This aim of this research programme is to understand better the evolution of the small bodies of rock and ice that lie in the outer parts of the solar system. These bodies are collectively called 'planetesimals', and are of great interest to planetary scientists because they have remained unchanged for most of the 4,500 million year history of the solar system. Thus, they can tell us much about how it formed and developed. This research project is concerned with planetesimals that contained liquid water, probably for a brief period soon after their formation. Pieces of these planetesimals have fallen to Earth as meteorites called 'carbonaceous chondrites', which are highly valued by scientists because their chemical compositions indicate that they are least altered rocks available for study. However, despite their very primitive chemistry, the carbonaceous chondrites are made mainly of minerals that were formed by water reacting with their parent planetesimal, and this process of aqueous alteration would be expected to have also modified the chemical composition of the rock. This contradiction between a primitive chemistry and secondary mineralogy can only be explained if water within the planetesimal was static. However, recent computer simulations of planetesimal evolution consistently predict that only the smallest bodies could have contained static water and in most it must have flowed through the rocks, modifying their chemical compositions along its path. In this research programme we will test the assumptions and predictions of these models by obtaining new information on the behavior and history of water within planetesimals using one group of carbonaceous chondrites called the CMs. These meteorites contain small crystals of minerals called carbonates that crystallized from the water. By examining the compositions, internal structures and distributions of carbonate crystals using a range of microscope-based techniques, we will address the following questions: Was the water stationary or did it flow in the same way that hot water moves through rocks on Earth? Did the water exist for only a brief period in a small body or was it present for millions of years within a larger planetesimal? Did planetesimal interiors contain water or was it present only close to their surface? Results of this research will increase our understanding of how planetesimals formed and evolved and will enable us and other scientists to assess and potentially modify the computer models of planetesimal interiors. Ultimately this work is significant for our understanding of the early history of the solar system but also of the present-day composition and internal structure of comets and asteroids. These bodies are currently the focus of a great deal of international research activity, having been visited recently by several space probes, and are targets for future unmanned and possibly manned exploration.

In response to the NERC Theme Action (TA) we propose a consortium among scientists at seven UK institutions and with three international partners centred on the 'The Volatile Legacy of the Early Earth'. Earth's habitability is strongly linked to its inventory and cycling of volatiles, which today are coupled to plate tectonics, but we still have little notion as to how our planet found itself in this near-ideal 'Goldilocks' state where the volatile mix is 'just right'. Was it simply a matter of being at the right solar distance with the right supply of volatiles? Or were the details of the chemistry and dynamics of early accretion and differentiation crucial to the eventual outcome? Such questions are of critical importance for understanding our own planets development, and given the burgeoning field of exo-planet discovery, they gain extra piquancy for gauging the probability of life elsewhere. In this proposal we investigate how the early evolution of volatiles on Earth set the stage for habitability. Planets grow by collisions and these violent events may lead to loss of the volatiles carried within the impacting bodies. We will explore with numerical modeling the conditions under which the volatiles are retained or lost in planetesimal collisions. We will also assess the likelihood that volatiles were delivered to Earth 'late', namely after the maelstrom of major collisions was finished and the planet was largely constructed, by studying the element S and notably its geochemical twin, Se. We will constrain the process of loss to the core and the isotopic signature imparted by this process. We will further use isotopic measurements as finger-prints of the origin of modern Se, and will find out whether it corresponds to any known meteorite type, or if it was possibly delivered by comets. The Moon provides further clues to the origin of the Earth, and Interrogating the significance of the recently refined volatile inventory of the Moon requires new experiments under appropriate conditions. The energy generated by planetary collisions inevitably results in large-scale melting. The solubility and chemical nature of volatiles within a magma ocean controls whether or not gases are carried into the interior of the planet or left in the atmosphere. Volatiles retained in the magma ocean may become part of a deep mantle volatile cycle or become permanently sequestered in deep reservoirs. We will redress this issue with a series of experiments that simulate conditions of the early magma ocean. We will further investigate the stability of phases in the lower mantle that can potentially hold volatile elements if delivered to great depths by solubility in a convecting magma ocean. Using seismic and modeling techniques, we will assess if any remnants of such stored volatiles are currently 'visible' in the deepest mantle. The influence of the core on volatile budgets is potentially great because of its size, but volatile solubility is poorly known. We will examine the solubility of hydrogen, carbon and nitrogen in liquid metal at high pressures and temperatures. In this consortium we will also create a cohort of PhD students and supervisors who work as part of a large team to piece together the evidence for Earth's volatile evolution using inclusions trapped in diamonds. These may be the key 'space-time' capsules that can link experimental and theoretical work on early Earth evolution to present-day volatile budgets and fluxes in the deep Earth. The questions raised in this proposal are complex and require a wide range of information in order to provide meaningful answers. It is our goal to establish a much-improved understanding of how Earth initially became a habitable planet, and to build a solid foundation on which further UK research can continue to lead the way in this exciting field. This will be the ultimate legacy of this consortium, and through links to other consortia, of the entire Theme Action.

