Sediment yield and storage variations in the Négron River catchment (south western Parisian Basin,France) during the Holocene period

In the Négron River catchment area (162 km²), surface‐sediment stores are composed of periglacial calcareous ‘grèze’ (5 × 10⁶ t) and loess (21 × 10⁶ t), and Holocene alluvium (12·6 × 10⁶ t), peat (0·6 × 10⁶ t) and colluvium (18·5 × 10⁶ t). Seventy‐five per cent of the Holocene sediments is stored along the thalwegs. Present net sediment yield, calculated from solid discharge at the Négron outlet, is low (0·6 t km^?2 a^?1) due to the dominance of carbonate rocks in the catchment. Mean sediment yield during the Holocene period is 7·0 t km^?2 a^?1 from alluvium stores and 7·6 t km^?2 a^?1 from colluvium stores. Thus, the gross sediment yield during the Holocene period is about 18·7 t km^?2 a^?1 and the sediment delivery ratio 3 per cent. The yield considerably varies from one sub‐basin to another (3·9 to 24·5 t km^?2 a^?1) according to lithology: about 25 per cent and 50 per cent of initial stores of periglacial grèze and loess respectively were reworked during the Holocene period. Sediment yield has increased by a factor of 6 in the last 1000 years, due to the development of agriculture. The very high rate of sediment storage on the slope during that period (88 per cent of the yield) can be accounted for by the formation of cultivation steps (‘rideaux’). It is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades. Subboreal and Subatlantic sediments and pollen assemblages in the Taligny marsh, where one‐third of the alluvium is stored, show the predominant influence of human activity during these periods in the Négron catchment. Copyright © 2002 John Wiley & Sons, Ltd.     

Experimental study of brucite dissolution and precipitation in aqueous solutions: surface speciation and chemical affinity control

Dissolution and precipitation rates of brucite (Mg(OH)₂) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10⁻⁴ to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10⁻⁶ to 5·10⁻⁴ M). Brucite surface charge and isoelectric point (pH_IEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pH_IEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m²) and lack of dependence on ionic strength predicts the dominance of >MgOH₂⁺ species at pH < 8 and their progressive replacement by >MgOH° and >MgO⁻ as pH increases to 10-12. Rates are proportional to the square of >MgOH₂⁺ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH₂⁺ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity:

     

Le Bassin lusitanien (Portugal) à l'Aptien supérieur–Albien : organisation séquentielle,proposition de corrélations, évolutionThe Lusitanian Basin (Portugal) during the late Aptian–Albian: sequential arrangement,proposal of correlations,evolution

. Three transgressive–regressive 2nd-order cycles were identified in the Upper Aptian–Albian fluvial and marine deposits of the Lusitanian Basin. Its widespread nature, probably including eustatic origin, allows correlation between the southern package, with precise stratigraphic positioning, and the northern series with a poorly constrained age. The main unconformities can be related to the onset of an oceanic crust, in the western margin of Galicia during the Late Aptian, in the bay of Biscay during the Early Albian, and, to the northwest of the Galicia triple point, during the Middle to Late Albian transition, and, at the Albian–Cenomanian boundary, to a probable compressive event with Africa due to the rotation of Iberia. To cite this article: J. Dinis et al., C. R. Geoscience 334 (2002) 757–764.     

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.

     

地球化学场精细结构解析方案与应用

为满足勘查地球化学对化探数据处理越来越精细的要求,把人工神经网络和统计分析结合起来形成地球化学场精细结构解析方案。该方案包括对地球化学样品的分类和对各类样品性质的详细研究,目的在于提供便捷的数学和计算机软件工具,以提取化探数据中的深层信息。可以用于:①研究各组样品中元素含量水平和组合特征以解释可能的矿种;②研究元素组合特征与指示元素的关联和区别以揭示可能的成矿作用过程;③研究异常样品的空间组合属性以揭示地球化学场的精细结构;④研究异常样品之间的差异性以缩小找矿靶区;⑤综合分析以确定剥蚀深度和找矿前景。该方案也适用于环境地球化学及矿产资源预测研究。     

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.     

