Continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings. In one well-studied, classic, flood basalt province, the Mesozoic Karoo province of southern Africa, magmatism occurred in the following tectonic settings: (a) continental rifting leading to ocean-floor spreading in the South Atlantic Ocean (Etendeka suite of Namibia); (b) stretched continental lithosphere and rifting not leading directly to ocean-floor formation (Lebombo suite of southeastern Africa); and (c) an a-tectonic, within-plate, continental setting characterized by an absence of faulting or warping (Lesotho highlands and Karoo dolerites of South Africa). By means of spidergrams of the elements Rb, Ba, Th, Nb, K, La, Ce, Sr, Nd, P, Hf, Zr, Sm, Ti, Tb, Y, V, Ni and Cr, uncontaminated tholeiites from (c) above [i.e. the Lesotho-type continental flood basalts (LTCFB)] are compared with mid-ocean ridge basalts (MORB), ocean-island tholeiites (OIT), and tholeiites and calc-alkali basalts from subduction environments. The comparison reveals the LTCFBs are geochemically distinct. The differences are reflected in relative enrichments or depletions of the more incompatible elements (Rb-Ce) to less incompatible elements (Ce-Y), i.e. the overall slope of the spidergrams, and in anomalous enrichments or depletions of one or more of the elements Th, K, Nb, Sr, Ti, Hf, and Zr. The distinctive geochemical character of the Lesotho LTCFBs is interpreted in terms of a lithospheric mantle source for the basalts. This is supported by isotopic data. There are no major geochemical differences between Lesotho CFBs and basalts of the rift-related Etendeka and Lebombo suites, although the latter are somewhat enriched in Rb, Ba and K. However, unlike the Lesotho basalts, the Lebombo and Etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late-stage dolerites having MORB/OIT (i.e. asthenospheric) geochemical characteristics. The flood basalt/silicic magmatism/late-stage dyke swarm association is characteristic of several rift or thinned lithosphere environments (e.g., Ethiopia, Skye, eastern Greenland) but in many of these the flood basalts have ocean-island basalt (OIT) geochemical characteristics. The Lesotho-type CFB geochemistry is exhibited by the Grande Ronde Basalt of the Columbia River Group (a possible subduction-related flood basalt province) and the basic rocks associated with Mesozoic rifting in the North and South Atlantic. Basalt geochemistry alone is unhelpful in determining the tectonic setting of CFBs although the rift-related environments may be identified by the petrology and geochemistry of the whole igneous suite. A two-source model is proposed for the mantle-derived basic rocks in rift-related CFB provinces. Early enriched basalts are derived from the lithosphere and, following pronounced lithospheric attenuation or rifting, later MORB-like melts are emplaced from the rising asthenosphere. The presence of both Lesotho- and OIT-type geochemical patterns in rift-related CFBs suggests that the lithosphere exhibits different styles of enrichment. 相似文献
Children are considered particularly important in debates about the possibilities and dangers of information and communication technologies (ICT). Discourses on ICT contain paradoxical representations of childhood. On the one hand, unlike most other understandings of child/adult relations, these discourses assume children to be equally, if not more, technologically competent than adults. On the other hand, children's very competence at using ICT is alleged to be putting them “at risk” of abuse or corruption. This paper addresses these moral panics about children and ICT by exploring to what extent and why parents are concerned about their children's safety in on‐line space. In doing so the paper reflects on the extent to which anxieties about children in cyberspace replicate concerns about public outdoor space and the way networked computers emerge as different tools in different households. 相似文献
The solubility of CO2CO fluids in a mid-ocean ridge basalt (morb) has been measured at 1200°C, 500–1500 bar, and oxygen fugacities between NNO and NNO-4. High oxygen fugacities, and thus CO2-rich fluids, were produced by using a starting material equilibrated at NNO, and Ag2C2O4 as the fluid source. Low oxygen fugacities were achieved by using graphite capsules, and MgCO3 as the fluid source. These graphite-saturated fluids have the lowest possibleC/O2CO ratio for a given pressure and temperature.
Experiments were run in a rapid-quench internally heated pressure vessel. Fluid compositions were measured using a simple vacuum technique and by Raman spectroscopy of fluid inclusions. The two techniques yielded comparable results. Fourier transform micro-infrared spectroscopy was used to identify and measure concentrations of dissolved volatiles in double-polished wafers of the quenched glasses. Carbonate was the only carbon-bearing species identified. Raman spectroscopic analysis of inclusion-free areas of glass confirmed the absence of dissolved molecular CO2, CO and carbon. The measured concentrations of dissolved CO2 in the glasses were proportional to the fugacity of CO2 during the experiments, calculated from the measured fluid compositions. The data were fit to the equationXCO2melt(ppm)= 0.492 fCO2 (bar).
