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341.
Eric Ferrage Christophe Tournassat Emmanuel Rinnert Bruno Lanson 《Geochimica et cosmochimica acta》2005,69(11):2797-2812
The hydration state of a <2 μm fraction of Ca-saturated SWy-2 montmorillonite was characterised after rapid equilibration (3 hours) under pH-controlled conditions (0.1-12.6 pH range). The solution composition was monitored together with the interlayer composition and X-ray diffraction (XRD) patterns were recorded on oriented preparations. Experimental XRD patterns were then fitted using a trial-and-error procedure to quantify the relative proportions of layers with different hydration states.The montmorillonite is mostly bi-hydrated in basic and near-neutral conditions whereas it is mostly mono-hydrated at low pH. The transition from the bi-hydrated to the mono-hydrated state occurs through very heterogeneous structures. However, the proportion of the different layer types determined from XRD profile modelling and that derived from chemical modelling using Phreeqc2 code strictly coincide. This correlation shows that the hydration modification is induced by a H3O+-for-Ca2+ exchange at low pH, the two species being distributed in different interlayers. This layer-by-layer exchange process occurs randomly in the layer stack.Under alkaline conditions, results from XRD profile modelling and from near infrared diffuse reflectance spectroscopy (NIR-DRS) clearly demonstrate that there is no CaOH+-for-Ca2+ exchange at high pH. The apparent increase in Ca sorption in smectite interlayers with increasing pH is thus probably related to the precipitation of Calcium-Silicate-Hydrate (CSH) phases, which also accounts for the decrease in Si concentration under high-pH conditions. This precipitation is thermodynamically favoured. 相似文献
342.
Christophe Cloquet Olivier Rouxel Jean Carignan Guy Libourel 《Geostandards and Geoanalytical Research》2005,29(1):95-106
In this study, the Cd isotopic composition of various geological reference materials and anthropogenic samples was investigated. The measurements were made by multicollector ICP-MS and instrumental mass fractionation was controlled using a "sample-standard bracketing" technique. Cadmium isotopic data are reported relative to an internal Cd solution (Cd Spex) and expressed as the 114 Cd/110 Cd delta value. Two other Cd solutions (Prolabo and JMC) were analysed and yielded the same 0% delta value. A fractionated Cd metal sample (Münster Cd) was used as a secondary reference material for Cd isotopic measurements and we obtained a 114 Cd/110 Cd delta value of 4.48% relative to Cd Spex solution. As opposed to multi-stage Cd purification previously published in the literature, a new one step anionic exchange purification using dilute HCl for the analysis of Cd isotopes in geological samples was developed. This method enabled a high recovery (> 95%) and effective separation of the sample matrix to be achieved. The long-term external reproducibility was evaluated at 0.12% (2 standard deviations) for the 114 Cd/110 Cd ratio, based on reference solutions and replicated measurements of samples over one year. The variation of Cd isotopic composition of natural terrestrial samples is restricted to a small range of 0.4%, which is similar to previously reported results. In contrast, large variations of Cd isotopic composition were found for anthropogenic samples with values as low as −0.64% for a dust sample issued from a lead smelter and values as high as +0.50% for NIST SRM 2711 (metal-rich soil). These variations are 10 times larger than the reproducibility and suggest that Cd isotopes can be useful as tracers of anthropogenic sources of Cd in the environment. 相似文献
343.
344.
Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis? 总被引:4,自引:0,他引:4
Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain-to-grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm- and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. 'Meniscus-type cements' (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus-type cements are commonly micritic, but meniscus-shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure. 相似文献
345.
The nth-order moments of the electromagnetic impulse response are useful for interpreting electromagnetic data. We have derived an analytic expression for the half-order moment of a conductive half-space. By inverting this expression, the measured half-order moment can be used to estimate an apparent conductivity of the ground. The first-order moment can also be used to estimate the half-space conductivity. A sensitivity analysis indicates that for an airborne EM configuration, the half-order moment will be most sensitive to material in the top 26–48 m, while the first-order moment will be sensitive to deeper material (down to depths between 66 and 127 m). 相似文献
346.