Earth's present belies its violent past. Catastrophic impacts during the Earth's first 500 million years generated enough energy to melt the planet's interior, creating planetary-scale volumes of melt, or "magma oceans". Their subsequent cooling and crystallisation would have set the chemistry of the Earth and its future long-term habitability. However, we do not know exactly where and how the Earth's magma oceans crystallised, what their composition was and whether remnants of early magma ocean material remain present in the Earth's deep interior, potentially acting as important reservoirs for volatiles and precious metals. A key piece of information may reside in the deep Earth: as the magma ocean cooled it would have started to crystallise, with the dense newly formed crystals sinking to the base of Earth's mantle. This would have generated strong chemical layering in the mantle, which could persist to today. This project focuses on finding the chemical evidence for these piles of dense magma ocean crystals, and thus identifying a key missing piece of evidence for Earth's earliest history. As the deepest mantle is inaccessible to direct sampling, we must rely on nature to do this for us. This occurs when regions of the mantle heat up, buoyantly rise and melt, ultimately producing volcanism; a phenomenon exhibited at Iceland, Hawaii and other "mantle plumes". We can use the chemistry of these lavas to probe the composition of the material that melted to form them, thereby gaining a window into the deep Earth. The chemical signals in both modern and ancient lavas have resulted in the paradigm of isolated and "primordial" regions of the Earth's interior, often presumed to be located at the very base of the Earth's mantle, at the boundary with the planet's central metallic core. It has been suggested that the mineralogy and composition of these deep mantle domains has allowed them to resist being entrained into the convecting mantle for billions of years, where they may store volatile- and heat-producing elements. Do these regions of the Earth's mantle have their origin in magma ocean crystallisation? Has magma ocean material always remained isolated from the convecting mantle? Can residual frozen melts or crystalline material left over from magma ocean crystallisation be transported into the upper mantle, and if so, can it melt and contribute to the chemistry of modern and ancient primitive lavas? To answer these questions, we need chemical tracers that, 1) respond directly to the type of minerals that would have formed during the crystallisation of a deep magma ocean, 2) are resistant to alteration when volcanic rocks are weathered at Earth's surface so that they can be applied to ancient lavas, and 3) reflect the bulk properties of the mantle that these lavas were derived from. We propose to use iron (Fe) and calcium (Ca) stable isotopes as tracers. Reconnaissance measurements of 3.7 billion year old rocks shows that these tracers are robust to the rocks' weathering history. The data also contain the tantalising suggestion that these volcanics were derived from melting material residual from a former magma ocean. We will use these tracers to explore the Earth's magma ocean history and its role in defining the chemical and physical state of the planet today. Important steps are: 1) Constraining the partitioning of Fe and Ca isotopes during magma ocean crystallisation. We will do this by high-pressure laboratory experiments, where we will simulate the conditions of magma ocean crystallisation and analyse the crystal residues that we produce. 2) Undertaking new Fe and Ca isotope analysis of volcanics ranging from 3.7 billion years old to the present. 3) Develop a series of thermodynamic models to track the Fe and Ca isotope effects of magma ocean crystallisation and to predict the composition of volcanics derived from the entrainment and melting of these magma ocean crystal piles in the upper mantle.

