Constraints on terrestrial differentiation from the isotopic fractionation of major elements

我们生活在地壳上，也就是地球的外层。确定位于地壳屏障之下的地球内部的组成是一个巨大的挑战。硅酸盐地幔的一些碎片（占地球的80%，位于地壳下方和中央核心上方）被运送到地表，这是板块构造命运的一个奇特之处。这些碎片为我们了解地球内部提供了宝贵的线索。我们需要从这个有限的资源中尽可能多地收集信息，以了解更深层次的星球。一个可以玩的科学游戏是考虑这些地幔碎片的组成是否可以合理地代表整个内部。如果是这样，那么地幔作为一个整体对流是合理的。如果没有，则需要一个未开发的“隐藏”储层锁定在我们的采样方法之外。因此，这些信息提供了关于地幔结构的关键信息。为了检验观测到的地幔是否能合理地代表整个地幔，需要与合理地代表构成地球的原材料的成分进行比较。这种参照以所谓的原始陨石的形式存在。这些来自小行星的样本记录了小天体的组成，这些小天体从未长大到足以融化并“分化”成化学上不同的层，这使得估计地球的整体构成变得如此困难。简单地将原始陨石与地球进行比较的一个问题是，许多过程可能会改变地表取样的地幔碎片的组成。因此，我们需要选择一种不易改变的化学特性，这种特性只能通过我们正在研究的特定过程来改变。这就是我们项目的目的。我们将对可接近的地幔和原始陨石碎片进行高精度的镁同位素测量。镁由3种稳定的同位素组成（质量分别为24,25和26），而26/ 24mg的比例预计在陨石和地球中是相同的。然而，我们最初的测量表明情况并非如此。我们需要更详细地研究这一令人惊讶和值得注意的观察结果。首先，我们要检查测量的准确性。要使Mg同位素测量达到我们所要求的精度，已知存在潜在的问题。我们已经开发了一种新技术来规避这些问题，并确保我们得到准确的测量结果。该项目的第一部分是使用这种方法对地球和陨石进行一系列测量。其次，我们认为地球和原始陨石中Mg同位素比值的差异可能是早期地球历史的一个指纹。地球是通过一系列巨大的撞击而形成的，最后一次撞击可能产生了月球。在这些撞击之后，地球很可能是熔融的，岩浆海洋的凝固很可能是从底部向上发生的。在这种高压结晶过程中生长的一些矿物的化学成分使我们相信，它们形成时的镁同位素比例可能与它们生长的液体不同。如果这些晶体沉入岩浆海洋的底部，并在地球历史上一直留在那里，那么我们偶尔在地质样本中看到的地幔的Mg同位素比率将与原始陨石不同。我们将通过实验室实验来验证这一想法，在适合全球岩浆海洋的条件下生长出这种高压晶体。我们将分析实验结果的Mg同位素比率，看看我们的理论是否正确。虽然我们将集中讨论Mg，但类似的故事也适用于Si及其同位素。因此，与此同时，我们将使用相同的样品和实验来研究Si，这也为核心的形成提供了线索。

We live on the crust, the outer skin of the Earth. Determining the composition of the Earth's interior that lies below this crustal barrier is a huge challenge. Some fragments of the silicate mantle (80% of the Earth, which lies beneath the crust and above the central core) are transported to the surface a quirk of plate tectonic fate. These fragments gives us precious clues about the Earth's interior. We need to glean as much as possible from this limited resource to gain an understanding of the deeper planet. One scientific game that can be played is to consider if the composition of these mantle fragments can reasonably represent the whole of the interior. If so, then plausibly the mantle convects as a whole. If not there needs to be an untapped, 'hidden' reservoir locked from our means of sampling. Such information therefore provides critical information on the structure of the mantle. To test if the observed mantle can reasonably represent the whole mantle requires a comparison with a composition that plausibly represents the raw material from which the Earth was made. Such a reference exists in the form of so-called primitive meteorites. These samples from asteroids record the compositions of small bodies that never grew large enough to melt and 'differentiate' into the chemically distinct layers which make estimating the overall make-up of the Earth so difficult. A problem in simply comparing primitive meteorites with the Earth is that many processes potentially change the composition of the bits of mantle sampled at the surface. Thus we need to select a chemical characteristic that is not readily altered, changed only by the specific process we are looking to investigate. This is the aim of our project. We will make highly precise Mg isotope measurements on accessible fragments of the Earth's mantle and primitive meteorite. Magnesium is comprised of 3 stable isotopes (with masses 24, 25 and 26) and the ratio of 26/24Mg is expected to be the same in meteorites and the Earth. However, our initial measurements suggest that this is not the case. We need to investigate this surprising and notable observation in more detail. Firstly, we want to check the accuracy of our measurements. There are known potential problems with making Mg isotope measurements to the precision we require. We have developed a novel technique to circumvent these problems and ensure we get accurate measurements. The first part of the project is to make a set of measurements on the Earth and meteorites using this approach. Secondly, we believe that the difference in the Mg isotope ratios on Earth and in the primitive meteorites is likely a fingerprint of the history of the earliest Earth. The Earth accreted via a series of giant impacts, the last of which likely produced the Moon. After these impacts the Earth was likely molten and the solidification of this magma ocean likely occurred from the bottom upward. Aspects of the chemistry of some of the minerals that grow during such high-pressure crystallisation make us believe that they may have formed with a different Mg isotope ratio to the liquid from which they grew. If these crystals sunk to the bottom of the magma ocean and have remained there over earth history, the Mg isotope ratio of the mantle we occasionally glimpse in geological samples would be different from the primitive meteorites. We will test this idea with laboratory experiments which grow such high pressure crystals under conditions appropriate to the global magma ocean. We will analyse Mg isotope ratios the products of the experiments to see if our theory is correct. Although we will concentrate on Mg and similar story pertains to Si and its isotopes. Thus at the same time, using the same samples and experiments we will investigate Si, which also casts light on the formation of the core.