Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA.
Nature. 2017 Jan 25;541(7638):521-524. doi: 10.1038/nature20830.
The Earth formed by accretion of Moon- to Mars-size embryos coming from various heliocentric distances. The isotopic nature of these bodies is unknown. However, taking meteorites as a guide, most models assume that the Earth must have formed from a heterogeneous assortment of embryos with distinct isotopic compositions. High-precision measurements, however, show that the Earth, the Moon and enstatite meteorites have almost indistinguishable isotopic compositions. Models have been proposed that reconcile the Earth-Moon similarity with the inferred heterogeneous nature of Earth-forming material, but these models either require specific geometries for the Moon-forming impact or can explain only one aspect of the Earth-Moon similarity (that is, O). Here I show that elements with distinct affinities for metal can be used to decipher the isotopic nature of the Earth's accreting material through time. I find that the mantle signatures of lithophile O, Ca, Ti and Nd, moderately siderophile Cr, Ni and Mo, and highly siderophile Ru record different stages of the Earth's accretion; yet all those elements point to material that was isotopically most similar to enstatite meteorites. This isotopic similarity indicates that the material accreted by the Earth always comprised a large fraction of enstatite-type impactors (about half were E-type in the first 60 per cent of the accretion and all of the impactors were E-type after that). Accordingly, the giant impactor that formed the Moon probably had an isotopic composition similar to that of the Earth, hence relaxing the constraints on models of lunar formation. Enstatite meteorites and the Earth were formed from the same isotopic reservoir but they diverged in their chemical evolution owing to subsequent fractionation by nebular and planetary processes.
地球通过来自不同日心距离的月球到火星大小的胚胎吸积形成。这些天体的同位素性质是未知的。然而，以陨石为指导，大多数模型假设地球必须由具有不同同位素组成的不同胚胎混合物形成。然而，高精度测量表明，地球、月球和顽辉石陨石具有几乎无法区分的同位素组成。已经提出了一些模型来协调地球-月球的相似性与地球形成物质推断的不均匀性质，但这些模型要么要求月球形成撞击的特定几何形状，要么只能解释地球-月球相似性的一个方面（即 O）。在这里，我表明，对金属具有不同亲和力的元素可以用于通过时间来解析地球吸积物质的同位素性质。我发现亲石 O、Ca、Ti 和 Nd、中等亲铁 Cr、Ni 和 Mo 以及高度亲铁 Ru 的地幔特征记录了地球吸积的不同阶段；然而，所有这些元素都指向与顽辉石陨石同位素最相似的物质。这种同位素相似性表明，地球吸积的物质总是包含大量顽辉石型撞击体（在吸积的前 60%中约有一半是 E 型，此后所有撞击体都是 E 型）。因此，形成月球的巨大撞击体可能具有与地球相似的同位素组成，从而放宽了对月球形成模型的限制。顽辉石陨石和地球是由相同的同位素库形成的，但由于随后的星云和行星过程的分馏，它们在化学演化中出现了分歧。
