On the cooling of a deep terrestrial magma ocean
On the cooling of a deep terrestrial magma ocean
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关于深层陆地岩浆海洋的冷却
DOI：
10.1016/j.epsl.2016.05.010
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发表时间：
2016
影响因子：
5.3
通讯作者：
H. Samuel
中科院分区：
文献类型：
--
作者：
J. Monteux;D. Andrault;H. Samuel
在地球演化的最初阶段，可能发生过几次完全融化的事件。我们已经开发了一个数值模式来监测从最初完全熔融的地幔冷却和结晶岩浆海洋的热和熔体分数的演变。为了这个目的，我们数值求解热方程在一维球形几何，占湍流传热，并整合最近和强大的实验约束矿物物理。我们探索了不同的初始岩浆海洋粘度，成分，热边界层厚度和初始核心温度。我们表明，厚的陆地岩浆海洋的冷却是一个快速的过程，与整个地幔变得更加粘在20 kyr。由于温度曲线与熔融曲线的斜率差异，熔融地幔的凝固过程是自下而上进行的。与此同时，由于高的地表辐射热流，地壳形成，最后一滴完全熔融的硅酸盐被限制在上地幔。在所研究的参数中，岩浆海洋的寿命主要取决于其粘度。根据核幔边界处的热边界层厚度，地核与岩浆海之间的热耦合既可以在岩浆海凝固期间隔离地核，有利于热的地核，也可以在岩浆海冷却的同时将热量排出地核。然而，热边界层的合理厚度表明，快速的核心冷却，直到核幔边界温度导致一个缓慢的最低地幔。一旦最低地幔的结晶变得显着，地核热损失的效率就会降低。由于更热的液相线有利于在更高温度下结晶，更热的深地幔液相线有利于核心内的热保持。在最初完全熔融的地幔的背景下，很难想象基底岩浆海洋的形成或防止地核的主要热量耗尽。因此，地球的地球发电机在4 Gyr期间仅由核心冷却维持似乎不太可能，需要调用其他运动源。
Several episodes of complete melting have probably occurred during the first stages of the Earth's evolution. We have developed a numerical model to monitor the thermal and melt fraction evolutions of a cooling and crystallizing magma ocean from an initially fully molten mantle. For this purpose, we numerically solve the heat equation in 1D spherical geometry, accounting for turbulent heat transfer, and integrating recent and strong experimental constraints from mineral physics. We have explored different initial magma ocean viscosities, compositions, thermal boundary layer thicknesses and initial core temperatures.We show that the cooling of a thick terrestrial magma ocean is a fast process, with the entire mantle becoming significantly more viscous within 20 kyr. Due to the slope difference between the adiabats and the melting curves, the solidification of the molten mantle occurs from the bottom up. In the meantime, a crust forms due to the high surface radiative heat flow, the last drop of fully molten silicate is restricted to the upper mantle. Among the studied parameters, the magma ocean lifetime is primarily governed by its viscosity. Depending on the thermal boundary layer thickness at the core–mantle boundary, the thermal coupling between the core and magma ocean can either insulate the core during the magma ocean solidification and favor a hot core or drain the heat out of the core simultaneously with the cooling of the magma ocean. Reasonable thickness for the thermal boundary layer, however, suggests rapid core cooling until the core–mantle boundary temperature results in a sluggish lowermost mantle. Once the crystallization of the lowermost mantle becomes significant, the efficiency of the core heat loss decreases. Since a hotter liquidus favors crystallization at hotter temperatures, a hotter deep mantle liquidus favors heat retention within the core. In the context of an initially fully molten mantle, it is difficult to envision the formation of a basal magma ocean or to prevent a major heat depletion of the core. As a consequence, an Earth's geodynamo sustained only by core cooling during 4 Gyr seems unlikely and other sources of motion need to be invoked.
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