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41.
The effect of thermal modulation on the onset of convection in a horizontal, anisotropic porous layer saturated by a viscoelastic
fluid is investigated by a linear stability analysis. Darcy’s law with viscoelastic correction is used to describe the fluid
motion. The perturbation method is used to find the critical Rayleigh number and the corresponding wavenumber for small amplitude
thermal modulation. The stability of the system characterized by a correction Rayleigh number is calculated as a function
of the thermal and mechanical anisotropy parameters, the viscoelastic parameters and the frequency of modulation. It is found
that the onset of convection can be delayed or advanced by the factors represented by these parameters. The results of the
problem have possible implications in mantle convection. 相似文献
42.
First-principles calculations using the GGA+U method have been made to investigate the spin transitions of iron in MgSiO3 perovskite up to 120 GPa, with Fe2+ and Fe3+ at the A and B sites involving different substitution mechanisms and various charge compensation configurations. Our results, unlike those from previous local density approximation and generalized gradient approximation calculations, are now consistent with available experimental data for the spin states of Fe3+ in perovskite. In particular, our calculations show that both Fe2+ and Fe3+ at the A site do not exhibit any spin transition at the lower mantle conditions. However, Fe3+ at the B site in both (Mg0.9375Al0.0625)(Si0.9375Fe0.0625)O3 and (Mg0.9375Fe0.0625)(Si0.9375Fe0.0625)O3 undergoes a high-spin to low-spin transition at ~ 35 GPa, and this spin-transition pressure is largely independent on substitution mechanisms and charge compensation configurations. 相似文献
43.
ABSTRACTAs part of an investigation of carbonate systems under mantle pressures and temperatures, phase relations in the K2CO3–MgCO3 system have been studied at 3?GPa and 800–1300°C. Subsolidus assemblages comprise the stability fields of K2CO3?+?K2Mg(CO3)2 and K2Mg(CO3)2?+?MgCO3 with the transition boundary near 50?mol% K2CO3 in the system. The K2CO3–K2Mg(CO3)2 eutectic is located at 840°C and 52?mol% K2CO3. The K2CO3 content in the melt coexisting with potassium carbonate increases to 85?mol% as temperature increases to 1050°C. K2CO3 remains solid up to 1250 and melts at 1300°C. K2Mg(CO3)2 melts incongruently at 890°C to produce magnesite and a liquid containing 51?mol% K2CO3. As temperature increases to 1300°C, the K2CO3 content in the liquid coexisting with magnesite decreases to 27?mol%. 相似文献
44.
Abstract The results of thermodynamic calculations in the FeO-MgO-SiO2 (FMS) system show that the elastic properties and density across the 400-km discontinuity might be consistent with a transition zone of isochemical pyrolite composition and do not require chemical stratification. However, phase changes in pyrolite do not explain the 650-km discontinuity. 相似文献
45.
This paper discusses studies of the migration over time of hypocenters from great depths to the Earth’s surface. The calculations are based on the notion that surface fracturing of the external lithosphere corresponds to brittle fracturing, while internal lithosphere fracturing, accompanied by a decrease of earthquake hypocenter depth, corresponds to ductile fracturing. A procedure for calculating periods of the rising of earthquake sources in an elastic-plastic environment is suggested. Finally, it was revealed that the rising of hypocenters to the surface takes approximately 8–10 years. 相似文献
46.
Polycrystalline aggregates lacking four independent systems for the glide of dislocations can deform in a purely viscoplastic regime only if additional deformation mechanisms (such as grain boundary sliding and diffusion) are activated. We introduce an implementation of the self-consistent scheme in which this additional physical mechanism, considered as a stress relaxation mechanism, is represented by a nonlinear isotropic viscoplastic potential. Several nonlinear extensions of the self-consistent scheme, including the second-order method of Ponte-Castañeda, are used to provide an estimate of the effective viscoplastic behavior of such polycrystals. The implementation of the method includes an approximation of the isotropic potential to ensure convergence of the attractive fixed-point numerical algorithm. The method is then applied to olivine polycrystals, the main constituent of the Earth's upper mantle. Due to the extreme local anisotropy of the local constitutive behavior and the subsequent intraphase stress and strain-rate field heterogeneities, the second-order method is the only extension providing qualitative and quantitative accurate results. The effective viscosity is strongly dependent on the strength of the relaxation mechanism. For olivine, a linear viscous relaxation (e.g. diffusion) could be relevant; in that case, the polycrystal stress sensitivity is reduced compared to that of dislocation glide, and the most active slip system is not necessarily the one with the smallest reference stress due to stress concentrations. This study reveals the significant importance of the strength and stress sensitivity of the additional relaxation mechanism for the rheology and lattice preferred orientation in such highly anisotropic polycrystalline aggregates. 相似文献
47.
Xenoliths of garnet and spinel-garnet 1herzolite from Mingxi, Eastern China have been studied in terms of comprehensive methods of mineralogy, petrology, fabrics, mierostructures and infrared spectrum. The temperature and pressure equilibration of the xenolith have been calculated using the TEMPEST program with the two-pyroxene geothermometer of Bertrand and Mercier (1985) combined with the geobarometer of Nickel and Green (1985). The temperature and pressure of xenoliths range from~1300 K at 1. 9 GPa (60 km) to ~1500 K at 2. 6 GPa (80 km). The geotherm inferred from xenolith is excellently consistent with the measured surface heat flow (70 mW/m2) in Mingxi. A character of geothermal curve and conductive type suggest that these xenoliths are from the lithosphere, not from the asthenosphere. The geotherm and mierostructures and preferred orientations of minerals in the xenolith are consistent with tecon-ic thinning of the lithosphere in the present-day extensional back-arc environment. 相似文献
48.
INVERSION OF MULTILAYERED VARIABLE DENSITY MODELS AND STUDY OF CRUSTAL AND UPPER MANTLE STRUCTURE 总被引:1,自引:0,他引:1
陈胜早 《中国科学B辑(英文版)》1990,(8)
This paper presents a direct gravity inverse method of multilayered variable density models, and gives gravity forward formulae under various conditions of density-depth functions such as the constant, exponential, linear, sine and cosine, cubic spline, and Fourier progression. The testing of theoretical models and the analysis of real examples have been made for the cubic spline function and have proved that the method is effective. Other formulae can be applied under corresponding conditions by means of optimization or other methods. 相似文献
49.
用阻抗匹配法和PZT压电探针技术,在100 GPa的冲击压力范围内测量了初始密度分别为1.375 g/cm3和2.001 g/cm3两种孔隙度叙永石样品的Hugoniot状态方程。根据其pH-ρH线所给出的高温高压相变点,用Grüneisen状态方程计算其相变点压力所对应的温度,并结合常压下受热相变的温度值,建立了“高岭石/Al2O3+SiO2+H2O”的温度-压力相平衡图。通过该相图与线性地热线的交点推断:高岭石至少可在上地幔50 km深处作为一种含水(OH-)矿物而稳定存在;或在俯冲板块中至少于133 km深处作为一种含水(OH-)泥质沉积物的过渡相而存在。 相似文献