Jacques-Marie Bardintzeff, Volcanologue : de la passion à la vocation

Cet ouvrage a comme sous-titre « De la passion à la vocation », mais cela aurait pu être « Itinéraire d’un volcanologue gâté ». En effet, Jacques-Marie Bardintzeff nous fait parcourir sa vie depuis son enfance dans la banlieue de Grenoble, ses années lycée et surtout revient sur son parcours professionnel depuis les classes préparatoires jusqu’à sa soutenance de thèse. Le lecteur croisera au détour d’une page, entre les nombreuses photos de l’auteur sur le terrain et de volcans, quelques gran..

First paleomagnetic and 40Ar/39Ar study of Paleoproterozoic rocks from the French Guyana (Camopi and Oyapok rivers), northeastern Guyana Shield

In order to understand the Paleoproterozoic geographic evolution of the Guyana Shield, paleomagnetic and 40Ar/39Ar investigations were carried out on granitoids and volcano-sedimentary rocks from the Oyapok and Camopi rivers (French Guyana–Brazil frontier). Scanning electronic microscope, thermomagnetic and isothermomagnetic experiments show that magnetite is the main magnetic remanent carrier in most of the samples. The metavolcano-sedimentary rocks (Paramaca) show a weak magnetization and scattered magnetic directions. Therefore, no reliable magnetic component could be isolated from these samples. Samples taken from tonalite and meta-ultrabasite rocks yield a characteristic magnetic direction, carried by subautomorphous magnetite, that is well defined and distinct from that of the present Earth field and that of nearby Jurassic dikes. A virtual geomagnetic pole (VGP) deduced from this probable primary remanence was calculated, namely pole OYA, λ=28.0°S, φ=346.0°E, N=5, k=31.9 and A95=13.8°. Four 40Ar/39Ar ages, ranging from 2052 to 1973 Ma, were obtained from amphiboles and biotites of tonalite rocks, showing a relatively slow cooling rate of ca 4.8+2.6/−2.1°C Ma−1. The linear extrapolation of this cooling rate to the magnetite unblocking temperature (540 to 580°C) yields a magnetization age of 2036±14 Ma for pole OYA. Pole OYA differs significantly from available paleomagnetic results from Venezuela of the West Guyana Shield dated at 2000±10 Ma. This difference may indicate an important latitudinal movement of the Guyana Shield between 2036 and 2000 Ma with a velocity of 9±7 cm/year

Alteration of As-bearing phases in a small watershed located on a high grade arsenic-geochemical anomaly (French Massif Central)

International audienceAt a watershed scale, sediments and soil weathering exerts a control on solid and dissolved transport of trace elements in surface waters and it can be considered as a source of pollution. The studied subwatershed (1.5 km2) was located on an As geochemical anomaly. The studied soil profile showed a significant decrease of As content from 1500 mg.kg-1 in the 135-165 cm deepest soil layer to 385 mg.kg-1 in the upper 0-5 cm soil layer. Directly in the stream, suspended matter and the <63 μm fraction of bed sediments had As concentrations greater than 400 mg.kg-1. In all these solid fractions, the main representative As-bearing phases were determined at two different observation scales: bulk analyses using X-ray absorption structure spectroscopy (XAS) and microanalyses using scanning electron microscope (SEM) and associated electron probe microanalyses (EPMA), as well as micro-Raman spectroscopy and synchrotron-based micro-scanning X-ray diffraction (μSXRD) characterization. Three main As-bearing phases were identified: (i) arsenates (mostly pharmacosiderite), the most concentrated phases As in both the coherent weathered bedrock and the 135-165 cm soil layer but not observed in the river solid fraction, (ii) Fe-oxyhydroxides with in situ As content up to 15.4 wt.% in the deepest soil layer, and (iii) aluminosilicates, the least concentrated As carriers. The mineralogical evolution of As-bearing phases in the soil profile, coupled with the decrease of bulk As content, may be related to pedogenesis processes, suggesting an evolution of arsenates into As-rich Fe-oxyhydroxides. Therefore, weathering and mineralogical evolution of these As-rich phases may release As to surface waters

Metals retention after weathering of a sulphide ore body: unexploited gossan versus mine tailings.

Numerous studies have highlighted the fact that the bioavailability of potentially toxic trace elements (PTTE), such as As, Cd, Cr, Cu, Ni, and Pb, is strongly related to the redox state and chemical form. Since dissolved forms of trace elements are the most bioavailable, the issue of greatest concern is to define the conditions under which these elements may be remobilized in solution up to levels higher than those recommended by National agencies as e.g. the Environmental Protection Agency (EPA). In order to understand and then ideally to prevent trace element mobilization, the characterisation of the contaminant sources, i.e. the solid fraction, is crucial