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Assuming a radially stratified Newtonian mantle in a steady-state approximation, we demonstrate that the permeability of a viscosity interface at 660-km depth strongly depends on the wavelength of buoyancy forces driving the flow. The flow induced by long-wavelength loads penetrates through the boundary freely even if the viscosity increases by two orders. In contrast, the boundary is practically impermeable for short-wavelength loads located in the upper mantle. Thus, a stepwise increase of viscosity is a significant obstacle for small descending features in the upper mantle, but huge upper mantle downwellings, or upwellings formed in the-lower mantle can overcome it easily. This indicates that certain care is necessary in interpreting the seismic structure of the mantle by means of flow models. The global tomographic image includes only the first few degrees of the harmonic series and, consequently, its interpretation in terms of a present-day flow field results in a predominantly whole-mantle circulation even for extreme viscosity contrasts. 相似文献
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The shear viscosities and 1 bar heat capacities of glasses and melts along the 67mol% silica isopleth in the system SiO2-Al2O3-Na2O-TiO2 have been determined in the temperature ranges 780-1140 K and 305-1090 K respectively. Anomalous behaviour of both these properties is observed for compositions rich in TiO2 and/or Al2O3, an observation attributed to liquid-liquid phase separation followed by anatase crystallization. For samples which do not show anomalous behaviour, it is found that the partial molar heat capacity of the TiO2 component previously determined in Al-free compositions reproduces our heat capacities to within 1.3%. Viscosity data show that addition of TiO2 tends to increase viscosity and melt fragility at constant temperature. Furthermore, heat capacity and viscosity data may be combined within the framework of the Adam-Gibbs theory to extract values of the configurational entropy of the liquids and qualitative estimates of the variation of the average energy barrier to viscous flow. Configurational entropy at 900K is inferred to decrease upon addition of TiO2, in contrast to previous results from Al-free systems. The compositional limit separating normal from anomalous behaviour, as well as the data for homogenous melts have been used to constrain the structural role of Ti in these samples. Our data are consistent with a majority of Ti in five-fold coordination associated with a titanyl bond, in agreement with previous spectroscopic studies. Furthermore, we find no evidence for a Ti-Al interaction in our samples, and we are led to the conclusion that Al and Ti are incompletely mixed, a hypothesis consistent with the observed reduction of configurational entropy upon addition of TiO2, suggesting an important role of medium range order in controlling the variations in thermodynamic properties. 相似文献
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岩石圈拆沉作用是指部分岩石圈由于重力不稳定性而沉入软流圈中的过程,与造山带的演化密切相关.本文基于非牛顿流体近似的有效黏度模型对岩石圈拆沉的过程进行了数值模拟,着重分析了岩石圈的黏度结构对拆沉作用的影响.数值模拟显示,下地壳控制着地壳与岩石圈地幔的耦合程度,对拆沉作用的过程和形态有很大的影响;在一定的初始重力不稳定性条件下,当岩石圈地幔相关的有效黏度在1022~1024Pa·s时,拆沉作用有可能在5~30 Ma时间范围内发生.从拆沉的形态看,在上述岩石圈地幔有效黏度范围内,黏度越大,重力不稳定性发展越慢,岩石圈剥离(peel away)范围越大.从拆沉的结果看,当拆沉块体与上部岩石圈完全断离时,造山带完成了从挤压构造到伸展构造的转化过程.最后,结合秦岭—大别—苏鲁造山带的岩浆事件和构造演化,讨论了岩石圈拆沉在该地区的应用. 相似文献
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