High pressure polymorphism of dicalcium silicate Ca2SiO4. A transmission electron microscopy study     

Corinne Remy  François Guyot  Michel Madon 《Physics and Chemistry of Minerals》1995,22(7):419-427

Ca₂SiO₄ dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca₂SiO₄ (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca₂SiO₄ usually produced with high temperatures, including α-Ca₂SiO₄, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca₂SiO₄ and α-Ca₂SiO₄ polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca₂SiO₄ has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca₂SiO₄ into CaSiO₃ and CaO is reported.  相似文献   

Multiple origin and regional significance of bedding parallel veins in a fold and thrust belt: The example of a carbonate slice along the Appalachian structural front     

Stephan Sjourn  Michel Malo  Martine M. Savard  Donna Kirkwood 《Tectonophysics》2005,407(3-4):189-209

The development and evolution of bedding parallel veins (BPV) are investigated in the Saint-Dominique carbonate slice (southern Québec Appalachian structural front), in order to emphasize the regional significance of BPV in a fold and thrust belt and their importance in establishing its structural and diagenetic evolution. Structural and microstructural analyses reveal that most BPV display a laminated structure locally crosscut by a massive one. Both structures show mutual crosscutting relationships with bedding parallel or bedding perpendicular stylolites and with bedding perpendicular veinlets, indicating that BPV cementation and deformation are continuous processes. Opening both sub-parallel and sub-perpendicular to the BPV walls are documented. Early BPV are planes of weakness that focus later reactivation, and evidence of successive and sometimes opposite senses of opening are locally preserved within a single BPV. The bedding parallel orientation of these veins proved particularly helpful in establishing the structural evolution of an area mostly characterized by front parallel structures with little crosscutting relationships otherwise. Petrographic and geochemical (δ¹⁸O _VPDB and δ¹³C) analyses of structural cements suggest a common, locally derived source for the fluids that percolated through BPV, non-bedding parallel veins and faults. The isotopic ratios also reveal a significant enrichment in ¹⁸O and ¹³C through time. This evolution is tentatively correlated with tectonic, syn-convergence exhumation of the Saint-Dominique slice during its imbrication along the Appalachian structural front, after its burial under Taconian thrust sheets.  相似文献   

Sm–Nd isotopic compositions as a proxy for magmatic processes during the Neoproterozoic of the southern Brazilian shield     

Maria do Carmo P. Gastal  Jean Michel Lafon  Lo Afraneo Hartmann  Edinei Koester 《Journal of South American Earth Sciences》2005,18(3-4):255-276

Combined analyses of Nd isotopes from a wide range of Neoarchaean–Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the São Gabriel accretionary orogeny (900–700 Ma) and the Dom Feliciano collisional orogeny (660–550 Ma). Rocks related to the formation of the São Gabriel arc (900–700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800–550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the São Gabriel arc and Taquarembó block. In the collisional belt, the old crust was intensely reworked during the São Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (São Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplate tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3–2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605–550 Ma) and younger suites from the eastern collisional belt (600–580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800–600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605–580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575–550 Ma).  相似文献   

Apport de la modélisation géophysique pour la compréhension de la structuration du pas de Calais     

Bruno Minguely  Jean-Louis Mansy  Michel Everaerts  Geoffrey M. Manby  Olivier Averbuch 《Comptes Rendus Geoscience》2005,337(3):305-313

This study is based on an analysis of seismic, gravimetric and magnetic data to model the structure of the Dover Straits area. The Dover Straits are set in a transitional position within the Brabant para-autochthon, which lies between the Ardennes allochthon to the south and the Brabant Massif to the north. The seismic profiles show that this area is characterised by several superimposed Mesozoic basins, whose evolution has been controlled by the reactivation of Lower Palaeozoic faults. In common with the adjacent Weald–Wessex and southern North Sea basins, the Dover Straits area records a Jurassic extensional phase and a post-Cretaceous tectonic inversion. The geophysical modelling reveals the presence of a basement to these Mesozoic basins that exhibits Ordovician to Silurian structures, which cannot be detected on the seismic reflection profiles. In contrast to earlier interpretations, the modelling presented here suggests that the Brabant Massif continues southward beyond the Dover Straits and under the Brabant para-autochthon. Gravity data also provides evidence for the presence of granitic body at depth, in the vicinity of Km 27 on the seismic profile, which may have driven the uplift of the Silurian rocks in the area of the Straits. Similar granitic intrusions have been proposed to explain gravity anomalies along the Rhenohercynian Margin in Belgium, eastern and southern England. The geophysical modelling demonstrates the continuity of the basement and Mesozoic basin structures across the Dover Straits, suggesting a common geological evolution of the adjacent southern England–northern France regions. To cite this article: B. Minguely et al., C. R. Geoscience 337 (2005).  相似文献   