象牙海岸Issia地区铌钽砂矿的研究(英文)

在西非的象牙海岸Issia地区分布着许多铌钽矿床,包括原生矿和砂矿。原生矿产在分带很好的伟晶岩中。这个地区发育着许多花岗岩和花岗伟晶岩。伟晶岩可分为:(1)分异不好的伟晶岩,无矿化;(2)含Be伟晶岩,分带较好,见绿柱石;(3)含Be-Nb-Ta的伟晶岩,分带很好,是矿体;(4)Be-Nb-Ta-Li伟晶岩,分带很好,是矿体。除了10多个原生矿外,还有许多砂矿。砂矿有两种类型:一种近原生矿的坡积砂矿,另一种是经河流冲刷搬运后的冲积砂矿。我们选择了Etienne-Meguhe砂矿作为近原生矿的坡积砂矿的代表,Bemadi矿作为河流冲积型的砂矿的代表,对这两种类型的砂矿的风化、搬运、沉积机理进行了比较研究。研究发现不同的搬运距离和搬运模式对铌钽砂矿的主要矿物——铌钽铁矿的化学性质没有什么影响,而其物理形态则发生了变化。坡积型砂矿的铌钽铁矿常与石英、长石等矿物连生,其颗粒大小与原生的Nb-Ta伟晶岩中的铌钽铁矿一致;经河流冲积而形成的砂矿中,铌钽铁矿颗粒变小、轮廓变得圆滑,几乎无连生矿物,但矿体集中,常在河流的冲积扇中分布,易开采。     

New chitinozoans from the historical type area of the Hirnantian Stage and additional key sections in the Wye Valley,Wales, UK

The type locality for several core elements of the Hirnantia brachiopod fauna is a small disused quarry on the western slopes of Cwm Hirnant. There, the Hirnant Limestone Member of the Foel‐y‐Ddinas Mudstone Formation yields a new, well‐preserved chitinozoan assemblage, attributed to the Spinachitina taugourdeaui Biozone. This allows tight correlation with the Hirnantian of Baltica and Laurentia and neatly ties the chitinozoan zonation with the classical brachiopod fauna. Nearby, the chitinozoan assemblage from the Caradoc Cymerig Limestone Member at Gelli‐grîn belongs to the Spinachitina cervicornis Biozone?, and is identical to that recovered from the Burrellian in the Onny Valley, Welsh Borderland. A Silurian assemblage higher up section, discovered in the Cwm‐yr‐Aethnen Formation, is attributed to the globally recognized Eisenackitina dolioliformis Biozone. Attempts to integrate the chitinozoan and graptolite biozonation, in the central Welsh Rhayader area, were less successful. Copyright © 2008 John Wiley & Sons, Ltd.     

Évolution géodynamique messino-pliocène en Languedoc central : le paléo-réseau hydrographique de l’Orb et de l’Hérault (Sud de la France)

Résumé

En Languedoc central, la cartographie des sédiments formés lors de la crise messinienne et de ceux de la séquence pliocène, ainsi que les données patées paléontologiques nouvelles (Aigues-Vives et Servian) permettent de proposer une reconstitution du tracé messimeli des rivières actuelles (Orb et Hérault) transformées en rias à la suite du relèvement eustatique pliocène. L’Hérault messinien se singularise par une incision beaucoup plus faible que celle des autres rivères. En effet, la nature karstique de l’arinère-pays immédiat permet un enfouissement des eaux météoriques au profit d’aquifères profonds sous-jacents au drainage aérien du réseau hydrographique messinien. Le Languedoc central illustre, entre Roussillon et Rhône, le modèle géodynamique messino-pliocène de Clauzon. © Elsevier, Paris