The insolubility of CO, compared to CO2, may be related to the fact that dissolution of CO requires reduction of another species in the melt, whereas dissolution of CO2 does not. Due to the fact that CO will be an important component of natural CO fluids at low pressures and low oxygen fugacities, equilibrium dissolved CO2 contents will be less than calculated assuming pure CO2 fluids, but as theC/O2CO ratio in a pure CO fluid at fixed pressure and temperature is a direct function of oxygen fugacity, measurement of the oxygen fugacity of quenched glasses or trapped fluids in natural samples should allow saturation concentrations to be calculated. Dissolved CO2 contents of somemorb are less than expected if they were in equilibrium with pure CO2. These samples must, therefore, have been more reduced than average if they were fluid-saturated. Together with results from other studies of CO2 and H2O solubilities in basalt, the results of this study provide a comprehensive framework for modelling CO2 solution inmorb. 相似文献
The lack of correspondence between traditional economic development measures, such as GNP per capita, and human welfare measures, such as the PQLI, has been well documented in the literature. We devised and analyzed an economic development-welfare disparity measure, and found considerable geographic variability, with actual levels of human welfare much less than expected in many Middle Eastern and African countries, given the level of economic development. Exploratory correlation and regression analysis revealed that inequalities in the distribution of economic power and governmental expenditures contributed to the geographic variability. More importantly, each region improved its standing on the development-welfare disparity measure when relevant factors were controlled via multivariate regression. 相似文献
In the past decade geographers have critiqued the exclusivity of idyllic representations of rurality and sought to explore the diverse experiences of Other social groups in the countryside. This paper builds upon that small but significant strand of research which has highlighted the whiteness of representations of rurality and the consequences of this for racialised minorities. These debates have been crucially important in forwarding our understanding of racially-exclusive constructions of idyllic country living; however, it is important that academics neither assume that such ideas are forever fixed, nor that they affect all minority ethnic groups in the same ways. This paper takes both the potential power and frailty of these ideas seriously, as it examines how the concepts of race, racism and rurality are deployed by different commentators as they debate the place of one specific minority ethnic group in the English countryside. The route taken into this is a consideration of print-media reporting of events in Firle, Sussex, where, in 2003, some white rural residents symbolically purged their village of Gypsy-Travellers by burning a mock caravan complete with effigies at their annual bonfire celebrations. In conclusion, the paper challenges the relative lack of attention to these issues within the white-dominated discipline of geography, and argues that it is imperative to trace the dynamic ways in which the whiteness of idyllic understandings of the rural are reinforced and challenged through contemporary social practice. 相似文献
Youths have increasingly experienced labor market problems over the last several decades. One fruitful line of explanation focuses on structural changes in the demand for labor stemming from deindustrialization, changing skill requirements for employees, and increasing supply competition from women and recent immigrants. While these explanations merit attention, they have not adequately considered facts that condition their impact. This paper considers the argument that intra-metropolitan residential location conditions the effects of metropolitan labor market structure on black and white male youths' employment probabilities. Using a sample of individual-level data drawn from the 1990 census combined with metropolitan-level indicators of economic structure, it was found that some structural effects varied between central-city and suburban male youths. The conditioning role of residential location, and the subsequent nature of the structural effects, varied considerably between black and white male youths. Interpretations of the conditioning role of residential location include a variety of social and institutional effects on individual residents and the stigmatizing effects of some neighborhoods, especially on black male youths. 相似文献
A 1600 year paleoecological record of environmental change is developed from a small lake in the Nothofagus forest of southern Chile (45.5°S, 72°W). High resolution fossil pollen, charcoal, sedimentological, and chrysophycean stomatocyst analyses are used to investigate the impacts of natural and anthropogenic disturbances on terrestrial and lacustrine environments. Chronological control is based on a combination of 210Pb and 14C dating. Temporal resolution during the past 150–200 yr is ca. 8 yr/sample. The macroscopic charcoal record correlates very closely with historical and dendroecological records of 20th century anthropogenic burning in this region. The chrysophyte stratigraphy indicates that this burning had immediate impacts on the lake itself, while the pollen record provides evidence for a succession of vegetation changes lagging slightly behind the disturbance. These palynological changes are very similar to the pollen signal of European disturbance in northeastern North America. Pre-European shifts in chrysophyte assemblages may be due to the influence of earthquake activity on the lake, though there is no corroborative evidence in the pollen or charcoal records. This study demonstrates that high resolution paleoecological methods can be used to help bridge the temporal gap between traditional ecological and paleoecological studies of environmental change in the temperate forests of southern South America. 相似文献