Sur l'assimilation variationnelle de données par un modèle d'écosystème marin de type NPZ     

Yann Leredde  Christine Lauer-Leredde  Frédéric Diaz 《Comptes Rendus Geoscience》2005,337(12):1055-1064

A variational data assimilation method is applied to a simplified marine ecosystem model of NPZ (Nutrient Phytoplankton Zooplankton) type, which implies five parameters. The method allows us to optimise these parameters by an iterative process of minimisation of a cost function which quantifies the quadratic discrepancy between observations and simulation results. The first part of this note describes how to obtain the adjoint model of the NPZ model allowing us to compute the gradient of the cost function relative to the parameters. Two experiments of artificial data assimilation then show the efficiency of this method, but also its limitations because of non-linearities and sensitivity problems of the model relative to the parameters. To cite this article: Y. Leredde et al., C. R. Geoscience 337 (2005).  相似文献   

Experimental sorption of Ni, Cs and Ln onto a montmorillonite up to 150°C     

Emmanuel Tertre  Gilles Berger  Michel Loubet 《Geochimica et cosmochimica acta》2005,69(21):4937-4948

The effect of temperature on the sorption of cations onto a dioctahedral smectite was investigated by running batch experiments at 25, 40, 80 and 150°C. We measured the distribution coefficient (Kd) of Cs⁺, Ni²⁺ and 14 lanthanides (Ln³⁺) between solutions and the montmorillonite fraction of the MX80 bentonite at various pH and ionic strengths. Up to 80°C we used a conventional experimental protocol derived from Coppin et al. (2002). At 150°C, the experiments were conducted in a PTFE reactor equipped with an internal filter allowing the sampling of clear aliquots of solution.The results show a weak but measurable influence of the temperature on the elements sorption. Kd’s for Ni²⁺ and Ln³⁺ increase by a factor 2 to 5 whereas temperature raises from 25 to 150°C. This effect seems higher at high ionic strength. The estimated apparent endothermic sorption enthalpies are 33 ± 10 kJ.mol⁻¹ and 39 ± 15 kJ.mol⁻¹ for Ni²⁺ and Eu³⁺, respectively. On the other hand, the temperature effect on Cs⁺ sorption is only evidenced at low ionic strength and under neutral conditions where the Kd decreases by a factor 3 between 25 and 150°C. Apparent exothermic sorption enthalpy for Cs⁺ on the montmorillonite is −19 ± 5 kJ.mol⁻¹.Experiments conducted at the four temperatures with the coexistence of all of the cations in the reacting solution (100 ppb of each element in the starting solution) or only one of them, produced similar values of Kd. This suggests the absence of competition between the sorbed cations, and consequently a low degree of saturation of the available sites. A fractionation of the lanthanides spectrum is also observed at high pH and high ionic strength whatever the temperature.The conclusion of this study is that the temperature dependence on sorption reflects, as the fractionation of REE or the pH and ionic strength effects, the chemical process which controls the overall reaction. In the case of an exchange dominated reaction (low pH and low ionic strength), the temperature effect is negligible. In the case of surface complexation (high pH and high ionic strength), the observed increase of Kd with temperature reflects either an increase of the sorption equilibrium constant with temperature or an endothermic property for reactions describing the montmorillonite surface chemistry.  相似文献   

Differentiating Future Commitments on the Basis of Countries’ Relative Historical Responsibility for Climate Change: Uncertainties in the ‘Brazilian Proposal’ in the Context of a Policy Implementation   总被引：1，自引：0，他引：1  

Michel?G.?J.?Den?ElzenEmail author  Michiel?Schaeffer  Paul?L.?Lucas 《Climatic change》2005,71(3):277-301

