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181.
A set of outcrop samples from the Lokoja and Patti Formations of the southern Bida Basin (Nigeria) was examined for grain size distribution, sedimentary and hydraulic characteristics. Most of the samples are well-sorted with an uniformity coefficient (U) ranging between 1.3 and 6.3. The mean effective grain diameter (d10) is in the order of 0.11 mm, the mean value of d90 was determined as 0.66 mm (both geometric mean) and the median grain size (d50) as 0.23 mm. Based on these values, the sedimentary sequence can be described as ‘fine to medium sized sand’, having minor amounts of either silt, or coarse sand and some gravel. The total porosity of the samples was determined by laboratory methods to be in the range of 9–29%. The hydraulic conductivity (K) was determined according to Hazen.and Beyer, and by Shepherd's formula, the last resulting in a geometric mean of 3.3 m d−1 or 3.3 darcy. The results are used to estimate local groundwater potential. The entire pore space (potential groundwater reservoir) for the area under study is estimated to be in the order of 290–430 km3. Because of higher hydraulic conductivities, it is recommended that the Lokoja Formation is concentrated on as a target for groundwater exploration. 相似文献
182.
贵州遵义牛蹄塘组黑色岩系的硒同位素变化及其环境指示初探 总被引:4,自引:0,他引:4
西南早寒武世牛蹄塘组是分布在中国南方扬子地台的一套黑色岩系,区域上发育有典型富集多金属元素的硫化物矿化层.选取遵义松林小竹牛蹄塘组下部富硒、钼、镍等元素的黑色岩系剖面,利用Se同位素初步探讨了该剖面岩石的沉积环境和硒的可能来源.结果表明,小竹牛蹄塘组下部剖面黑色岩系的δ82/76SeSRM3149比值变化较大,下部底层含碳斑脱岩与磷块岩的变化范围在-4.35‰~ 4.11‰之间;中间镍钼层及碳质页岩、碳质碳酸盐岩的δ82/76SeSRM3149变化范围窄,平均值为0.9‰±0.23‰(n=4);上层碳质页岩为-1.24‰.结合已发表的钼同位素数据,认为中间岩石沉积于缺氧/无氧环境,但存在盆地海水与热液或充氧水团的混合;下层的岩石曾一度位于充氧与贫氧环境的边界面,局部岩石曾暴露地表经历了较强的风化和蚀变作用,海水中硒有可能来自底部富硒斑脱岩的氧化淋滤或海底热液.据此推测遵义松林小竹牛蹄塘组下部岩石的沉积环境极可能处在局限盆地靠近陆地的边缘部分,经历了充氧→贫氧→缺氧/无氧→贫氧的演化阶段. 相似文献
183.
New Pb-, Sr-, and Nd-isotopic data have been obtained for the rocks of volcanoes overlying a wide range of depths (100–580 km) to the Wadati-Benioff Zone (WBZ) in the New Britain island arc, Papua New Guinea. Well-defined trends consistent with two-component mixing are observed in combined Pb-isotope/trace-element plots. One of the components is believed to represent a slab contribution whose isotopic signature, unlike those noted for several other arcs, appears to be dominated by subducted, altered, oceanic crust rather than by sediment. This conclusion is consistent with the results of a recent Be–B study of New Britain rocks. The influence of the slab component is considered to decrease as depth to the WBZ increases. Higher abundances of high-field-strength elements correlate with increasing depths to the WBZ, and may be indicative of smaller degrees of partial melting of the mantle wedge as WBZ depths increase. Abundances of other incompatible elements appear to reflect a complex interplay between the slab-derived flux and melting process. 相似文献
184.
Economic and social development indicators suggest that the small West African state of Sierra Leone is among the poorest
countries in the world. Sierra Leone’s economy and quality of life deteriorated rapidly during a decade of political instability
and civil war in the 1990s, when many people fled their homes and abandoned their livelihoods due to the rebel insurgency.
This paper examines the post-war reconstruction scenario in Sierra Leone and presents recent evidence from two rural communities
in the Eastern Province that were badly affected by the conflict. The paper considers the links between the farming and diamond
mining sectors, which, despite severe dislocation during the conflict period, have proved to be remarkably resilient. It is
argued that seasonal labour mobility associated with this dual economy has not only continued to be a key ingredient in sustaining
livelihood portfolios, but is actually an essential pre-condition for the creation of an enabling environment for sustainable
post-conflict return.
相似文献
Tony BinnsEmail: |
185.
