Carbon mobilized at shallow depths in subduction zones by carbonatitic liquids

More than half a gigaton of CO 2 is subducted into Earth's interior each year. At least 40% of this CO 2 is returned to the atmosphere by arc volcanism. Processes that are known to release carbon from subducting slabs - decarbonation or carbonate dissolution in fluids - can account for only a portion of the CO 2 released at arc volcanoes. Carbonatitic liquids may form from the subducting crust, but are thought to form only at very high temperatures. Melting of carbonated rocks could restrict the subduction of carbon into the deeper Earth. However, the behaviour of such rock types in subduction zones is unclear. Here I use laboratory experiments to show that calcium-rich hydrous carbonatitic liquids can form at temperatures as low as 870 to 900 °C, which corresponds to shallow depths of just 120 km beneath subduction zone arcs, in warm thermal regimes. I find that water strongly depresses the solidus for hydrous carbonate gabbro and limestone rocks, creating carbonatitic liquids that efficiently scavenge volatile elements, calcium and silicon, from the slab. These extremely mobile and reactive liquids are expected to percolate into the mantle wedge, and create a CO 2 source for subduction zone magmatism. Carbonatitic liquids thus provide a potentially significant pathway for carbon recycling at shallow depths beneath arcs.