During the negotiations on the Kyoto Protocol, Brazil proposed allocating the greenhouse gas emission reductions of Annex I Parties according to the relative effect of a country’s historical emissions on global temperature increase. This paper analyses the impact of scientific uncertainties and of different options in policy implementation (policy choices) on the contribution of countries’ historical emissions to indicators of historical responsibility for climate change. The influence of policy choices was found to be at least as large as the impact of the scientific uncertainties analysed here. Building on this, the paper then proceeds to explore the implications of applying the Brazilian Proposal as a climate regime for differentiation of future commitments on the global scale combined with an income threshold for participation of the non-Annex I regions. Under stringent climate targets, such a regime leads to high emission reductions for Annex I regions by 2050, in particular for Europe and Japan. The income threshold assumptions strongly affect the Annex I reductions, even more than the impact of another burden-sharing key. A variant of the Brazilian Proposal, allocating emission reductions on the basis of cumulative emissions since 1990, would lead to a more balanced distribution of emission reductions.  相似文献   

Natural speciation of Mn, Ni, and Zn at the micrometer scale in a clayey paddy soil using X-ray fluorescence, absorption, and diffraction   总被引：1，自引：0，他引：1  

Alain Manceau  Caterina Tommaseo  Nicolas Geoffroy  Michel Schlegel  Matthew A. Marcus  Zueng-Sang Chen 《Geochimica et cosmochimica acta》2005,69(16):4007-4034

The natural speciation of Mn (0.19 g/kg), Ni (46 mg/kg), and Zn (42 mg/kg) in the argillic horizon (120 cm depth, pH = 5.6) of an Ultisol from a paddy soil in northern Taiwan was investigated by advanced X-ray synchrotron techniques. Microchemical associations were imaged by synchrotron-based X-ray microfluorescence, host minerals were identified by standard and micrometer-resolved X-ray diffraction, and the local coordination environment of Mn, Ni, and Zn was probed using extended X-ray absorption fine structure (EXAFS) spectroscopy on a powdered sample and a soil thin section, and polarized EXAFS spectroscopy on a highly textured self-supporting clay film from the <2 μm fraction of the soil. Manganese was concentrated in Fe-Mn soft mottles (44.4 g/kg) as turbostratic hexagonal birnessite and lithiophorite having Mn³⁺/Mn⁴⁺ atomic ratios of ∼20% and 50%, respectively. Quantitative analysis of high-order scattering paths of the EXAFS spectrum for natural and synthetic lithiophorite revealed that Mn³⁺ and Mn⁴⁺ are ordered in the layer. A structural model is proposed, in which Mn⁴⁺ and Mn³⁺ are ordered similarly to Al and Li in the layer, with Mn³⁺ cations being surrounded by six Mn⁴⁺, and Mn⁴⁺ cations by three Mn³⁺ and three Mn⁴⁺. Similar cation ordering in the manganese and aluminum layers likely provides a more homogeneous local balance of the excess and deficit of charges in each layer and increases the stability of lithiophorite. Ni (r = 0.70 Å) substitutes for Mn (r(Mn⁴⁺) = 0.54 Å, r(Mn³⁺) = 0.65 Å) in the manganese layer in the natural lithiophorite. In contrast, Zn (r = 0.74 Å) fills vacant sites in the gibbsitic layer of natural lithiophorite, in a similar manner as lithium (r = 0.74 Å) in synthetic lithiophorite. The partitioning of Ni and Zn between the two layers is a result of the general preference of Ni, whose size is intermediate between those of Mn³⁺ and Li⁺, for slightly smaller sites. In contrast with nickel, which is detected only where there is lithiophorite, the Zn-lithiophorite association found in Fe-Mn mottles is not representative of the bulk soil. The combined use of X-ray diffraction, and powder and polarized EXAFS spectroscopy revealed that Zn is predominantly bound to hydroxy-Al interlayers sandwiched between 2:1 vermiculite layers in the fine soil matrix. The incorporation of Zn in the gibbsitic layer of both lithiophorite and vermiculite helps increase the stability of these minerals by providing positive charge to balance the negative charge from the 2:1 phyllosilicate layer and the layer of lithiophorite. This binding environment for zinc is probably the main mechanism by which zinc is sequestered in acidic to near-neutral aluminum-rich clayey soils.  相似文献   

A Special Issue Dedicated to Michel David (1945–2000)     

Roussos?DimitrakopoulosEmail author  Michel?Dagbert 《Mathematical Geosciences》2005,37(5):449-450

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Autonomous space resection using Point- and Line-Based representation of FREE-FORM control Linear Features   总被引：1，自引：0，他引：1  

Ayman Habib  Hsiang Tseng Lin  Michel Morgan 《The Photogrammetric Record》2003,18(103):244-258

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