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91.
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Susan Ellsworth Alexandra Navrotsky Rodney C. Ewing 《Physics and Chemistry of Minerals》1994,21(3):140-149
Transposed temperature drop calorimetry at 1000 °C was performed on natural zircons (ZrSiO4) from Sri Lanka that were partially to completely metamict due to α-decay event damage (0.06 to 11.7×1015 α-decay events/mg). The enthalpy of annealing at room temperature (ΔHanneal) varies sigmoidally as a function of radiation dose. ΔHanneal reaches a saturation plateau at radiation doses greater than 5×1015 α-decay events/mg. The annealing of several samples to a crystalline structure with broadened diffraction peaks does not significantly affect the enthalpy of annealing. The large magnitude of the enthalpy of annealing plateau, ?59±3 kJ/mol, suggests that the damage to the structure is pervasive on the scale of Angstroms, consistent with the loss of mid-range order characteristic of a glass. The energetics are consistent with, but do not require, chemical heterogeneity caused by micro-domains of amorphous SiO2-rich and ZrO2-rich regions in the metamict state. 相似文献
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African education controlled by the state's Department of Education and Training has collapsed. Insufficient spending, low standards and persistent student boycotts have made township schools undesirable places of learning. African students with any financial means now seek secondary education at multi-racial private schools in affluent white suburbs or at street-academies in the centre of Johannesburg. The establishment of schooling services for African pupils in the traditionally white central business district contributes to the wider process of desegregation of South Africa's major urban area, Johannesburg. 相似文献
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Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama 总被引:3,自引:0,他引:3
Mare J. Defant Lee F. Clark Robert H. Stewart Mark S. Drummond Jelle Z. de Boer René C. Maury Hervé Bellon Thomas E. Jackson Juan F. Restrepo 《Contributions to Mineralogy and Petrology》1991,106(3):309-324
The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava
flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic
activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows
0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites.
The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young
group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed
in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent
among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or
polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples
from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics.
We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within
the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The
young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features
of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial
melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual
garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated
apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano
is related to the subduction of relatively hot lithosphere. 相似文献
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Urban systems are increasingly dominated by large cities that house a disproportionate share of telecommunications infrastructure. This paper investigates whether telecommunications is defining a technological divide between large and smaller cities. The paper provides a conceptual basis for large-city dominance and presents the results of surveys that investigate the use of telecommunications in medium and small Indiana cities. The survey data indicate that telecommunications is used by the set of Indiana cities, but is less prevalent among the set of smallest cities. Chi-square tests, however, suggest that city size does not adequately explain these variations in use. 相似文献