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For a long time, granites have been considered as passive bodies ascending under intrinsic negative density and viscosity contrasts with their host rocks. Chemical variations within a granitic body resulted from in situ differentiation and crystal fractionation. Since the mid 1980s, this global view has been significantly modified by (i) shifting melting from water-saturated conditions to fluid-absent reactions, (ii) increasing the role played by the mantle during granite generation, (iii) reassessing the rheology of partially molten rocks, (iv) demonstrating stepwise segregation and ascent of magmas by analogue and numerical models, (v) combining structural, geophysical and geochemical studies to reveal the internal structures in granitic plutons. It results that a granitic body is built up by a discontinuous accumulation of successive magma intrusions. The discontinuous nature of magma emplacement has also significant consequences for its ability to generate ore. The processes that lead to ore deposits are examined, with a brief review of the magmatic and fluid phases that concentrate ore forming elements. Examples are taken from crustal-derived granites and porphyry-type deposits. Those are considered as the two end-members of magmatic and hydrothermal ore deposits. The source characteristics of the magma, the emplacement mechanisms and magma mixing processes are the frame that controls the potential to carry base metals with the magma. The distribution of elements is controlled by diffusion, partition between minerals and melt, solubility and redox conditions. Variations of those parameters are examined by considering their activation energy which controls the exponential dependence with temperature. A characteristic length depending on the activation energy, temperature variation and time is estimated for a characteristic time lag of 30 ka. The intrusion of a magma into a magma chamber of similar composition, hence temperature, has few effects on diffusion, partition coefficient and redox conditions, because of a too low temperature contrast. The intrusion of a mafic magma into a felsic one induces a variation of 300 °C in both magmas. The characteristic length of diffusion may vary by up to two orders of magnitude, whereas the variation of partition coefficients is only one order of magnitude. The redox conditions are about 2.5 log unit in the mafic magma, but they can vary by 7 log units in the felsic magma. Hence, a strong decrease in δD values is observed in porphyry-type deposits. The effect is a removal of the elements with higher activation energy (W, Sn, Zr) from the mafic to the felsic magma. Deformation during the late stages of emplacement also controls ore formation. 相似文献
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1998年夏季副热带高压的短期结构特征及形成机制 总被引：19，自引：3，他引：19
通过对 1998年 7月副热带高压短期结构及其演变的诊断 ,指出 5 0 0hPa西太平洋副热带高压内并非均一的下沉运动 ,而是多处、多时存在上升运动 ,这是由西太平洋副热带高压的复杂的动力和热力性所决定。诊断还发现 ,在西太平洋副热带高压脊区下方的近地面层 ,多存在一个辐散下沉层 ,这是副热带高压内天气一般多晴空或少云的主要原因。利用动力学理论 ,研究副热带高压的复杂结构的成因 ,指出在 5 0 0hPa副热带高压的西部 ,高低空风场的分布决定了该区域必然多上升运动 ;高空强负涡度平流的存在是副热带高压内出现下沉运动的主要强迫机制 相似文献
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J. W. Elder 《Pure and Applied Geophysics》1978,117(1-2):3-14
The role of the buoyancy barrier of the stratified crust in controlling the ascent of magmas is represented by a model which operates close to lithostatic equilibrium and in which fractional crystallization of the magma or partial melting of the ambient rock can occur. The density structure of the crust has a powerful effect in trapping magma and thereby in controlling the occurrence of high level geothermal systems. The model is tested in Part II (this volume). 相似文献
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Launch strategy for Indian lunar mission and precision injection to the Moon using genetic algorithm
V. Adimurthy R. V. Ramanan S. R. Tandon C. Ravikumar 《Journal of Earth System Science》2005,114(6):711-716
The Indian lunar mission Chandrayaan-1 will have a mass of 523 kg in a 100 km circular polar orbit around the Moon. The main
factors that dictate the design of the Indian Moon mission are to use the present capability of launch vehicles and to achieve
the scientific objectives in the minimum development time and cost. The detailed mission planning involves trade-off studies
in payload optimization and the transfer trajectory determination that accomplishes these requirements. Recent studies indicate
that for an optimal use of the existing launch vehicle and space-craft systems, highly elliptical inclined orbits are preferable.
This indeed is true for the Indian Moon mission Chandrayaan-1. The proposed launch scenario of the Indian Moon mission program
and capabilities of this mission are described in this paper, highlighting the design challenges and innovations. Further,
to reach the target accurately, appropriate initial transfer trajectory characteristics must be chosen. A numerical search
for the initial conditions combined with numerical integration produces the near accurate solution for this problem. The design
of such transfer trajectories is discussed in this paper. 相似文献
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Results from radiosoundings, performed both over land and over sea, show that the ascent rate of a radiosounding balloon, the vertical velocity of the balloon, can be used to determine the height of the boundary layer. In many cases the balloon has a higher ascent rate in the boundary layer and a lower, less variable, ascent rate above. The decrease in ascending velocity appears as a jump at the top of the boundary layer. Two examples of potential temperature profiles for unstable stratification and one profile for stable conditions are shown with the corresponding ascent rates. A comparison between the boundary-layer height determined from potential temperature profiles and from ascent rates is presented for a larger dataset. The different ascent rates of the balloon in the boundary layer and above can be explained by a decrease in drag on the balloon in combination with a lowering of the critical Reynolds number in the boundary layer caused by turbulence. Hence, by simply logging the time from release of a radiosonde, it is possible to obtain additional information that can be used to estimate the height of both the unstable and stable boundary layers. 相似文献
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