mn atomic contents.It is worthy to note that a new peak (named the “An peak”) has been reported for the first time in the part with oriented columbite inclusions.The vibration frequency varies from 827 to 830 cm^-1.The presence of th enew peak may be attributed to structural changes of cassiterite due to the excess of Nb and Ta in the lattice and the exsolution of columbite inclusions in cassiterite.     

Paper 2. agricultural impacts of and responses to climate change in the Missouri-Iowa-Nebraska-Kansas (MINK) region

The climate of the 1930s was used as an analog of the climate that might occur in Missouri, Iowa, Nebraska and Kansas (the MINK region) as a consequence of global warming. The analog climate was imposed on the agriculture of the region under technological and economic conditions prevailing in 1984/87 and again under a scenario of conditions that might prevail in 2030. The EPIC model of Williamset al. (1984), modified to allow consideration of the yield enhancing effects of CO₂ enrichment, was used to evaluate the impacts of the analog climate on the productivity and water use of some 50 representative farm enterprises. Before farm level adjustments and adaptations to the changed climate, and absent CO₂ enrichment (from 350 to 450 ppm), production of corn, sorghum and soybeans was depressed by the analog climate in about the same percent under both current and 2030 conditions. Production of dryland wheat was unaffected. Irrigated wheat production actually increased. Farm level adjustments using low-cost currently available technologies, combined with CO₂ enrichment, eliminated about 80% of the negative impact of the analog climate on 1984/87 baseline crop production. The same farm level adjustments, plus new technologies developed in response to the analog climate, when combined with CO₂ enrichment, converted the negative impact on 2030 crop production to a small increase. The analog climate would have little direct effect on animal production in MINK. The effect, if any, would be by way of the impact on production of feed-grains and soybeans. Since this impact would be small after on-farm adjustments and CO₂ enrichment, animal production in MINK would be little affected by the analog climate.     

Timing and duration of partial melting and magmatism in the Variscan Montagne Noire gneiss dome (French Massif Central)

International Journal of Earth Sciences - Unravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides...     

In situ identification,pairing, and classification of meteorites from Antarctica through magnetic susceptibility measurements

Abstract— We report on the effectiveness of using magnetic measurements in the search for meteorites on the Antarctic ice sheet, which is thus far the Earth's most productive terrain. Magnetic susceptibility measurements carried out with a pocket meter (SM30) during the 2003/04 PNRA meteorite collection expedition to northern Victoria Land (Antarctica) proved to be a rapid, sensitive, non‐destructive means for the in situ identification, pairing, and classification of meteorites. In blue ice fields characterized by the presence of moraines and glacial drifts (e.g., Miller Butte, Roberts Butte, and Frontier Mountain), magnetic susceptibility measurements allowed discrimination of meteorites from abundant terrestrial stones that look like meteorites thanks to the relatively high magnetic susceptibility of the former with respect to terrestrial rocks. Comparative measurements helped identify 16 paired fragments found at Johannessen Nunataks, thereby reducing unnecessary duplication of laboratory analyses and statistical bias. Following classifications schemes developed by us in this and previous works, magnetic susceptibility measurements also helped classify stony meteorites directly in the field, thereby providing a means for selecting samples with higher research priority. A magnetic gradiometer capable of detecting perturbations in the Earth's magnetic field induced by the presence of meteorites was an efficient tool for locating meteorites buried in snow along the downwind margin of the Frontier Mountain blue ice field. Based on these results, we believe that magnetic sensors should constitute an additional payload for robotic search for meteorites on the Antarctic ice sheet and, by extension, on the surface of Mars where meteorite accumulations are predicted by theoretical works. Lastly, magnetic susceptibility data was successfully used to crosscheck the later petrographic classification of the 123 recovered meteorites, allowing the detection of misclassified or peculiar specimens.     

L'extension à Madagascar du Néogène à l'Actuel: arguments structuraux et géophysiques

Field studies and observations from aerial photographs, remote sensing and numerical field models show that the Meso-Cenozoic and Recent sedimentary sequences of Madagascar are affected by normal faults. These faults, north-south, north-northeast-south-southwest and north-northwest-south-southwest trending, are responsible for the following morphological effects: scarps, block tilting and opening of faulted basins, the most important of which being the Lake Alaotra Basin. The general orientation of these brittle structures and some observations in the field show tha the horizontal maximum extension isroughly east-west directed. Other lines of evidence, seismic and gravimetric, confirm that Madagascar is presently submitted to a crustal and lithospheric extension, parallel to that in East Africa and the Mozambique Channel.     

The impact of atmospheric sodium on erodibility of clay in a coastal Mediterranean region

 Heavy rainfalls, between 25 and 100 mm·h–1, were simulated on Pliocene/Quaternary sediments. To reproduce the heterogeneity of natural environments, 231 small plots of various sizes (between 2.5 and 3.5 m²; mean: about 3 m²) were used. The duration of all simulations was 1 h. We used water that had been collected during natural rainfall. The concentration of clay particles in the sheet wash depended upon the concentration of dissolved sodium in the wash (for about 42%) and of the sheet wash quantity (for about 37%). Under natural water conditions colloidal matter, like clay minerals, is charged negatively and therefore is destabilized by metal cations such as in the case of Na⁺. Results suggest that relatively higher concentrations of montmorrillonite were related to higher concentrations of sodium as opposed to illite and kaolinite. Microflakes of up to 25 μ were observed to vary between face-to-edge and face-to-face modes (competition between protons and other cations). The concentration of dissolved sodium (Na⁺) in the runoff water depends on water and sodium balances such as atmospheric input, infiltration, evaporation and surface water runoff. The reduction of vegetation cover increases the amount of salt and amorphous matter in/on the topsoil between heavy rainfall generations. The best predictor to explain montmorillonite, illite and kaolinite in % of mineral clay-sized matter in the surface water runoff (sheet wash) is the percentage of each clay mineral in the topsoil. As opposed to illite and kaolinite, more sheet wash indicate for montmorillonite relatively higher concentrations in the wash. The results of model simulations were confirmed on different field plots of about 1 ha and small catchments during natural heavy rainfall events. Models can also be used to understand and to better simulate sheet, rill and gully erosion, micropedimentation; and pedimentation.     

Investigating strong ground-motion variability using analysis of variance and two-way-fit plots

A statistical method to quantitatively assess the relative importance of unmodelled site and source effects on the observed variability (σ) in ground motions is presented. The method consists of analysis of variance (ANOVA) using the computed residuals with respect to an empirical ground-motion model for strong-motion records of various earthquakes recorded at a common set of stations. ANOVA divides the overall variance (σ ²) into the components due to site and source effects (respectively σ _S ² and σ _E ²) not modelled by the ground-motion model plus the residual variance not explained by these effects (σ _R ²). To test this procedure, four sets of observed strong-motion records: two from Italy (Umbria-Marche and Molise), one from the French Antilles and one from Turkey, are used. It is found that for the data from Italy, the vast majority of the observed variance is attributable to unmodelled site effects. In contrast, the variation in ground motions in the French Antilles and Turkey data is largely attributable, especially at short periods, to source effects not modelled by the ground-motion estimation equations used.