Differentiated planetesimal impacts into a terrestrial magma ocean: Fate of the iron core
Differentiated planetesimal impacts into a terrestrial magma ocean: Fate of the iron core
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DOI：
10.1016/j.epsl.2016.05.012
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发表时间：
2016-08-15
影响因子：
5.3
通讯作者：
Melosh, H. J.
中科院分区：
文献类型：
--
作者：
Kendall, Jordan D.;Melosh, H. J.
地幔中中等亲铁元素（“亲铁”；例如钴、镍）的丰度比低压下硅酸盐熔体和铁之间的化学平衡预测的丰度高 10 到 100 倍，但它确实符合高压和高温下平衡的预期。最近对差异化星子撞击的研究假设，星子核心在撞击中完好无损，作为集中质量，从零初始速度被动沉降，并在全球岩浆海洋中经历湍流夹带；在这些条件下，如果没有足够深的岩浆海，直径大于10公里的岩心就无法完全混合。我们进行了水电编码模拟，修正了这一假设，并更清晰地了解了具有半径 = 100 公里的铁核的微星撞击岩浆海洋的撞击过程。撞击过程剥离了星子的硅酸盐地幔，然后拉伸铁核，将液态铁分散到更大体积的下面的液态硅酸盐地幔中。当撞击拉伸并分散铁芯时，拉格朗日示踪粒子会追踪最初完好的铁芯。最初最接近的示踪剂对的最终位移距离给出了核心拉伸的度量。拉伸的统计数据意味着混合，将铁芯分成片状、韧带和更小的碎片，尺寸为 10 公里或更小。当铁水碎片穿过岩浆海洋并沉入地球更深处时，撞击分散的核心碎片通过湍流夹带进一步混合。因此，我们的结果支持这样的观点：即使是大型分化星子的核心中的铁也可以在陆地岩浆海洋深处实现化学平衡。 (C) 2016 Elsevier B.V. 保留所有权利。
The abundance of moderately siderophile elements ("iron-loving"; e.g. Co, Ni) in the Earth's mantle is 10 to 100 times larger than predicted by chemical equilibrium between silicate melt and iron at low pressure, but it does match expectation for equilibrium at high pressure and temperature. Recent studies of differentiated planetesimal impacts assume that planetesimal cores survive the impact intact as concentrated masses that passively settle from a zero initial velocity and undergo turbulent entrainment in a global magma ocean; under these conditions, cores greater than 10 km in diameter do not fully mix without a sufficiently deep magma ocean. We have performed hydrocode simulations that revise this assumption and yield a clearer picture of the impact process for differentiated planetesimals possessing iron cores with radius = 100 km that impact into magma oceans. The impact process strips away the silicate mantle of the planetesimal and then stretches the iron core, dispersing the liquid iron into a much larger volume of the underlying liquid silicate mantle. Lagrangian tracer particles track the initially intact iron core as the impact stretches and disperses the core. The final displacement distance of initially closest tracer pairs gives a metric of core stretching. The statistics of stretching imply mixing that separates the iron core into sheets, ligaments, and smaller fragments, on a scale of 10 km or less. The impact dispersed core fragments undergo further mixing through turbulent entrainment as the molten iron fragments rain through the magma ocean and settle deeper into the planet. Our results thus support the idea that iron in the cores of even large differentiated planetesimals can chemically equilibrate deep in a terrestrial magma ocean. (C) 2016 Elsevier B.V. All rights reserved.