Percolative core formation in planetesimals enabled by hysteresis in metal connectivity
Percolative core formation in planetesimals enabled by hysteresis in metal connectivity
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DOI：
10.1073/pnas.1707580114
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发表时间：
2017-12-19
影响因子：
11.1
通讯作者：
Prodanovic, Masa
中科院分区：
文献类型：
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作者：
Ghanbarzadeh, Soheil;Hesse, Marc A.;Prodanovic, Masa
致密的成核熔体通过多孔流的分离是早期微行星核形成的一种自然机制。然而，实验观察表明，纹理平衡的金属熔体不润湿的硅酸盐晶界，并倾向于驻留在孤立的口袋，防止渗滤。在这里，我们使用孔隙尺度模拟来确定诱导多孔流动所需的最小熔体分数，渗流阈值。类地行星的组成表明，典型的微行星含有足够的金属来克服这个门槛。然而，目前认为，熔体偏析是防止夹断熔体分数略低于逾渗阈值。与以前的工作相比，我们对不规则晶粒几何形状的模拟表明，纹理平衡的熔体网络仍然连接到只有1至2%的熔体分数。这种熔体连通性的滞后现象允许在含有足够金属的微行星中形成扩散核，以超过逾渗阈值。原始无球粒陨石西北非洲（NWA）2993的X射线显微断层摄影提供的证据，金属熔体的渗流。微观结构分析表明，金属-硅酸盐界面具有预期的纹理平衡的孔隙网络与二面角类似于85度的特性。因此，尽管经历了复杂的历史，熔体网络仍然接近织构平衡。这表明熔体连通性的滞后是原始无球粒陨石母体中结晶核形成的一个可行过程。
The segregation of dense core-forming melts by porous flow is a natural mechanism for core formation in early planetesimals. However, experimental observations show that texturally equilibrated metallic melt does not wet the silicate grain boundaries and tends to reside in isolated pockets that prevent percolation. Here we use pore-scale simulations to determine the minimum melt fraction required to induce porous flow, the percolation threshold. The composition of terrestrial planets suggests that typical planetesimals contain enough metal to overcome this threshold. Nevertheless, it is currently thought that melt segregation is prevented by a pinch-off at melt fractions slightly below the percolation threshold. In contrast to previous work, our simulations on irregular grain geometries reveal that a texturally equilibrated melt network remains connected down to melt fractions of only 1 to 2%. This hysteresis in melt connectivity allows percolative core formation in planetesimals that contain enough metal to exceed the percolation threshold. Evidence for the percolation of metallic melt is provided by X-ray microtomography of primitive achondrite Northwest Africa (NWA) 2993. Microstructural analysis shows that the metal-silicate interface has characteristics expected for a texturally equilibrated pore network with a dihedral angle of similar to 85 degrees. The melt network therefore remained close to textural equilibrium despite a complex history. This suggests that the hysteresis in melt connectivity is a viable process for percolative core formation in the parent bodies of primitive achondrites.