Turbulent mixing of metal and silicate during planet accretion - And interpretation of the Hf-W chronometer
Turbulent mixing of metal and silicate during planet accretion - And interpretation of the Hf-W chronometer
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DOI：
10.1016/j.epsl.2010.03.038
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发表时间：
2010-06-15
影响因子：
5.3
通讯作者：
Stevenson, David J.
中科院分区：
文献类型：
--
作者：
Dahl, Tais W.;Stevenson, David J.
在当前的行星形成观点中，地球的最终形成涉及原行星（半径>1000千米）之间的巨大碰撞，月球就是由一次这样的撞击形成的。在这个阶段，碰撞体可能已经分化为一个被硅酸盐地幔包围的金属核。在月球形成的撞击过程中，几乎所有的金属都沉入了地球的核心。我们研究大型自引力铁核在多大程度上能与周围的硅酸盐混合，以及这如何影响用于推断月球年龄的短寿命计时器铪 - 钨（Hf - W）。我们提出了全液态系统中湍流混合的流体动力学模型，试图对混合程度加以限制。由瑞利 - 泰勒不稳定性驱动的下沉核的侵蚀确实会导致紧密混合和平衡，但大的团块（直径>10千米）不会完全乳化。如果大部分增生的金属核在穿过地球地幔下降过程中变形为薄结构，乳化作用会增强。然而，在地球增生过程中，只有1% - 20%的地球核心会与硅酸盐达到平衡。撞击体的初始速度并不重要。我们接着评估剪切不稳定性的混合潜力，在这种情况下，硅酸盐在垂直壁上的夹带会导致混合。湍流结构表明，涡旋保持在最大尺度，不会混合到厘米长度尺度，而在厘米尺度上扩散起作用且同位素能够达到平衡。因此，增生的铁核可能会出现不完全乳化和平衡的情况。金属 - 硅酸盐平衡的程度为解释亲铁元素预算以及使用铪 - 钨计时器确定核心形成的时间提供了关键信息。由铪 - 钨计时器得出的核心形成时间尺度通常与最后一次主要的金属 - 硅酸盐再平衡相关联，据信这与月球形成的撞击时间一致。然而，我们表明，大的核心在硅酸盐地球中重置铪 - 钨系统的能力有限。整体硅酸盐地球中过量的钨 - 182对早期核心形成过程比对最后一次巨大撞击后的放射性增长更为敏感。（C）2010爱思唯尔有限公司。保留所有权利。
In the current view of planet formation, the final assembly of the Earth involved giant collisions between proto-planets (>1000 km radius), with the Moon formed as a result of one such impact. At this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle. During the Moon-forming impact, nearly all metal sank into the Earth's core. We investigate to what extent large self-gravitating iron cores can mix with surrounding silicate and how this influences the short-lived chronometer, Hf-W, used to infer the age of the Moon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to place constraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh-Taylor instability does lead to intimate mixing and equilibration, but large blobs (> 10 km diameter) do not emulsify entirely. Emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the Earth's mantle. Yet, only 1-20% of Earth's core would equilibrate with silicate during Earth's accretion. The initial speed of the impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale, where diffusion operates and isotopes can equilibrate. Thus, incomplete emulsification and equilibration of accreting iron cores is likely to occur.The extent of metal-silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the Hf-W chronometer. The time scale of core formation derived from the Hf-W chronometer is usually tied to the last major metal-silicate re-equilibration, believed to coincide with time of the Moon-forming impact. However, we show that large cores have limited ability to reset the Hf-W system in the silicate Earth. Excess W-182 in bulk silicate Earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact. (C) 2010 Elsevier B.V. All rights reserved.