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

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.

There are several audiences that may benefit from the anticipated outcomes of the proposed research. The main beneficiaries of the proposed research are academic beneficiaries, analytical (industry) beneficiaries, and wider public audiences. These audiences will benefit from the results generated from this study in different ways, as outlined below.

A. Academic Beneficiaries. This research will inform on the origin, delivery and distribution of halogens, and more broadly, other volatile elements, on Earth. This research has the potential to inform on the role of halogens in broad scale planetary evolution and the overall habitability potential of planetary surfaces. Results will be relevant to a broad range of Earth scientists, including geochemists, sedimentologists, and geomicrobiologists. This group will benefit from sharing and presentation of results annually at international and national conferences and in the wide availability of the large, high-quality data sets generated in this project, which will be available through publication in international, open-access journals.

B. Analytical (Industry) Beneficiaries. I will share any analytical (mass spectrometry) or technical (sample extraction) innovations with the industrial sector that we typically work with (for instance, Thermo Fisher Scientific, Teledyne-Cetac Technologies, FEI, JEOL). Any advances (or identified future requirements) in the analysis of small samples and inclusions will be widely relevant to this sector. This will be particularly relevant to the development of micro-scale analytical capabilities, especially laser probe technology.

C. Public Audiences. Successful impact in the public sector is reliant upon effective dissemination of scientific results using varied vehicles of knowledge transfer, such as social media, large-scale outreach events, and smaller hands on opportunities. The nature and subject of the proposed work is ideally suited for fostering excellent outreach and public engagement opportunities. I am particularity interested in using my research to target younger, female audiences who are currently underrepresented in STEM fields. Luckily, within the University of Manchester, support and infrastructure for public engagement is particularly strong, with the need for community interaction and engagement highlighted as part of the University's 2020 strategic plan. Within the IGC group, there is a strong outreach component and the resources and contacts are in place for effective science outreach. The IGC group hosts a science blog (Earth and Solar System Blog), which reaches wide and varied audiences. Long standing links with the Manchester Museum, the Museum of Science and Industry and Jodrell Bank are in place and events, such as the autumn Science Spectacular as part of the Manchester Science Festival (Manchester Museum), Girls Night Out and bluedot (Jodrell Bank) are organized annually. These events have good attendance and are well regarded by the public.