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

挥发性元素，如水、碳和氮，对行星演化的许多方面都很重要。例如，我们可以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，了解行星分化、火山活动和大气演化。一组中等挥发性的元素，卤素Cl，Br和I，是一个特别有用的地球化学示踪剂，调查挥发性演化在陆地环境中，是我的研究的主要课题。卤素元素在大多数陆地物质中的丰度很低，存在于非常特殊的地球化学储层中。它们也是亲水性的，有用地跟踪水，并作为其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过程的独特地球化学指纹。大约45亿年前（Ga），小的岩石星子碰撞形成更大的天体，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。一个“火星大小”的撞击物和一个更大的原地球之间的一次特殊撞击被认为形成了地月系统。这个动态的开始，行星地球，时间被称为冥古宙（4.5-4.0 Ga）引起了地球上的第一次火山活动，以及古代海洋的发展。然而，地球历史上这一最早篇章的岩石记录没有证据保存到今天。我们所拥有的最古老的岩石来自地球上较年轻的太古代（4.0-2.7 Ga）。这些岩石很罕见，但今天可以在稳定的大陆上找到。太古代岩石为我们提供了一个独特而令人兴奋的窗口，让我们了解地球最早的历史，使我们能够研究地球第一个地壳的性质和化学，并寻找生命形成的（可能的）第一个环境的证据。具体的研究问题，我的目标是解决包括：（1）什么是卤素组成的陆地积木（原始陨石），这是否符合我们所了解的其他挥发性元素？(2)地球上的卤素是如何从吸积、分化、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以及如何形成的？这些环境看起来像什么？早期地球卤素地球化学对这些环境的发展有多重要？我的目标是通过有针对性的研究，使用稀有的原始球粒陨石，pallasite（代表残余金属硅酸盐的石质陨石，小行星的地幔-核心边界样本）和我们最古老的地球幸存下来的碎片惰性气体和卤素分析来解决这些问题;来自太古代的岩石，包括Isua Supracrustal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，该方法用于测量非常小样品（<1 mg）中非常低浓度的卤素（<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P实验相结合，将使我们深入了解地球历史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，包括描述可能承载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.