Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 2011 Oct 25;108(43):17604-9. doi: 10.1073/pnas.1108544108. Epub 2011 Oct 17.
Constraints on the formation history of the Earth are critical for understanding of planet formation processes. (182)Hf-(182)W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and (182)Hf-(182)W and (87)Rb-(87)Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to > 100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the (182)W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (> 52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in < 10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.
地球形成历史的约束条件对于理解行星形成过程至关重要。（182）Hf-（182）W 时标测定陆地岩石表明，地球大约在太阳系形成后 30 Myr 内形成，随后立即发生了形成月球的大碰撞（MGI）。然而，一些 N 体模拟以及一些月球岩石的（182）Hf-（182）W 和（87）Rb-（87）Sr 年代学已经被用于论证月球形成时间较晚，为 52 至 > 100 Myr。这种差异通常用大碰撞期间的金属-硅酸盐不平衡来解释。在这里，我们描述了一个（182）W 同位素演化模型，包括核心形成期间难熔亲铁元素（Ni、Co、W、V 和 Nb）分配的约束条件，这可以解释这种差异。我们的模型表明，地幔中亲铁元素的浓度与地球在一个陆地岩浆海洋中高压金属-硅酸盐平衡的浓度一致。我们的分析表明，MGI 的时间与地球主要吸积阶段的时间尺度呈反比关系。具体来说，MGI 最早可能发生在大约 30 Myr 时，与大约 30 Myr 时主要吸积阶段的结束相对应。晚期 MGI（> 52 Myr）需要地球的主要吸积阶段在 < 10.7 ± 2.5 Myr 内迅速完成。这是两个极端解决方案，在这两个极端之间存在一个连续的解决方案。
