Heterogeneous accretion, composition and core-mantle differentiation of the Earth
Heterogeneous accretion, composition and core-mantle differentiation of the Earth
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DOI：
10.1016/j.epsl.2010.11.030
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发表时间：
2011-01-03
影响因子：
5.3
通讯作者：
Palme, Herbert
中科院分区：
文献类型：
--
作者：
Rubie, David C.;Frost, Daniel J.;Palme, Herbert
提出了一个地核形成的模型，该模型涉及地球通过一系列与较小分化体的撞击非均匀地吸积。每次碰撞都会导致撞击器的金属核与岩浆海洋发生反应，然后与地球的原核合并。吸积星子的总体组成由非挥发性元素（即ci -球粒体）的平均太阳系丰度表示，难熔元素和氧含量增加22%，主要由Fe金属/FeO硅酸盐比定义。基于无水体积化学，利用实验确定的Fe、Si、O、Ni、Co、W、Nb、v、Ta和Cr元素的金属/硅酸盐分配系数，通过质量平衡计算了形成核心的共存金属液体和橄榄岩硅酸盐液体的组成。氧逸度由铁在金属和硅酸盐之间的分配确定，并取决于温度、压力和起始成分的氧含量。利用最小二乘法将计算得到的地幔成分与原始地幔成分拟合，确定模型参数。涉及均匀吸积或单阶段岩心形成的模型不能提供可接受的拟合。在最成功的模型中，涉及24个撞击体，地球最初的60-70%（按质量计算）是从高度还原的物质中吸积的，最后30-40%的吸积质量被更多地氧化，这与动态吸积模拟的结果一致。为了获得满意的Ni， Co和W的拟合，要求较大（和较晚）的撞击核在与地球原核合并之前不能完全平衡，这是先前基于Hf-W同位素研究提出的。估计的平衡条件可能与岩浆海洋延伸到核幔边界相一致，从而使岩心的形成非常有效。该模型能够预测整个吸积过程中地幔和地核的成分演化。这一结果与地球水库存的晚期增加相一致，可能是在岩心形成完成后的晚期贴面。最后，预测岩心中含有5%的Ni， 8%的Si， 2%的S和0.5%的o。(C) 2010 Elsevier B.V.版权所有。
A model of core formation is presented that involves the Earth accreting heterogeneously through a series of impacts with smaller differentiated bodies. Each collision results in the impactor's metallic core reacting with a magma ocean before merging with the Earth's proto-core. The bulk compositions of accreting planetesimals are represented by average solar system abundances of non-volatile elements (i.e. CI-chondritic), with 22% enhancement of refractory elements and oxygen contents that are defined mainly by the Fe metal/FeO silicate ratio. Based on an anhydrous bulk chemistry, the compositions of coexisting core-forming metallic liquid and peridotitic silicate liquid are calculated by mass balance using experimentally-determined metal/silicate partition coefficients for the elements Fe, Si, O, Ni, Co, W, Nb, V. Ta and Cr. Oxygen fugacity is fixed by the partitioning of Fe between metal and silicate and depends on temperature, pressure and the oxygen content of the starting composition. Model parameters are determined by fitting the calculated mantle composition to the primitive mantle composition using least squares minimization. Models that involve homogeneous accretion or single-stage core formation do not provide acceptable fits. In the most successful models, involving 24 impacting bodies, the initial 60-70% (by mass) of the Earth accretes from highly-reduced material with the final 30-40% of accreted mass being more oxidised, which is consistent with results of dynamical accretion simulations. In order to obtain satisfactory fits for Ni, Co and W, it is required that the larger (and later) impactor cores fail to equilibrate completely before merging with the Earth's proto-core, as proposed previously on the basis of Hf-W isotopic studies. Estimated equilibration conditions may be consistent with magma oceans extending to the core-mantle boundary, thus making core formation extremely efficient. The model enables the compositional evolution of the Earth's mantle and core to be predicted throughout the course of accretion. The results are consistent with the late accretion of the Earth's water inventory, possibly with a late veneer after core formation was complete. Finally, the core is predicted to contain similar to 5 wt.% Ni, similar to 8 wt.% Si, similar to 2 wt.% S and similar to 0.5 wt.% O. (C) 2010 Elsevier B.V. All rights reserved.