A multiphase model of core formation
A multiphase model of core formation
复制标题
核心形成的多相模型
DOI：
10.1111/j.1365-246x.2010.04528.x
复制
发表时间：
2010
影响因子：
2.8
通讯作者：
F. Dubuffet
中科院分区：
文献类型：
--
作者：
O. Šrámek;Y. Ricard;F. Dubuffet
总结
固体行星的分化与金属从硅酸盐中的分离发生在冥古宙时期，而行星仍在通过吸积生长。当至少金属相是液体时，金属的分离发生，并且通过底辟不稳定性和更扩散的渗滤流的组合进行。在本文中，我们开发了一个形式主义来自Bercovici等人。可以同时处理两个组件，硅酸盐和金属，其中金属可以存在于固体和液体状态。力学方程是非Boussinesq的横向密度变化是同一数量级的密度本身。当金属是固体时，金属和硅酸盐被锁定在一起，我们将它们的混合物视为单相流体，其中密度是组成（铁硅酸盐比例）的函数。当金属是液体时，它可以从硅酸盐中分离出来，两相通过剪切应力（例如达西流）和法向应力相互作用。铁液体积比的演变受固液两相压力差的控制。能量守恒方程考虑了引力能以热的形式耗散的不同机制。我们实现的2-D笛卡尔数值代码来解决这些方程，利用以前没有被用于地球物理两相建模的数值技术，我们讨论的数值方面和基准的解决方案。我们提出的模拟核幔分异表明，第一个影响，熔化铁相附近的表面是潜在的能够触发整个核幔分离的失控现象。这种不稳定性的影响和行星的大小和初始行星温度的阈值进行了研究。金属的偏析发生的机制，以前没有建议，这是中间的通常的底辟不稳定性和孔隙波。虽然我们不能探索我们的数值模型的整个参数空间，我们展示了各种模拟，阐明了最重要的参数，如固体和金属粘度或重力的深度依赖性的作用。
SUMMARY
The differentiation of solid planets with segregation of metal from silicates happens during the Hadean time while the planet is still growing by accretion. The separation of metal occurs when at least the metallic phase is liquid and proceeds by a combination of transport by diapiric instabilities and by more diffuse percolation flow. In this paper we develop a formalism derived from Bercovici et al. that can handle simultaneously two components, silicates and metal, and where the metal can be present both in solid and liquid states. The mechanical equations are non-Boussinesq as the lateral density variations are of the same order as the density itself. When the metal is solid, the metal and the silicates are locked together and we treat their mixture as a single-phase fluid where density is function of composition (iron–silicate proportions). When metal is liquid, it can separate from the silicates and the two phases interact through shear stress (e.g. Darcy flow) and normal stress. The evolution of the volume proportion of liquid iron is controlled by the difference of pressure between the solid and liquid phases. The energy conservation equation takes into account the different mechanisms by which the gravitational energy is dissipated as heat. The 2-D Cartesian numerical code that we implemented to solve these equations makes use of numerical techniques that have not been previously used in geophysical two-phase modelling; we discuss the numerical aspects and benchmark the solutions. We present simulations of core–mantle differentiation showing that the first impact that melts the iron phase near the surface is potentially able to trigger the whole core–mantle segregation in a runaway phenomenon. The threshold of this instability in terms of the impactor and planetary size and the initial planetary temperature is investigated. The segregation of the metal occurs by a mechanism that was not suggested before and which is intermediate between the usual diapir instability and a porosity wave. Although we cannot explore the whole parameter space of our numerical model, we show various simulations that clarify the role of the most important parameters, such as the solid and metal viscosities or the depth dependence of gravity.