Testing the Hadean Hidden Reservoir Model

测试冥古宙隐伏储层模型

• 批准号: NE/H011927/1
• 负责人: Timothy Elliott
• 金额: $ 7.87万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2010
• 资助国家: 英国
• 起止时间: 2010 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FH011927%2F1
• 关键词: Testing; Hadean; Hidden; Reservoir; Model

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

大型行星体生长所涉及的能量几乎不可避免地导致它们的熔化。由此产生的“岩浆海洋”的结晶从根本上塑造了行星的结构。长久以来，人们一直认为月球经历了这样一个动荡的“岩浆海洋”阶段。事实上，月球表面主要的明亮白色是月球形成后不久，浅色矿物斜长石漂浮到密度更大的岩浆海洋表面的结果。月球内部结晶的互补演化可能产生了一堆不同的晶体层，每一层都具有独特的化学性质，最终形成了熔融的固化残渣。后来淹没月球撞击坑以提供其较暗表面阴影的熔体来自这个分层晶体堆的不同部分，并反映了该模型预测的广泛成分。地球也应该是从岩浆海洋阶段进化而来的，但后来经历了一个更加活跃的历史，消除了月球上仍然明显的这种关键的古老特征。尽管如此，这种戏剧性的最初历史应该留下一些化学指纹，但直到最近，似乎几乎没有这种过程的痕迹。这种零观测被用来论证地球内部通过对流进行混合的效率，这是板块构造中明显的过程的一部分，在月球上显然不存在。然而，对142 Nd/144 Nd同位素比的新的、非常高精度的测量（误差小到百万分之几）却讲述了一个不同的故事。卡内基研究所（美国华盛顿）的两名工作人员表明，所有来自地球样品的142 Nd/144 Nd测量结果都与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了地球的平均成分（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）。他们认为，这一观测结果可以解释为，如果地壳在地球历史的前3000万年通过岩浆海洋表面的凝固而形成，并在地幔底部（地表以下约3000公里）下沉和积累。这种“隐藏的水库”模型在许多方面都很有吸引力，因为它解释了地球上已知水库（地壳，地幔和地核）中许多元素长期存在的预算缺失问题，但它要求地幔的最低层在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，而是拒绝地球具有与原始陨石相同的大块成分的范式。虽然这解决了一个问题，但它通过消除行星地球化学的核心原则而产生了更多问题。我们建议对“隐藏水库”模型进行独立测试，从而解决行星历史上的一阶问题。我们将通过观察另一种同位素比值182 W/184 W的变化来做到这一点，就像142 Nd/144 Nd一样，它只受地球历史开始时发生的过程的影响。研究发现，不仅地球的142 Nd/144 Nd与陨石不同，而且地球上最古老的岩石的142 Nd/144 Nd甚至比现代样品更高。这可以通过35亿年前一部分隐藏的水库混合回到地幔的其余部分来解释。如果是这种情况，它预测182 W/184 W的互补变化。我们相信我们已经发现了这些变化，但要清楚地发现这些变化需要更高精度的测量。我们想通过一系列类似的额外测量来检验我们最初的发现。如果测量结果成立，我们将证实最近在了解地球结构方面最重要的进展之一，即在地幔底部存在一个古老的“隐藏”水库