The atomic-scale interactions that occur between cations and the metal-binding cell wall components common to many gram-positive bacteria were investigated using molecular simulations techniques. We examined the adsorption of Cd and Pb onto peptidoglycan and teichoic acid components of the bacterial cell wall using classical energy force field methods. Within the framework of molecular mechanics and the Cerius2 modeling software, we used energy minimization, conformational analysis, and molecular dynamics to examine the different components of the cell wall and to determine relative binding energies and structural configurations of the cell wall components, both with and without the metals present. Electronic structure calculations of representative metal-organic complexes validate the more practical classical methods required in simulating the large number of atoms associated with the cell wall components. The classical force field simulations were conducted in both gas phase and solvated periodic cells. Force field-based simulation techniques can adequately describe the interactions of Cd with the cell wall, defining both metal ion coordinations and binding distances. However, the classical force field approach is inconsistent in describing the observed Pb-cell wall interactions due to possible limitations in the force field parameters, the propensity for Pb to form hydroxides at circumneutral pH, or the dominance of other adsorption mechanisms. 相似文献
186.
We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at 8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp. 相似文献
187.
Marcelo Assumpo Meijian An Marcelo Bianchi George S.L. Frana Marcelo Rocha Jos Roberto Barbosa Jesús Berrocal 《Tectonophysics》2004,388(1-4):173
The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate. 相似文献
188.
Optical absorption spectra are presented for taramellite, traskite and neptunite, all of which have both Fe2+ and Ti4+ as major elements. The spectra of each of these minerals are dominated by a single, intense absorption band in the 415 to 460 nm region with 7000 to 9000 cm?1 halfwidth. These transitions, assigned to Fe2+-Ti4+ intervalence charge transfer, showed little difference in intensity at 80 and 300 K and have molar absorptivities which range from ~100 to ~1300 M?1 cm?1. The Fe2+-Ti4+ absorptions in these standards generally compare well to other mineral spectra in which Fe2+ — Ti4+ intervalence absorption has previously been proposed with the exception of the most cited example, blue corundum. 相似文献
189.
Juraj Majzlan Peter Glasnák Robert A. Fisher Mary Anne White Michel B. Johnson Brian Woodfield Juliana Boerio-Goates 《Physics and Chemistry of Minerals》2010,37(9):635-651
Jarosite phases are common minerals in acidic, sulfate-rich environments. Here, we report heat capacities (C
p) and standard entropies (S°) for a number of jarosite samples. Most samples are close to the nominal composition AFe3(SO4)2(OH)6, where A = K, Na, Rb, and NH4. One of the samples has a significant number of defects on the Fe sites and is called the defect jarosite; others are referred
to as A-jarosite. The samples, their compositions, and the entropies at T = 298.15 K are:
There are additional configurational entropies of 13.14 and 8.23 J mol−1 K−1 in defect and NH4-jarosite, respectively. A detailed analysis of the synchrotron X-ray diffraction patterns showed a large anisotropic peak
broadening for defect and NH4-jarosite. The fits to the low-temperature (approx. <12 K) C
p data showed that our samples can be divided into two groups. The first group is populated by the K-, Na-, Rb-, and NH4-jarosite samples, antiferromagnetic at low temperatures. The second group contains the H3O-jarosite (studied previously) and the defect jarosite. H3O- and defect jarosite are spin glasses and their low-T
C
p was fit with the expression C
p = γT + ΣB
j
T
j
, where j = (3, 5, 7, 9). The linear term is typical for spin glasses and the sum represents the lattice contribution to C
p. Surprisingly, the C
p of the K-, Na-, Rb-, and NH4-jarosite samples, which are usually considered to be antiferromagnetic at low temperatures, also contains a large linear
term. This finding suggests that even these phases do not order completely, but have a partial spin-glass character below
their Néel transition temperature. 相似文献
Sample | Chemical composition | S o/(J mol−1 K−1) |
---|---|---|
K-jarosite | K0.92(H3O)0.08Fe2.97(SO4)2(OH)5.90(H2O)0.10 | 427.4 ± 0.7 |
Na-jarosite | Na0.95(H3O)0.05Fe3.00(SO4)2(OH)6.00 | 436.4 ± 4.4 |
Rb-jarosite | RbFe2.98(SO4)2(OH)5.95(H2O)0.05 | 411.9 ± 4.1 |
NH4-jarosite | (NH4)0.87(H3O)0.13Fe3.00(SO4)2(OH)6.00 | 447.2 ± 4.5 |
Defect jarosite | K0.94(H3O)0.06Fe2.34(SO4)2(OH)4.01(H2O)1.99 | 412.7 ± 4.1 |
190.