Collisional stripping of planetary crusts
Collisional stripping of planetary crusts
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行星地壳的碰撞剥离
DOI：
10.1016/j.epsl.2017.12.012
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发表时间：
2018
影响因子：
5.3
通讯作者：
Carter P
中科院分区：
文献类型：
--
作者：
Carter P
行星吸积和演化的地球化学研究援引了不同程度的碰撞侵蚀来解释行星和球粒陨石之间体积组成的差异。在这里，我们对吸积过程中的“地壳剥离”及其关键的地球化学后果进行了全面的动态评估。地壳预计将包含地球预算中不相容元素的很大一部分，其中包括主要的产热核素。我们提出的平滑粒子流体动力学模拟碰撞之间的分化岩石星子和行星胚胎。我们发现，在撞击过程中，地壳相对于地幔优先消失，我们基于这些模拟开发了一个比例定律，该定律近似于最大残骸中保留的地壳质量。利用这一比例定律和最近一组类地行星形成的n体模拟，我们估计了在行星胚胎增生期间地壳剥离对不相容元素丰度的最大影响。我们发现，当胚胎增生时，平均约有三分之一的初始地壳被剥离，这导致如果剥离的地壳不增生，产热元素的预算减少约20%。地壳的侵蚀可以导致不相容元素的非球粒比例，但这种影响的大小敏感地取决于星子上地壳形成熔化过程的细节。Lu/Hf系统被划分为广泛的地壳形成情景。我们利用长长辉长岩（微行星硅酸盐熔融的产物，被认为代表灶神星的地壳）作为微行星地壳在吸积过程中部分丢失的Lu/Hf的指南，预测地球目前可能演化为超球粒质176hf /177Hf（万分之3-5）。这样的数值与地球整体的成分估计是一致的。在吸积过程中，行星地壳的剥离可以导致可检测到的亲石元素的总体组成变化，但分馏相对微妙，并且对再吸积的效率敏感。
Geochemical studies of planetary accretion and evolution have invoked various degrees of collisional erosion to explain differences in bulk composition between planets and chondrites. Here we undertake a full, dynamical evaluation of ‘crustal stripping’ during accretion and its key geochemical consequences. Crusts are expected to contain a significant fraction of planetary budgets of incompatible elements, which include the major heat producing nuclides. We present smoothed particle hydrodynamics simulations of collisions between differentiated rocky planetesimals and planetary embryos. We find that the crust is preferentially lost relative to the mantle during impacts, and we have developed a scaling law based on these simulations that approximates the mass of crust that remains in the largest remnant. Using this scaling law and a recent set ofN-body simulations of terrestrial planet formation, we have estimated the maximum effect of crustal stripping on incompatible element abundances during the accretion of planetary embryos. We find that on average approximately one third of the initial crust is stripped from embryos as they accrete, which leads to a reduction of ∼20% in the budgets of the heat producing elements if the stripped crust does not reaccrete. Erosion of crusts can lead to non-chondritic ratios of incompatible elements, but the magnitude of this effect depends sensitively on the details of the crust-forming melting process on the planetesimals. The Lu/Hf system is fractionated for a wide range of crustal formation scenarios. Using eucrites (the products of planetesimal silicate melting, thought to represent the crust of Vesta) as a guide to the Lu/Hf of planetesimal crust partially lost during accretion, we predict the Earth could evolve to a superchondritic176Hf/177Hf (3–5 parts per ten thousand) at present day. Such values are in keeping with compositional estimates of the bulk Earth. Stripping of planetary crusts during accretion can lead to detectable changes in bulk composition of lithophile elements, but the fractionation is relatively subtle, and sensitive to the efficiency of reaccretion.
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