Incompatible and volatile elements like noble gases and the halogens are continually brought from the mantle to the Earth's surface at mid ocean ridges and other regions of volcanic activity. It has always been assumed that this is a one-way process for these species. This assumption is made when using these tracers to investigate planetary volatile origin, accretionary processes, mantle dynamics and atmosphere evolution in the case of the noble gases, or tracking the halogen budget of the oceans and assessing the role of ocean salinity in the evolution and sustenance of life in the oceans. Recent Manchester work suggests that surface noble gases are being subducted into the deep mantle and that this source dominates the mantle heavy noble gas budget (Ballentine et al., Nature 2005; Holland and Ballentine, Nature 2006). What is surprising is that the mantle shows an Ar/Kr/Xe signature identical to that of seawater. This is an elemental/isotopic composition unique in the solar system and we can rule this out as an original accretionary mantle component. Because of the volatile nature of noble gases we might expect this seawater ratio to be perturbed during the process of subduction. What then is the mechanism that preserves the seawater signature? Where in the oceanic crust are these gases found? How and why are they preserved in the subducting slab? We present new pilot data in our proposal showing that we can identify the same seawater signature in fluids released from dewatering slabs that have been taken to at least 100km depth in the mantle. With a technique pioneered at Manchester, we also show that the halogens in this fluid, which are equally susceptible to fractionation, are identical to values found in marine pore fluids. It would appear that noble gas and halogen subduction are linked. We propose the first complete and systematic analytical survey of the oceanic crust and associated sediment to identify the location and elemental character of the phases or hosts that dominate the noble gas and halogen budget of subducting material. For little extra effort we will also obtain the abundance and hydrogen isotopic composition of the associated water. We also propose to extend the pilot study results to two more terrains, with contrasting thermal regimes, where we expect to be able to sample and identify the noble gas and halogen composition of deep (~100km) subducted fluids. This combined data set will be the first to link noble gases, halogens and related water in a systematic way from subducting to subducted fluid composition in differernt thermal settings. This will enable us to identify the major carrier/ers of noble gas into the mantle and use our understanding of noble gas concentrations and convection behaviour in the mantle to start to model and identify the associated subducted halogen and water impact on the respective total mantle budget and the evolution of these tracers in the mantle system - systems that underpin our understanding of the Earth.

Melting in the Earth has a huge effect on its chemical and physical state. For instance, the chemistry of the crust, the mantle and the atmosphere are largely controlled by melting and crystalisation at mid-ocean ridges, hotspots or island arcs. There has, therefore, been an enormous effort in the last decades to understand these shallow melting processes. Yet much deeper melts may have been equally influential in the evolution of the Earth. For instance, it is generally accepted that a deep magma ocean perhaps extending to the Earth's centre, existed early its history. This was the result of multiple impacts as the Earth accreted. From this magma ocean, iron melts separated from silicate melts to form the core, volatiles degassed to form an early atmosphere, and a proto-crust may have formed. It is also accepted that the Earth was hit by a Mars-sized body to create the moon; this too would have caused enormous amounts of melting in the deep Earth. Moreover, there is some evidence for melting in the deep Earth now. It is possible, therefore, that melts in the deepest Earth have existed throughout Earth's history. However, many basic data on the physical and chemical properties of deep melting do not exist. For instance, we don't know the melting curves for mantle minerals and rocks at the pressure and temperatures of the deep Earth. We don't know which minerals crystalise from these melts first (the liquidus phases). We don't know the composition of partial melts of deep mantle rocks or rocks which have been subducted. We don't know the relative densities of the rocks and their melts, and so we do not even know whether minerals float of sink in these deep melts. This lack of data has led to much speculation on the effect of deep melts on the Earth's evolution. For instance, it has been suggested that geophysical and geochemical anomalies in the Earth's mantle have deep early melts as their origin. But these models depend of the chemical and physical properties of the melts and crystalline solids, properties that are simply not known. This project will use novel experiments in conjunction with ab initio modelling obtain these data. The data will provide the chemical and physical foundation on which all future models of the Earths early crystallization and subsequent evolution will be based.