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

涉及大型行星体的能量几乎不可避免地会导致它们的融化。最终的“岩浆海洋”的结晶从根本上塑造了行星的结构。长期以来，月亮经历了如此动荡的“岩浆海洋”舞台。的确，月球表面的主要亮白色是浅色矿物斜长石的结果，在月球形成后不久，它不久就漂浮在浓密的岩浆海洋表面上。结晶月内部的互补演化可能会产生不同晶体层的堆叠，每个晶体具有特征性的化学性质，结束于最终的熔体融化糊剂。融化后，后来淹没了月亮的冲击坑以提供其深色的表面阴影，并源自该分层晶体桩的不同部分，并反映了该模型预测的广泛组合物。地球也应该从岩浆海洋舞台演变而来，但后来经历了更加活跃的历史，这已经消除了这种关键的古老特征在月球上仍然很明显。然而，应该仍然存在一些如此戏剧性的初始历史的化学指纹，但直到最近，这种过程的遗迹似乎很少。这种无效观察已被用来通过对流来争辩地球内部混合的效率，这是板块构造中明显的过程的一部分，这在月球上显然不存在。然而，新的，非常高精度的测量值（在100万个中的错误中，错误的误差为142/144，但讲述了一个不同的故事。卡内基研究所（美国华盛顿特区）的两名工人表明，对地球样本的142/144 nd的所有测量与原始陨石不同，长期以来一直认为这些陨石代表了行星的平均组成（如果地球，核心，核心，地幔和甲壳的所有不同部分都可能混合在一起）。他们认为，如果在地球底部的前3000万年形成地壳，则可以解释这种观察结果，该观察是在地球底部的岩浆海面固化而形成的（在地幔底部约3000km）。这种“隐藏的水库”模型在许多方面都具有吸引力，因为它解决了地球上已知的储层（地壳，地幔和核心）中许多元素缺少预算的长期问题，但是它要求地幔的最低层仍然隔离了超过45.3亿年的Mantle Contection。许多人认为这是令人难以置信的，而是拒绝了地球具有与原始陨石相同的庞大组成的范式。尽管这解决了一个问题，但它通过消除行星地球化学的中心宗旨而产生更多问题。我们建议对“隐藏的储层”模型进行独立测试，然后解决行星历史上的一阶问题。我们将通过查看另一种同位素比（182W/184W）的变化来做到这一点，而这种变化仅受到地球历史开始时发生的过程的影响，而这种变化仅受到142/144的影响。已经发现，地球不仅与陨石不同，但地球上最古老的岩石的142/144也比现代样品更高。这是通过将隐藏的水库的一小部分混合到35亿年前的其他地幔中来解释的。如果是这种情况，它可以预测182W/184W的互补变化。我们相信我们已经确定了此类变化，但是清楚地确定了这些变化需要更高级的测量。我们想通过进行一系列类似的其他测量来检查我们的初步发现。如果测量得出，我们将验证了解地球结构的最新发展之一，即在地幔底部存在一个古老的“隐藏”水库的存在