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A pinch of salt: Earth's halogen distribution and the habitability potential of planets

一小撮盐：地球的卤素分布和行星的宜居潜力

基本信息

批准号：
MR/S03465X/1
负责人：
Patricia Clay
金额：
$ 65.04万
依托单位：
University of Manchester
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2020
资助国家：
英国
起止时间：
2020 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=MR%2FS03465X%2F1
关键词：
pinch salt Earth halogen distribution

项目摘要

Volatile elements, like water, C, and N, are important to many aspects of planetary evolution. We can learn much about, for instance, planetary differentiation, volcanism and atmosphere evolution by studying volatile element abundances, their distribution and behavior, in terrestrial and meteoritic materials. One group of moderately volatile elements, the halogens Cl, Br and I, are a particularly useful set of geochemical tracers for investigating volatile evolution in terrestrial environments and are the main subject of my research. The halogen elements are present in very low abundance in most terrestrial materials and exist in very specific geochemical reservoirs. They are also hydrophilic, usefully tracking with water, and acting as a record of its presence in planetary systems. These factors make the halogens unique geochemical fingerprints for tracing large scale planetary processing. Approximately 4.5 billion years (Ga) ago, small rocky planetesimals collided to form larger bodies and over time, rapid accretion led to formation of the terrestrial planets that we observe today. One particular impact between a 'Mars-sized' impactor and a larger proto-Earth is believed to have formed the Earth-Moon system. This dynamic beginning to planet Earth, the time known as the Hadean (4.5-4.0 Ga) gave rise the first volcanism on Earth, and the development of ancient oceans. However, no evidence of the rock record of this earliest chapter in Earth's history has survived to the present day. The oldest rocks we have come from the younger Archean period (4.0-2.7 Ga) of our planet. These rocks are rare but can be found distributed across the stable continents today. Archean rocks offer us a unique and exciting window into Earth's earliest history, enabling us to study, amongst other things, the nature and chemistry of some of Earth's first crust and search for evidence of the (likely) first environments where life formed. Specific research questions I aim to tackle include: (1) What is the halogen composition of terrestrial building blocks (primitive meteorites) and does this fit with what we understand from other volatile elements? (2) how did the Earth's halogens evolve and distribute from accretion, differentiation, core formation? and (3) where and how did life form on early Earth? What did these environments look like and how important was early Earth halogen geochemistry to the development of these environments? I aim to address these questions through targeted research using noble gas and halogen analyses on rare primitive chondrite meteorites, pallasites (stony meteorites that represent remnant metal-silicate, mantle-core boundary samples of asteroids) and our oldest surviving pieces of Earth; rocks from the Archean including, the Isua Supracrustal Belt (Greenland) and the Barberton Greenstone Belt (South Africa). My research involves a novel method called Neutron-Irradiation Noble Gas Mass Spectrometry, or NI-NGMS, that was developed to measure very low concentrations of halogens (<1 ppb) in very small samples (<1 mg). Coupling this geochemical approach with detailed mineral chemical information and high P experiments relevant to conditions of planetary differentiation processes, will give insight into volatile behavior and distribution during this earliest period of Earth's history. Providing answers to the above outlined research questions is critically important for the advancement of our understanding of early Earth evolution, including characterizing the unique environments that likely hosted first life.

挥发性元素，如水、碳和氮，对行星演化的许多方面都很重要。例如，通过研究陆地和陨石材料中的挥发性元素丰度、分布和行为，我们可以了解很多关于行星分异、火山活动和大气演化的知识。一组中等挥发性元素，即卤素 Cl、Br 和 I，是一组特别有用的地球化学示踪剂，用于研究陆地环境中的挥发物演化，也是我研究的主要主题。卤素元素在大多数陆地物质中的丰度非常低，并且存在于非常特定的地球化学储库中。它们也是亲水性的，可以有效地追踪水，并作为水在行星系统中存在的记录。这些因素使卤素成为追踪大规模行星加工的独特地球化学指纹。大约 45 亿年前（Ga），小型岩石星子碰撞形成更大的天体，随着时间的推移，快速吸积导致了我们今天观察到的类地行星的形成。据信，“火星大小”的撞击体与更大的原地球之间的一次特殊撞击形成了地月系统。地球的这种动态开始，即被称为冥古宙（4.5-4.0 Ga）的时间，引发了地球上第一次火山活动，并形成了古代海洋。然而，地球历史上这一最早篇章的岩石记录至今还没有任何证据保存下来。我们发现的最古老的岩石来自地球较年轻的太古代时期（4.0-2.7 Ga）。这些岩石很罕见，但今天可以在稳定的大陆上发现它们的分布。太古宙岩石为我们提供了了解地球最早历史的独特而令人兴奋的窗口，使我们能够研究地球最早的一些地壳的性质和化学，并寻找（可能的）生命形成的第一个环境的证据。我要解决的具体研究问题包括：（1）陆地构件（原始陨石）的卤素成分是什么？这是否符合我们对其他挥发性元素的理解？（2）地球的卤素是如何从吸积、分异、核心形成演化和分布的？（3）早期地球上生命是在哪里以及如何形成的？这些环境是什么样的？早期地球卤素地球化学对这些环境的发展有多重要？我的目标是通过对稀有的原始球粒陨石、橄榄石（代表小行星残余金属硅酸盐、地幔核边界样本的石陨石）和我们现存最古老的地球碎片进行有针对性的研究，使用稀有气体和卤素分析来解决这些问题；来自太古宙的岩石，包括伊苏亚地壳带（格陵兰岛）和巴伯顿绿岩带（南非）。我的研究涉及一种称为中子辐照惰性气体质谱法 (NI-NGMS) 的新方法，该方法旨在测量非常小的样品 (<1 mg) 中的极低浓度的卤素 (<1 ppb)。将这种地球化学方法与详细的矿物化学信息和与行星分化过程条件相关的高磷实验相结合，将有助于深入了解地球历史最早时期的挥发性行为和分布。回答上述研究问题对于增进我们对早期地球演化的理解至关重要，包括描述可能孕育第一个生命的独特环境。