We have brought together a consortium of UK investigators and international partners with the key objective of providing a new process based understanding of volatile element (e.g. H2O, C, S, noble gases and halogens) fluxes into the deep mantle at subduction zones and out of the mantle at mid ocean ridges and ocean island settings. The mantle is by many orders of magnitude the largest silicate reservoir for carbon, nitrogen and sulphur on Earth and the input and output of volatiles (e.g., H2O, C, N, S, P, and halogens) at plate boundaries provides long-term controls on the climate and the biosphere. Nevertheless, our understanding of the deep-Earth volatile cycle is crude. In part because we have a very poor understanding of the relative contribution of recycled to primordial volatiles in the mantle system and how this might vary in different mantle reservoirs. In part this is because volatile elements are extensively lost during the eruptive process from many sample types making it hard to identify the controlling processes necessary to develop coherent models. To address our objective the consortium combines several advances in new sample resources and analytical tools: i) The recognition that rapidly quenched melt inclusions (MIs) within erupted material often preserve mantle-source volatile compositions; ii) The ability to determine sulphur and boron isotopes in addition to major volatiles in the MIs; iii) The discovery that boron isotopes can track the extent of volatile loss to the surface from subducting slabs and preserve this signal in the deeper mantle; iv) The innovations in noble gas isotope determination that allow us to resolve recycled volatiles from those trapped during accretion and provide links to halogens, H2O and C; v) The development of non-traditional stable isotopes such as Fe, Cu and Se to identify system oxidation state (a key variable in understanding sulphur) and chalcophile trace element determinations; vi) The advances in computing power and techniques that allow better representation of mantle-like systems. By coordinating the combined consortium expertise and analytical resources on the same sample suites in two thermally contrasting subduction regimes (Kamchatka (cool) and Southern Chile (hot)) we plan to investigate how both the processes and thermal setting control the efficiency and geochemical character (isotopic composition and relative abundance to other volatiles) of volatile subduction into the deep mantle. This allows us to take into consideration changes in subduction temperature as the Earth cools in the development of flux models that run for the age of the Earth. At mid ocean ridges and ocean island settings with different geochemical provenance (e.g. HIMU, EMI, EMII, FOZO) we will determine the proportion and character of volatile elements that have been recycled compared to those that were incorporated into the mantle during its formation (primitive volatiles). This is an essential component in building our understanding of the volatile flux into the mantle required to support the signals in the mantle today. New experimental partitioning developed within the consortium and our ability to track oxidation state will allow us to make a step change in understanding the sulphur cycle - barely understood to date but critical in understanding climate and commercial mineral deposit formation. Numerical simulations of mantle transport for suites of geochemical elements, iterating the geophysical parameters to approach matches for the geochemical observables, will allow us to identify the key geophysical processes in subduction zones and during whole mantle convection that control the geochemical distribution of subducted vs. primordial volatiles in the mantle. Together, these will lead to a significant advance in reconstructing the deep Earth volatile fluxes over Earth history - a grand science challenge.

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.

This proposal seeks to quantify the water content of parental mafic magmas in arcs, using a new method involving the measurement of H+ and Al in pyroxenes. The method utilises the linear relationship between the melt-pyroxene partition coefficient for water, and the Al content of the pyroxene, as determined experimentally. In the past, this method has been applied primarily to mantle xenoliths for inferring the water content of the mantle, with the exception of one study which looked at volcanic rocks from Central America. At Soufriere Hills Volcano and many other intermediate-silicic volcanoes in arc settings, mafic intrusion is thought to trigger and sustain eruptions. At SHV, the mafic magma supplies sulphur and heat to the crystal-rich resident andesite, provoking convective self-mixing and eruption. Gas emissions and ground deformation datasets all point to intrusion of magma at 6-12 km depth, and petrological studies show streaked and banded andesites, or rounded enclaves in the erupted products, suggesting that 'mingling', rather than mixing occurs between the two end member magmas. If the intruding magma has a high volatile content, it will supply a significant vapour phase to the system on quenching and degassing at the interface between the magmas. Magma with a high fluid content is highly compressible, damping ground deformation signals. Sulphur partitions strongly into vapour at depth in oxidised systems, so a high fluid content has a higher potential for 'stripping out' sulphur from the melt, segregating it and allowing it to migrate to the surface or to cause explosive eruption. This proposal seeks to compare pyroxenes from mafic magma (which occurs as enclaves) with those from the resident andesite, to assess the pre-eruptive H2O contents of both end member magmas and further, to evaluate whether it is possible to assess melt water contents through time using profiles across the pyroxenes and accounting for diffusive loss. The results will have broad implications for arc magmas in general.

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.

Natural diamonds are formed at high pressures and temperatures deep within the Earth's interior. When diamonds form, probably from carbonate-rich fluids and melts in the mantle, they sometimes encapsulate small pieces of the minerals that occur at great depth in the Earth. These are called mineral inclusions. The diamonds are then transported from Earth's deep mantle to the surface in uncommon magmas called kimberlites. Diamonds that contain these mineral inclusions are very rare, and offer a truly unique glimpse into what is an otherwise inaccessible portion of the Earth. Some very rare inclusions provide direct samples of lithologies present in the mantle transition zone (400 - 660 km) and the lower mantle (>660 km) - these are often called superdeep diamonds. The chemistry of the inclusions along with mineral phase relations yield important information about the kinds of lithologies they originated in, and constrain the conditions of diamond formation and the depth at which kimberlite magmas form. Thus, superdeep diamonds are very important for studying the types of materials that occur in the deep Earth, for elucidating deep mantle processes, and for understanding how carbon is cycled from the surface to the mantle and back to the surface again - the deep carbon cycle. For example, some diamonds contain materials that are very similar to those occurring near the earth's surface, such as minerals akin to oceanic crust or sediments, and these often have carbon isotopic compositions akin to organic carbon - although this is a controversial subject. From this, we can conclude that surface materials can be transported to great depth, helping to constrain models of mass transfer in Earth by mantle convection. Further, by dating when the diamonds formed, for example by dating of inclusions, we can effectively place time constraints in the geodynamic processes involved in diamond formation and uplift in the mantle. Inclusion-bearing diamonds suitable for study are very hard to come by. We are very fortunate to be in possession of several large suites (over 200 inclusion-bearing diamonds in all!) of diamonds from kimberlite pipes in the famous Juina region of Brazil, a region known for its superdeep diamonds. Our previous study on diamonds from the Juina region has yielded some fascinating results, and has led to a model of material recycling beneath Brazil that we have recently published in the journal Nature and in Contributions to Mineralogy and Petrology. We now wish to extend our investigations by studying new suites of diamonds from Juina to test our current model, and to make high-pressure temperature experiments that will allow us to determine at what depths the inclusions formed and equilibrated, and will provide information needed to constrain the rates at which diamonds were transported in the solid-state mantle, possibly in a mantle plume. Here, we propose a three-year project for a comprehensive mineralogical, geochemical, isotopic and experimental investigation of these unique diamonds and their mineral inclusions.