Constraints on terrestrial differentiation from the isotopic fractionation of major elements

我们生活在地壳上，地球的外层。确定位于这一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(80%的地球，位于地壳之下和中央核心之上)被输送到地表，这是板块构造命运的奇特之处。这些碎片为我们提供了有关地球内部的宝贵线索。我们需要从这一有限的资源中尽可能多地收集信息，以了解地球更深的地方。可以玩的一个科学游戏是考虑这些地幔碎片的组成是否能合理地代表整个内部。如果是这样，那么地幔似乎是作为一个整体对流的。如果没有，就需要有一个未开发的“隐蔽”水库，使其不能被我们的采样手段所利用。因此，这些信息提供了有关地幔结构的关键信息。为了测试观测到的地幔是否能合理地代表整个地幔，需要将其与可信地代表构成地球的原材料的成分进行比较。这样的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，这些小天体从未变得足够大，无法融化并“分化”成不同的化学层，这使得估计地球的整体构成变得非常困难。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在地表采样的地幔碎片的组成。因此，我们需要选择一种不容易改变的化学特征，这种特征只会被我们要研究的特定过程改变。这就是我们项目的目标。我们将对可接近的地幔碎片和原始陨石进行高精度的镁同位素测量。镁由3个稳定同位素组成(质量分别为24、25和26)，26/24 mg的比例预计在陨石和地球中是相同的。然而，我们的初步测量表明，情况并非如此。我们需要更详细地调查这一令人惊讶和值得注意的观察结果。首先，我们想检查我们测量的准确性。要使镁同位素测量达到我们所要求的精度，存在一些已知的潜在问题。我们已经开发了一种新的技术来绕过这些问题，并确保我们得到准确的测量。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，地球上的镁同位素比值与原始陨石中的镁同位素比率的差异很可能是地球最早的历史的指纹。地球是通过一系列巨大的撞击形成的，其中最后一次很可能产生了月球。在这些撞击之后，地球很可能是熔化的，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方面使我们相信，它们可能是以不同的镁同位素比率形成的，而不是它们生长的液体。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，我们偶尔在地质样品中看到的地幔的镁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，这些实验在适合全球岩浆海洋的条件下生长这种高压晶体。我们将分析实验产品的镁同位素比率，看看我们的理论是否正确。虽然我们将集中在镁和类似的故事与硅和它的同位素。因此，与此同时，使用相同的样品和实验，我们将研究硅，这也揭示了核心的形成。

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.