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101.
The influence of the Landau-Pomeranchuk effect on the development of a shower generated by ultrarelativistic particles bombarding the surface of a pulsar is discussed. Because of this effect, the path length of the shower increases while low-energy photon generation is strongly suppressed. In view of this, the mechanism of pair production suggested by Cheng, Ruderman, and Jones for the pulsar magnetosphere, may be essential only for pulsars whose magnetic field intensity at the surface lies in a relatively narrow range of aroundB 1012 G. 相似文献
102.
Burnley IH 《Applied geography (Sevenoaks, England)》1983,3(4):277-301
This paper is concerned with the methodology of forecasting population change and structures of large cities and their major subregions. "The model isolates quantifiable base populations and their growth, and international, inter-state and city-hinterland migration streams and the national increase attributable to each, and intra-urban mobility, as components of growth and change." The model is applied to data for Sydney, Australia, and its applicability to other urban systems is considered. 相似文献
103.
A series of hydrodynamical models of type-II supernova outbursts (SNII) has been calculated. Approximate relations connecting the total outburst energy ε, the mass of envelope ejectedM, the presupernova radiusR, and the amount of ionizing quanta radiated by the supernovaeN H with such values as the duration of the light curve plateau Δt, and absolute magnitude in the wavelength bandV and photospheric velocityU PH observed near the middle of the plateau have been established. Advantage has been taken of the relations to obtain a preliminary evaluation for the characteristics of the average SN II: ε=7×1050 erg,M=6M ⊙,R=500R ⊙,N H=2×1058. The SNIIs with plateau-like light curves seem to be accounted for by thermonuclear explosions of degenerate cores of red giant stars and result in a total disruption of the star without any stellar remnant. To the contrary, SNIIs with linear light curves have substantially different properties (in particular, they throw considerably less massive envelopes off). These SNII must signify the birth of collapsed objects—neutron stars (pulsars) or black holes. 相似文献
104.
A. I. Konyukhov 《Lithology and Mineral Resources》2008,43(4):380-393
The Gulf of Mexico basin occupies a vast region encompassing the southern continental margin of North America, a considerable part of the Greater Antilles, and the intervening Sigsbee Deep with the oceanic crust. In the north, the basin is contiguous with spurs of the Hercynian Appalachians, the Mississippi Interior and Permian basins. The Mississippi Fan, one of the largest in the world, governs the bottom topography and structure in the eastern Gulf of Mexico. The abyssal basin is surrounded in many areas by steep continental slopes passing in places into escarpments: Sigsbee, Campeche, and others. It is only in the Yucatan Peninsula region that the continental slope merges with a wide shelf. The Cuban-North Haiti meganticlinorium frames the basin on the Cuba Island side. 相似文献
105.
In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized
zone of pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change
from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly
perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope
system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of
2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave.
−7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local
contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the
Platreef pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef
has generally higher plagioclase and pyroxene δ
18O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic
evidence for sub-solidus interaction with fluids, and the Δ
plagioclase–pyroxene values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope
system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative
δD values suggest a greater involvement of meteoric water. Consistently higher values of Δ
plagioclase–pyroxene in the Platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO2-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction
is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale.
The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination
played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO2 produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation. 相似文献
106.
The Mordor Alkaline Igneous Complex (MAIC) is a composite intrusion comprising a body of syenite and a funnel-shaped layered
mafic–ultramafic intrusion of lamprophyric parentage, the Mordor Mafic–Ultramafic Intrusion or MMUI. The MMUI is highly unusual
among intrusions of lamprophyric or potassic parentage in containing primary magmatic platinum-group element (PGE)-enriched
sulfides. The MMUI sequence consists largely of phlogopite-rich pyroxenitic cumulates, with an inward dipping conformable
layer of olivine-bearing cumulates divisible into a number of cyclic units. Stratiform-disseminated sulfide accumulations
are of two types: disseminated layers at the base of cyclic units, with relatively high PGE tenors; and patchy PGE-poor disseminations
within magnetite-bearing upper parts of cyclic units. Sulfide-enriched layers at cycle bases contain anomalous platinum group
element contents with grades up to 1.5 g/t Pt+Pd+Au over 1-m intervals, returning to background values of low parts per billion
(ppb) on a meter scale. They correspond to reversals in normal fractionation trends and are interpreted as the result of new
magma influxes into a continuously replenished magma chamber. Basal layers have decoupled Cu and PGE peaks reflecting increasing
PGE tenors up-section, due to increasing R factors during the replenishment episode, or progressive mixing of between resident PGE-poor magma and more PGE-enriched
replenishing magma. The presence of PGE enriched sulfides in cumulates from a lamprophyric magma implies that low-degree partial
melts do not necessarily leave sulfides and PGEs in the mantle restite during partial melting.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
107.
A. A. Bulychev D. A. Gilod M. I. Vereshchagina 《Moscow University Geology Bulletin》2008,63(5):320-326
This paper discusses the structure of the American-Antarctic Ridge based on analysis of the gravity field. Analysis of the gravity field in different reductions and its transforms allowed the revelation of zonal, regional, and local specific features in the field’s structure and their correlation with contemporary notions on the tectonic zoning of the American-Antarctic Ridge and adjacent regions. The lineament structures of the field allowed locating the areas of spreading zones and principal transform and fracture zones. Comparison between the location of these structural features and the interpretation results of shipboard magnetic measurements confirmed the reliability of the received data. Results of the performed investigations presented in the form of structural schemes are a basis for constructing a model of the tectonosphere under the American-Antarctic Ridge. 相似文献
108.
109.
Evolution of melt composition during intrusion of basalts into a silicic magma chamber 总被引:3,自引:0,他引:3
P. Yu. Plechov I. S. Fomin O. E. Mel’nik N. V. Gorokhova 《Moscow University Geology Bulletin》2008,63(4):247-257
The article describes heat exchange between basaltic and rhyolite melts accompanied by fractional crystallization of phases in a basaltic melt. A numerical model has been developed for the homogenization mechanism of magma composition during intrusion of basaltic magma batches into felsic magma chambers. The results of numerical modeling demonstrate that the time needed for cooling the basalts and their fractionation to rhyolite melts is much shorter than the time required for chemical interaction based on diffusive mechanisms. 相似文献
110.
G. A. Golodkovskaya I. V. Kalinichenko Yu. L. Filimonov V. G. Khloptsov 《Moscow University Geology Bulletin》2008,63(3):162-170
This paper discusses issues of the decline of the reservoir properties of arenaceous-argillaceous rocks as a result of declining porosity due to long-term operation of underground gas storage facilities. An analysis of the many-year operation of storage facilities, as well as calculation, has revealed that the active capacity of a storage reservoir gradually decreases under certain conditions of underground storage operations. We performed a series of experiments with model specimens in order to support the hypothesis of decreasing reservoir (capacity-filtration) properties because of changes in the value and structure of the pore space. These experiments showed that the cyclic loading and unloading of arenaceous-silty rocks during long-term operation of underground gas storage facilities can significantly decrease the reservoir parameters of reservoirs created within worked out gas-and-gas condensate fields. Laboratory studies of model specimens corresponding to feldspar sandstone in their composition, porosity, and strength proved that porosity considerably decreases in such reservoirs at actually existing values of formation pressure. Tests of sand performed under conditions close to those existing during the development of hydrocarbon fields also showed that their permeability gradually decreases in the process of cyclic changes of effective pressure. 相似文献