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841.
Bryan G. Mark Geoffrey O. Seltzer Donald T. Rodbell Adam Y. Goodman 《Quaternary Research》2002,57(3):287
Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10−5 to 114×10−5 km3 yr−1 and 112×10−5 to 247×10−5 km3 yr−1 for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10−5 to 220×10−5 km3 yr−1, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent. 相似文献
842.
843.
We have investigated a well-ordered sample of natural Cr-bearing dickite from Nowa Ruda (Lower Silesia, Poland) using electron
paramagnetic resonance (EPR) at X- and Q-band frequencies (9.42 and 33.97 GHz, respectively) and optical diffuse reflectance
spectroscopy. The observation of the spin-forbidden transitions at 15500 and 14690 cm−1 allows us to unambiguously identify the major contribution of octahedrally coordinated Cr3+ ions in the optical spectrum. The X- and Q-band EPR spectra show two superposed Cr3+ signals. The corresponding fine-structure parameters were determined at room temperature and 145 K. These results suggest
the substitution of Cr3+ for Al3+ in equal proportions in the two unequivalent octahedral sites of the dickite structure. In kaolin group minerals, the distortion
around Cr3+ ions (λ≈ 0.2–0.4) in Al sites is significantly less rhombic than that observed around Fe3+ ions (λ≈ 0.6–0.8).
Received: 29 June 2001 / Accepted: 22 October 2001 相似文献
844.
Rigoberto G. Sanabria Castro Sandra M. C. Malta Abimael F. D. Loula Luiz Landau 《Computational Geosciences》2002,5(4):301-330
A finite element formulation is proposed to approximate a nonlinear system of partial differential equations, composed by an elliptic subsystem for the pressure–velocity and a transport equation (convection–diffusion) for the concentration, which models the incompressible miscible displacement of one fluid by another in a rigid porous media. The pressure is approximated by the classical Galerkin method and the velocity is calculated by a post-processing technique. Then, the concentration is obtained by a Galerkin/least-squares space–time (GLS/ST) finite element method. A numerical analysis is developed for the concentration approximation. Then, stability, convergence and numerical results are presented confirming the a priori error estimates. 相似文献
845.
G. M. da Costa E. Van San E. De Grave R. E. Vandenberghe V. Barrón L. Datas 《Physics and Chemistry of Minerals》2002,29(2):122-131
A novel method for synthesis of aluminium hematites, based upon the homogeneous precipitation of Fe and Al oxinates in various proportions, is presented. The precursor precipitates are heated in air at 700?°C. X-ray diffraction, thermal analyses, BET, FTIR, optical reflection analysis, TEM and Mössbauer spectroscopy at room temperature and 80?K of the resulting products indicate that single-phase hematites are formed with structural Al substitution of up to 10 at%. Interestingly, the particle size (>100?nm) is not substantially reduced by the Al content. Although it remains difficult to obtain a homogeneously distributed Al substitution in the final hematite, this processing line offers a unique opportunity to separate the effects of grain size and Al substitution on the Morin transition temperature (T M) of Al hematite. From the comparison between the present hematites and a series of Al-substituted hematites with lepidocrocite as precursor, it could be shown that the effect on T M, associated with a change of a factor 10 in grain size, is about 1/3 of the effect caused by a change of 10 in the degree of substitution. Finally, it is suggested that proper thermal treatments under different conditions of the same precursors are likely to produce spinel phases. 相似文献
846.
M. A. Bouhifd G. Gruener B. O. Mysen P. Richet 《Physics and Chemistry of Minerals》2002,29(10):655-662
Premelting effects in gehlenite (Ca2Al2SiO7) have been studied by Raman spectroscopy and calorimetry, and in gehlenite and pseudowollastonite (CaSiO3) by electrical conductivity. The enthalpy of premelting of gehlenite is 17.3 kJ mol−1 and represents 9% of the reported enthalpy of fusion, which is in the range of the reported fraction of other minerals. The
Raman and electrical conductivity experiments at high temperatures, for gehlenite and pseudowollastonite, show that the premelting
effects of both compositions are associated with enhanced dynamics of calcium atoms near the melting point. This conclusion
agrees with the results obtained for other minerals like diopside, but contrasts with those found for sodium metasilicate
in which the weaker bonding of sodium allows the silicate framework to distort near the melting temperature and deform in
such a way to prefigure the silicate entities present in the melt.
