The key aspect of the Earth’s deep carbon cycle is how exogene carbon is recycled into the deeper earth via subduction of altered, carbonated, mafic oceanic crust. We aim to use ultrahigh-pressure experimental petrology to investigate the behaviour of subducted carbonate at pressures corresponding to the deep upper mantle, the mantle transition zone and the uppermost lower mantle. Earlier experimental studies showed that some residual crystalline carbonate in oceanic crust remains stable in subducting oceanic crust without decarbonation or melting, and may be transported to very deep levels in the mantle. It may form carbonate eclogite in the upper mantle and carbonate garnetite in the transition zone and uppermost lower mantle. How far carbon can survive this journey to extreme depths depends on the relationship between the pressure-temperature path followed by deeply subducting carbonated oceanic crust and its melting relations and solidus temperatures. We aim to use multi-anvil experimentation at the University of Bristol to determine melting relations in the deep upper mantle, transition zone, and uppermost lower mantle (9-21 GPa) of carbonate eclogite. We will explore the influences of pressure, temperature, oxygen fugacity and key bulk compositional variables such as Na2O/CO2 on very deep subduction of carbonate and on the volumes and compositions, and fates of carbonated partial melts. Fundamental research outcomes will include understanding of (1) the role of bulk composition in determining melting temperatures of deeply subducted, carbonate-bearing oceanic crust, and hence how deep carbonate melting can occur (2) how these carbonate melts interact with surrounding ambient peridotite mantle and what sort of geochemical sources and deeply derived magmas/fluids could be so generated (kimberlites, carbonatites, CH4-fluids), and (3) the formation of sublithospheric diamonds and their mineral inclusion suites.