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

• 批准号: NE/S014802/1
• 负责人: Patricia Clay
• 金额: $ 81.26万
• 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: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FS014802%2F1
• 关键词: pinch; salt; Earth; halogen; distribution

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. Usefully, the halogens are hydrophilic, tracking with water and acting as a record of its presence in planetary systems. These factors make the halogens unique geochemical fingerprints for 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 (4.0-2.7Ga) 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 the halogen geochemistry of these environments? I aim to address these questions through targeted research using noble gas and halogen analyses on rare primitive meteorites, pallasites (stony meteorites that are the remnant metal-silicate, mantle-core boundary samples of asteroids) and our oldest surviving pieces of Earth; rocks from the Arcehan supracrustal belts from the Isua Supracrustal Belt (Greenland) and the Barberton Greenstone Belt (South Africa). My research involves a 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 characterising 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物质中挥发性元素的丰度、分布和行为，我们可以学到很多关于行星分化、火山作用和大气演化的知识。一组中等挥发性元素，卤素Cl， Br和I，是研究陆地环境中挥发演化的一组特别有用的地球化学示踪剂，也是我研究的主要课题。卤素元素在大多数陆相物质中丰度很低，存在于非常特殊的地球化学储层中。有用的是，卤素是亲水的，可以与水一起追踪，并作为行星系统中水存在的记录。这些因素使卤素成为大规模行星处理的独特地球化学指纹。大约45亿年前，小的岩石星子碰撞形成了更大的天体，随着时间的推移，快速的吸积导致了我们今天观察到的类地行星的形成。据信，一个“火星大小”的撞击器和一个更大的原地球之间的一次特殊撞击形成了地球-月球系统。这个充满活力的地球开始，被称为冥古宙（4.5-4.0 Ga）的时间产生了地球上第一次火山活动，以及古代海洋的发展。然而，地球历史上最早的一章的岩石记录的证据没有保存到今天。最古老的岩石来自较年轻的太古宙时期（4.0-2.7Ga）的行星。这些岩石很罕见，但如今分布在稳定的大陆上。太古代岩石为我们提供了一扇独特而令人兴奋的窗口，让我们了解地球最早的历史，使我们能够研究一些地球第一地壳的性质和化学成分，并寻找（可能的）生命形成的第一个环境的证据。我希望解决的具体研究问题包括：(1)陆地构造块（原始陨石）的卤素成分是什么，这与我们从其他挥发性元素中了解到的是否相符？(2)地球卤素在吸积、分异、地核形成过程中是如何演化和分布的？(3)早期地球上的生命是在哪里以及如何形成的？这些环境是什么样的，这些环境中的卤素地球化学有多重要？我的目标是通过有针对性的研究来解决这些问题，这些研究使用稀有原始陨石，pallasites（残余金属硅酸盐的石质陨石，小行星的地幔-核边界样本）和我们最古老的地球碎片；来自格陵兰岛伊苏亚上地壳带和南非巴伯顿绿岩带的太古宙上地壳带的岩石。我的研究涉及一种称为中子辐照惰性气体质谱法（NI-NGMS）的方法，该方法是为了测量非常小的样品（<1毫克）中非常低浓度的卤素（<1 ppb）而开发的。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，将深入了解地球历史上最早时期的挥发性行为和分布。为上述概述的研究问题提供答案对于提高我们对早期地球进化的理解至关重要，包括描述可能孕育第一个生命的独特环境。