Received: 30 April 2002 / Accepted: 7 August 2002
Acknowledgements We thank Y. Linard for help with DSC measurements and two anonymous reviewers for their constructive comments. This work
has been partly supported by the EU Marie-Curie fellowship contract no. HPMF-CT-1999-00329, the CNRS-Carnegie Institution
of Washington program PICS no.192, and the NSF grants EAR-9614432 and EAR-9901886 to B.O.M. 相似文献
847.
This paper presents an improved generalisation of cation distribution determination based on an accurate fit of all crystal-chemical
parameters. Cations are assigned to the tetrahedral and octahedral sites of the structure according to their scattering power
and a set of bond distances optimised for spinel structure. A database of 295 spinels was prepared from the literature and
unpublished data. Selected compositions include the following cations: Mg2+, Al3+, Si4+, Ti4+, V3+, Cr3+, Mn2+, Mn3+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+ and vacancies. Bond distance optimisation reveals a definite lengthening in tetrahedral distance when large amounts of Fe3+ or Ni2+ are present in the octahedral site. This means that these cations modify the octahedral angle and hence the shared octahedral
edge, causing an increase in the tetrahedral distance with respect to the size of the cations entering it. Some applications
to published data are discussed, showing the capacity and limitations of the method for calculating cation distribution, and
for identifying inconsistencies and inaccuracies in experimental data.
Received: 19 February 2001 / Accepted: 1 June 2001 相似文献
848.
A three-dimensional parametric study of the use of soil nails for stabilising tunnel faces 总被引:4,自引:0,他引:4
Applying an effective nailing system at a tunnel heading, not only improves the stability of the tunnel heading and limits deformation at the tunnel face, but it also reduces volume loss during excavation and hence reduces ground surface settlement. The effectiveness of a soil nail system is affected by many factors such as the diameter and stiffness of the nails. In this paper, a systematic parametric study was conducted to study the axial rigidity of a nail, EnAn, for improving the stability of tunnel headings and reducing ground movements in stiff clay. The parametric study involved a series of three-dimensional elasto-plastic coupled-consolidation finite element analyses. The stability of the tunnel face is improved with increasing EnAn. For a given nail density applied at the tunnel face, an optimum axial rigidity of the nail (EnAn)opt can be identified. The efficiency of the nailing system diminishes when (EnAn)opt is reached. The use of a soil nailing system reduces the magnitude of stress relief at the tunnel heading during excavation. Thus, this reduction of stress relief minimises the amount of soil yielding and excess pore water pressure generated in the soil around the tunnel heading. 相似文献
849.
850.
Validation of a fish farm waste resuspension model by use of a particulate tracer discharged from a point source in a coastal environment 总被引:2,自引:0,他引:2
Chris J. Cromey Thom D. Nickell Kenneth D. Black Paul G. Provost Colin R. Griffiths 《Estuaries and Coasts》2002,25(5):916-929
To validate a resuspension model of particulate material (salmonid farm wastes), a UV fluorescent particle tracer was selected with similar settling characteristics. Tracer was introduced to the seabed (water depth ≈30 m) and sediment samples taken on days 0, 3, 10, 17 and 30 to measure the horizontal and vertical distribution of tracer in sediments. A concentric sampling grid was established at radii of 25, 50, 100, 150, 200, 400, 700 and 1, 000 m from the source on transects 30° apart. The bulk of the deployed tracer was initially concentrated in an area 25 m radius from the release point; tracer was observed to steadily decrease to zero over a period of 30 days. In a 200 m region measured from the release point in the direction of the residual current, the redeposition of tracer was low. A Lagrangian particle tracking model was validated using these observed data by varying resuspension model parameters within limits to obtain the best agreement between spatial and temporal distributions. The validated model generally gave good predictions of total mass budgets (±7% of total tracer released), particulary where tracer concentrations were high near the release point. Best fit model parameters (critical erosion shear stress=0.018 N m−2, erodibility constan=60 g m−2 d−1) are at the low end of reported parameters for coastal resuspension models. Such a low critical erosion shear stress indicates that the frequency of resuspension and deposition events for freshly deposited material is high. 相似文献