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

挥发性元素，如水、碳和氮，对行星进化的许多方面都很重要。通过研究地球和陨石物质中挥发性元素的丰度、分布和行为，我们可以了解许多关于行星分化、火山作用和大气演化的信息。一组中等挥发性的元素，卤素氯、溴和碘，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性演化，也是我研究的主要对象。卤素元素以非常低的丰度存在于大多数陆地物质中，并存在于非常特殊的地球化学库中。它们也是亲水性的，用水进行有效的跟踪，并作为它在行星系统中存在的记录。这些因素使卤素具有独特的地球化学指纹，可用于追踪大规模的行星加工。大约45亿年前，小型岩石行星相互碰撞，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，火星大小的撞击器和更大的原始地球之间的一次特殊碰撞形成了地月系统。这一动力开始于地球，当时被称为Hadean(4.5-4.0Ga)，导致了地球上第一次火山活动，以及古代海洋的发展。然而，没有证据表明地球历史上这一最早的篇章的岩石记录保存到了今天。我们发现的最古老的岩石来自地球的年轻太古宙(4.0-2.7Ga)。这些岩石很罕见，但今天可以在稳定的大陆上找到。太古宙岩石为我们提供了一个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初地壳的一些性质和化学成分，并寻找(可能)生命形成的最初环境的证据。我打算解决的具体研究问题包括：(1)地球积木(原始陨石)的卤素组成是什么，这与我们从其他挥发性元素中所理解的是否相符？(2)地球上的卤素是如何从吸积、分异、核心形成演化和分布的？以及(3)早期地球上的生命是在哪里形成的，是如何形成的？这些环境是什么样子的？早期地球卤素地球化学对这些环境的发展有多重要？我旨在通过对稀有原始球粒陨石、软陨石(代表残留的金属硅酸盐、地幔核边界小行星样本的石质陨石)和地球上现存最古老的部分；太古代岩石，包括伊苏阿表壳带(格陵兰)和巴伯顿绿岩带(南非)进行的有针对性的研究，利用惰性气体和卤素分析来解决这些问题。我的研究涉及一种名为中子辐照稀有气体质谱仪(NI-NGMS)的新方法，该方法是为了在非常小的样品(&lt；1 mg)中测量非常低的卤素浓度(&lt；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有关的高P实验相结合，将深入了解地球历史上这一最早时期的挥发性行为和分布。提供上述研究问题的答案对于促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生命的独特环境。

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.