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61.
62.
Compositional and thermal convection in magma chambers 总被引:7,自引:1,他引:7
Daniel Martin Ross W. Griffiths Ian H. Campbell 《Contributions to Mineralogy and Petrology》1987,96(4):465-475
Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 106 times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included.The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage. 相似文献
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64.
We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate. 相似文献
65.
Patterns in seasonal abundance (no. per m2 surface area), growth and biomass (g per m2 surface area) of an annual fish, the Atlantic silverside, Menidia menidia (L.) were investigated in a marsh and more seaward bay region of Essex Bay, Massachusetts from August 1976 to May 1978 using a quantitative beach seining technique. Silverside abundance varied greatly by season and year class during the study period. Abundance was high in 1976 but winter mortality (99%) left an adult density of only .01 per m2 surface area in the marsh during spring 1977. Resultant 1977 year class density in the marsh was 1.88 per m2 by late fall 1977 but winter mortality again produced an adult density of .01 per m2 in spring 1978. Abundance was generally higher in the marsh than in the bay region especially during spring and late fall when catches in the bay were negligible. Based on catch rate comparisons, the summer and fall juvenile abundance of the 1976 year class was much higher than the juvenile abundance of the 1977 year class. Coincidentally, mean lengths and condition of the abundant 1976 year class in the late fall were significantly lower than those of the 1977 year class, suggesting density dependent population regulation. In both years, juveniles grew rapidly and reached full adult size by November when an offshore movement to deeper waters outside Essex Bay occurred. Biomass peaked in the marsh region in late fall 1977 at 7.8 g per m2 wet weight. Winter mortality was size selective, favoring larger individuals. The annual life history design of M. menidia including an offshore winter movement and high winter mortality suggests that silversides represent an important pathway of energy flow from marsh to offshore trophic systems. 相似文献
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67.
Gerald M. Ross 《Precambrian Research》1983,20(2-4)
The Hornby Bay Group is a Middle Proterozoic 2.5 km-thick succession of terrestrial siliciclastics overlain by marine siliciclastics and carbonates. A sequence of conglomeratic and arenaceous rocks at the base of the group contains more than 500 m of mature hematitic quartz arenite interpreted to have been deposited by migrating aeolian bedforms. Bedforms and facies patterns of modern aeolian deposits provided a basis for recognizing two sequences of aeolian arenite. Both sequences interfinger with alluvial—wadi fan conglomerates and arenites deposited by braided streams. Depositional processes, facies patterns and paleotopographic position of the arenites are consistent with modern sand sea dynamics.Distal aeolian facies in both sequences are composed of trough crossbed megasets deposited by climbing, sinuous-crested, transverse dunes. Megasets comprise a gradational assemblage of tabular to wedge-planar cosets formed by deflation/reactivation of dune lee slopes and migration of smaller superposed aeolian bedforms (small dunes and wind ripples). Megasets in the proximal facies are thinner, display composite internal stratification and have a tabular-planar geometry which suggests that they were formed by smaller, straight-crested transverse dunes. Most stratification within the crossbeds is inferred to have formed by the downwind climbing of aeolian ripples across the lee slopes of dunes.Remarkably few Precambrian aeolian deposits have been reported previously. This seems anomalous, because most Precambrian fluvial sediments appear to have been deposited by low sinuosity (braided) streams, the emergent parts of which are prime areas for aeolian deflation. Frequent floods and rapid lateral migration of Precambrian humid climate fluvial systems probably restricted aeolianite deposition to arid paleoclimates. Thus the apparent anomaly may reflect non-recognition and/or non-preservation of aeolianites and/or variations in some aspect of sand sea formation and migration unique to the Precambrian. Reconstruction of the Hornby Bay Group aeolianites using recently developed criteria for their recognition suggests that the latter reason did not exert a strong influence. 相似文献
68.
Prior to the penetration of the European capitalist economy into central and northern Australia, the Aboriginal mode of production was one of hunting and gathering. Spiritual, economic and social relationships betwen people and land produced a spatial distribution of small bands of people living on their traditional lands. To the incoming white settlers the low population density and type of land use appeared uneconomic. This observation was used as ideological justification for the expropriation and development of the land for commercial pastoralism. This paper discusses the articulation of the Aboriginal and European modes of production in the East Kimberley region of northwest Australia. It argues that the introduced capitalist economy virtually destroyed the Aboriginal means of subsistence, incorporating Aborigines into the new mode of production within various subordinate roles. The exploitation of Aborigines, formerly as labour and now predominantely as consumers, is fundamental to the maintenance of the capitalist mode of production in pastoral areas. The spatial effects of the dominant mode of production have been to centralise Aborigines in locations suited to the various phases of development of the European economy, undermining the traditional Aboriginal social-spatial structure. The centralisation of the Aboriginal population also enhances the opportunities for the further development of the mining industry, which is in direct competition with Aboriginal interests in land. Aboriginal resistance to these spatial tendencies is intense in many quarters, but efforts to return to living on traditional lands and to develop alternative ways of living are hampered by the dominant mode of production. 相似文献
69.
Low Energy Transfer to the Moon 总被引:15,自引:0,他引:15
W. S. Koon M. W. Lo J. E. Marsden S. D. Ross 《Celestial Mechanics and Dynamical Astronomy》2001,81(1-2):63-73
In 1991, the Japanese Hiten mission used a low energy transfer with a ballistic capture at the Moon which required less Vthan a standard Hohmann transfer. In this paper, we apply the dynamical systems techniques developed in our earlier work to reproduce systematically a Hiten-like mission. We approximate the Sun–Earth–Moon-spacecraft 4-body system as two 3-body systems. Using the invariant manifold structures of the Lagrange points of the 3-body systems, we are able to construct low energy transfer trajectories from the Earth which execute ballistic capture at the Moon. The techniques used in the design and construction of this trajectory may be applied in many situations. 相似文献
70.