With subduction zones being the main loci of ingassing of carbon into Earth’s interior, it is critical to constrain the efficiency of deep carbon subduction versus carbon recycling back through volcanic arcs. What are the latest constraints on the fate of carbon during subduction of the oceanic lithosphere with crust and sediments? How much carbon gets subducted deeper past the arc processing zones for each lithology (sediment, basalt, mantle peridotite), and how much gets released from the subducting slab? What is the expected flux of carbon out of the slab because of carbonate dissolution in fluids, metamorphic decarbonation, and hydrous slab melting? What constraints exist based on experimental petrology, thermodynamic modeling of fluid–rock interactions, and dissolution? How much carbon is expected to come out of the slab and how does that compare with the estimated flux of CO2 out of present-day volcanic arcs? These questions are addressed in this chapter.

This chapter reviews the systematics of partial melting of mantle lithologies – like peridotite and eclogite – in the presence of carbon dioxide. It discusses the composition of mantle-derived magmas generated in the presence of carbon dioxide and whether magmas erupted on Earth’s surface resemble carbonated magmas from the mantle. It reviews how the production of carbon dioxide-rich magma in the mantle varies as a function of tectonic settings – beneath continents and oceans and in subduction zones – and time.

Significant investment in new capacities for experimental research at high temperatures and pressures have provided new levels of understanding about the physical properties of carbon in fluids and melts, including its viscosity, electrical conductivity, and density. This chapter reviews the physical properties of carbon-bearing melts and fluids at high temperatures and pressures and highlights remaining unknowns left to be explored. The chapter also reviews how the remote sensing of the inaccessible parts of the Earth via various geophysical techniques – seismic shear wave velocity, attenuation, and electromagnetic signals of mantle depths – can be reconciled with the potential presence of carbon-bearing melts or fluids.