Catastrophic planetary collisions during the Earth’s first 500 million years provided enough energy to melt its interior, creating planetary-scale volumes of melt, or magma oceans. Their cooling and crystallisation determined the chemistry of the Earth and its long-term habitability. However, we do not know where and how the Earth’s magma oceans crystallised, whether remnants of early magma ocean melts and crystals, or the iron-sulfide liquid that may have separated from them, are still preserved in the mantle today, or what their role is in storing the Earth’s volatiles and rare metals. We also do not know if this residual material remained inert or whether it interacted with mantle melting events during the course of Earth history, potentially transferring its precious cargo to the planet’s surface.<br/><br/>The main barrier to studying magma oceans is that most of the evidence of them on Earth has been erased by the tectonic mixing processes that have operated over the past ~3 billion years. EarthMelt will address this issue by developing novel isotopic tools to study ancient magma ocean events. Iron and calcium stable isotopes show high-pressure phase-specific partitioning effects that can identify the molten and crystalline residues of magma oceans. Copper and platinum stable isotopes can be used to trace the separation of iron-sulfide melt from the silicate mantle and its incorporation in modern and ancient mantle melting regimes. EarthMelt will combine ultra-high precision measurements of these novel isotope systems with experiments simulating the conditions of magma ocean cooling and crystallisation and will apply these isotope tracers to rare samples of the Earth’s interior. <br/><br/>EarthMelt will determine how the Earth’s magma ocean crystallised and how this influenced the physical structure and chemical composition of our planet, opening up a new approach to the study of magma oceans and their role in controlling terrestrial planet chemistry and habitability.