Constraints on terrestrial differentiation from the isotopic fractionation of major elements

主要元素同位素分馏对陆地分化的限制

• 批准号: NE/L007428/1
• 负责人: Timothy Elliott
• 金额: $ 64.4万
• 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: 2014
• 资助国家: 英国
• 起止时间: 2014 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FL007428%2F1
• 关键词: Constraints; terrestrial; differentiation; isotopic; fractionation

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.

我们生活在地壳，地上的外皮上。确定地球内部的组成位于地壳屏障以下是一个巨大的挑战。硅酸盐地幔的一些碎片（地球下的80％的地球下方和中央芯上方）被运输到表面上，构造了板块构造命运。这些碎片为我们提供了有关地球内部的宝贵线索。我们需要从这个有限的资源中尽可能多地收集到对较深星球的理解。可以玩的一个科学游戏是要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，那么地幔总体上是整体上的。如果没有，则需要从我们的采样手段中锁定一个未开发的“隐藏”储层。因此，此类信息提供了有关地幔结构的关键信息。为了测试观察到的地幔是否可以合理地表示整个地幔，需要与合理代表地球的原材料的组成进行比较。这样的参考以所谓的原始陨石的形式存在。这些来自小行星的样品记录了从未成长的小物体的组成，这些小体的成分从来没有足够大，可以融化并“区分”化学上不同的层，从而使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问题是，许多过程可能会改变表面采样的地幔碎片的组成。因此，我们需要选择一个不容易改变的化学特性，仅通过我们要研究的特定过程而改变。这是我们项目的目的。我们将对地球地幔和原始陨石的可及碎片进行高度精确的MG同位素测量。镁由3个稳定的同位素组成（质量为24、25和26），预计在陨石和地球中，26/24mg的比率相同。但是，我们的初始测量表明事实并非如此。我们需要更详细地研究这种令人惊讶和值得注意的观察。首先，我们想检查测量的准确性。对我们需要的精度进行MG同位素测量存在已知的潜在问题。我们已经开发了一种新颖的技术来解决这些问题并确保我们获得准确的测量。该项目的第一部分是使用这种方法在地球和陨石上进行一组测量。其次，我们认为地球和原始陨石中Mg同位素比的差异可能是地球历史的指纹。地球通过一系列巨大的影响积聚，其中最后一个可能产生月球。在这些影响之后，地球很可能熔融，岩浆海洋的凝固可能是从底部向上发生的。在如此高压结晶过程中生长的某些矿物质的化学方面使我们相信它们可能以不同的Mg同位素比与生长的液体形成。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品中瞥见地幔的MG同位素比与原始的陨石不同。我们将通过实验室实验测试这个想法，这些实验实验在适合全球岩浆海洋的条件下生长如此高压晶体。我们将分析MG同位素比率的实验产物，以查看我们的理论是否正确。尽管我们将专注于MG和类似的故事与SI及其同位素有关。因此，同时，使用相同的样品和实验，我们将研究SI，这也阐明了核心的形成。

项目成果

Molybdenum isotope fractionation between Mo4+ and Mo6+ in silicate liquid and metallic Mo

• DOI: 10.1016/j.chemgeo.2018.11.014
• 发表时间: 2019-01
• 期刊: Chemical Geology
• 影响因子: 3.9
• 作者: R. Hin;Antony D. Burnham;D. Gianolio;M. Walter;T. Elliott

Magnesium isotope evidence that accretional vapour loss shapes planetary compositions.

• DOI: 10.1038/nature23899
• 发表时间: 2017-09-27
• 期刊: Nature
• 影响因子: 64.8
• 作者: Hin RC;Coath CD;Carter PJ;Nimmo F;Lai YJ;Pogge von Strandmann PAE;Willbold M;Leinhardt ZM;Walter MJ;Elliott T
• 通讯作者: Elliott T

Titanium-rich basaltic melts on the Moon modulated by reactive flow processes

• DOI: 10.1038/s41561-023-01362-5
• 发表时间: 2024-01
• 期刊: Nature Geoscience
• 影响因子: 18.3
• 作者: M. Klaver;S. Klemme;Xiao-Ning Liu;R. Hin;C. Coath;M. Anand;C. Lissenberg;J. Berndt;Tim Elliott

The magnesium isotopic composition of the mantle

• DOI: 10.1016/j.gca.2023.08.011
• 发表时间: 2023-08
• 期刊: Geochimica et Cosmochimica Acta
• 影响因子: 5
• 作者: Xiao-Ning Liu;R. Hin;C. Coath;M. Bizimis;Li Su;D. Ionov;E. Takazawa;R. Brooker;Tim Elliott-

The influence of crustal recycling on the molybdenum isotope composition of the Earth's mantle

• DOI: 10.1016/j.epsl.2022.117760
• 发表时间: 2022-10
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: R. Hin;K. Hibbert;Shuo Chen;M. Willbold;M. B. Andersen;E. Kiseeva;B. Wood;Y. Niu;K. Sims