Testing the Hadean Hidden Reservoir Model

生长大型行星体所涉及的能量几乎不可避免地会导致它们的融化。由此产生的“岩浆海洋”的结晶从根本上塑造了行星的结构。长期以来，人们一直认为月球经历了如此动荡的“岩浆海洋”阶段。事实上，月球表面占主导地位的明亮白色是在月球形成后不久漂浮到密度更高的岩浆海洋表面的浅色矿物斜长石的结果。月球内部结晶的互补演化很可能产生了一堆不同层的晶体，每一层都有独特的化学成分，最终形成了凝固的熔渣。后来涌入月球撞击坑以提供更暗表面阴影的熔体来自这个层状晶体堆的不同部分，反映了该模型预测的广泛成分。地球也应该从岩浆海洋阶段进化而来，但经历了一个更加活跃的后续历史，消除了月球上仍然明显存在的这些关键的、古老的特征。尽管如此，这种戏剧性的初始历史应该会保留一些化学指纹，但直到最近，这种过程的痕迹似乎都很少。这种零观测被用来论证地球内部通过对流混合的效率，这是板块构造中明显的过程的一部分，而月球上显然没有板块构造。然而，新的非常高精度的测量(误差小到百万分之几)142Nd/144Nd的同位素比率，讲述了一个不同的故事。来自卡内基研究所(美国华盛顿特区)的两名工作人员表明，来自地球样本的142Nd/144Nd的所有测量结果都与来自原始陨石的测量结果不同，原始陨石长期以来一直被认为代表地球的平均组成(如果地球的所有不同部分、核心、地幔和地壳可以混合在一起的话)。他们认为，如果地壳在地球历史的前3000万年通过岩浆海洋表面的固化而形成，并在地幔底部(地表以下约3000公里)下沉和积累，就可以解释这种观察结果。这一“隐藏库”模型在很多方面都很有吸引力，因为它解释了地球上已知的储藏库(地壳、地幔和地核)中许多元素长期缺乏预算的问题，但它要求地幔的这一最低层在45.3亿年的地幔对流中保持隔离。许多人认为这是不可信的，相反，他们拒绝接受地球具有与原始陨石相同的大宗成分的范例。虽然这解决了一个问题，但通过消除行星地球化学的这一中心原则，它产生了更多的问题。我们建议对“隐藏库”模型进行独立检验，从而解决这个行星历史上的一阶问题。我们将通过观察另一个同位素比182W/184W的变化来实现这一点，该同位素比与142W/144Nd类似，只受地球历史开始时发生的过程的影响。已经发现，地球不仅具有与陨石不同的142Nd/144Nd，而且地球上最古老的岩石的142Nd/144Nd甚至比现代样品更高。这可以通过35亿多年前隐藏的一小部分储藏层混合回到地幔的其余部分来解释。如果是这样的话，它预测182W/184W将出现互补性变化。我们相信我们已经发现了这种变化，但清楚地确定这种变化需要更多非常高精度的测量。我们想通过进行一系列类似的额外测量来检查我们最初的发现。如果测量结果成立，我们将证实在了解地球结构方面最重要的最新进展之一，即在地幔底部存在一个古老的“隐藏”的储集层

The energy involved in growing large planetary bodies almost inevitably results in their melting. The crystallisation of resultant 'magma oceans' fundamentally shapes the structure of planets. It has long been accepted that the Moon underwent such a tumultuous 'magma ocean' stage. Indeed the dominant bright white of the Moon's surface is the result of the light-coloured mineral plagioclase having floated to the surface of a denser magma ocean shortly after the Moon formed. The complementary evolution of the crystallising lunar interior likely produced a stack of different layers of crystals, each with a characteristic chemistry, ending in the final solidified dregs of melt. The melts that later flooded the Moon's impact craters to provide its darker surface shading, were derived from different parts of this layered crystal pile and reflect the wide range of compositions predicted by this model. The Earth too should have evolved from a magma ocean stage, but has undergone a much more active subsequent history that has eradicated such key, ancient features still evident on the Moon. Nevertheless, there should remain some chemical fingerprints of such a dramatic initial history but until recently there appeared to be few vestiges of such processes. This null-observation has been used to argue for efficiency of mixing in the Earth's interior by convection, part of the process evident in plate tectonics, which is clearly absent on the Moon. New, very high-precision measurements (with errors as small as a few parts in one million) of the isotope ratio of 142Nd/144Nd, however, tell a different story. Two workers from the Carnegie Institute (Washington DC, USA) showed that all measurements of 142Nd/144Nd on samples from Earth are different to those from primitive meteorites, which have long been assumed to represent the average composition of the planet (if all the different parts of the Earth, core, mantle and crust could be mixed back together). They argued that this observation could be explained if crust, formed in the first 30 million years of Earth history by solidification of the magma ocean surface, sunk and accumulated at the bottom of the mantle (at ~3000km below the surface). This 'hidden reservoir' model is attractive in many ways as it accounts for long-standing problems of missing budgets of many elements within the known reservoirs on Earth (crust, mantle and core) but it requires that this lowermost layer of the mantle has remained isolated over 4.53 billion years of mantle convection. Many have seen this as implausible and instead reject the paradigm that the Earth has a bulk composition the same as primitive meteorites. Although this solves one problem, it generates many more by removing this central tenet of planetary geochemistry. We propose to make an independent test of the 'hidden reservoir' model and so resolve this first order problem in planetary history. We will do this by looking at variations in another isotope ratio, 182W/184W, that like 142Nd/144Nd is influenced only by processes that occur at the beginning of Earth History. It has been found that not only does the Earth have different 142Nd/144Nd to meteorites but that the very oldest rocks on Earth have even higher 142Nd/144Nd than modern samples. This has been explained by the mixing of a fraction of the hidden reservoir back into the rest of the mantle more than 3.5 billion years ago. If this is the case, it predicts complementary changes in 182W/184W. We believe we have identified such changes, but clearly identifying this changes requires more very high-precision measurements. We want to check our initial finding by making a series of similar additional measurements. If the measurements hold, we will have verified one of the most significant recent developments in understanding the structure of the Earth, namely the existence of an ancient 'hidden' reservoir at the bottom of the mantle