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

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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.