Christophe Drouet 《Geochimica et cosmochimica acta》2003,67(11):2063-2076
The thermochemistry of well-characterized synthetic K-H3O, Na-H3O and K-Na-H3O jarosites was investigated. These phases are solid solutions that obey Vegard’s law. Electron probe microanalyses indicated lower alkali and iron contents than predicted from the theoretical end-member compositions, in agreement with thermal analyses, suggesting the presence of hydronium and “additional” water. The standard enthalpies of formation (ΔH°f) of K-H3O, Na-H3O and K-Na-H3O jarosites were determined by high-temperature oxide melt solution calorimetry. These enthalpies vary linearly with the K/H3O, Na/H3O and K/Na ratio, respectively. The enthalpy of formation of pure hydronium jarosite was also determined experimentally (ΔH°f = −3741.6 ± 8.3 kJ.mol−1), and it was used to evaluate ΔH°f for the end-members KFe3(SO4)2(OH)6 (ΔH°f = −3829.6 ± 8.3 kJ.mol−1) and NaFe3(SO4)2(OH)6 (ΔH°f = −3783.4 ± 8.3 kJ.mol−1). Finally, enthalpies of dehydration (loss of the “additional” water) of some jarosites were determined and found to be near the enthalpy of vaporization of water, suggesting that the “additional” water is weakly bonded in the structure. 相似文献
347.
Larbi Boudad Lahcen Kabiri Samer Farkh Christophe Falguères Louis Rousseau Jacques Beauchamp Élisabeth Nicot Guillaume Cairanne 《Comptes Rendus Geoscience》2003,335(5):469-478
A large travertine outcrop south of Errachidia, southern Morocco, was studied and U/Th dated. The carbonate fraction was provided by groundwaters then, as now, from the eastern High Atlas percolating through the regional Infra-Cenomanian aquifer. There were two main periods of accumulation at ca 262 kyr BP and 20–11.5 kyr BP separated by a long discontinuity with some limited weathering and erosion and correlated in part with a period of erosion at 30–20 kyr BP further to the west. The two travertine-deposition periods suggest increased rainfall and/or cooler thermal conditions in the eastern High Atlas source regions. Massive travertine accumulation ceased at the end of the Upper Pleistocene. To cite this article: L. Boudad et al., C. R. Geoscience 335 (2003).To cite this article: L. Boudad et al., C. R. Geoscience 335 (2003). 相似文献
348.
Influence of rainfall spatial variability on flood prediction 总被引:9,自引:0,他引:9
Patrick Arnaud Christophe Bouvier Leonardo Cisneros Ramon Dominguez 《Journal of Hydrology》2002,260(1-4):216-230
This paper deals with the sensitivity of distributed hydrological models to different patterns that account for the spatial distribution of rainfall: spatially averaged rainfall or rainfall field. The rainfall data come from a dense network of recording rain gauges that cover approximately 2000 km2 around Mexico City. The reference rain sample accounts for the 50 most significant events, whose mean duration is about 10 h and maximal point depth 170 mm. Three models were tested using different runoff production models: storm-runoff coefficient, complete or partial interception. These models were then applied to four fictitious homogeneous basins, whose sizes range from 20 to 1500 km2. For each test, the sensitivity of the model is expressed as the relative differences between the empirical distribution of the peak flows (and runoff volumes), calculated according to the two patterns of rainfall input: uniform or non-uniform. Differences in flows range from 10 to 80%, depending on the type of runoff production model used, the size of the basin and the return period of the event. The differences are generally moderate for extreme events. In the local context, this means that uniform design rainfall combining point rainfall distribution and the probabilistic concept of the areal reduction factor could be sufficient to estimate major flood probability. Differences are more significant for more frequent events. This can generate problems in calibrating the hydrological model when spatial rainfall localization is not taken into account: a bias in the estimation of parameters makes their physical interpretation difficult and leads to overestimation of extreme flows. 相似文献
349.
Erosion and flexural uplift along transform faults 总被引:1,自引:0,他引:1
350.