Deep carbon cycling in subduction zones constrained by partial melting of phengite-bearing carbonated eclogite

Abstract Partial melting of carbonated rocks in subduction zones is generally controlled by the availability of fluid or the stability of hydrous minerals. Few experiments have addressed the conditions of dehydration melting of carbonated rocks, despite their potential importance for carbon (C) cycling in subduction zones and arc magmatism. Here, we present high-pressure experiments on a carbonated eclogite with H2O stored in phengite to illustrate the decarbonation behavior of subducted crust. The experimental results show that phengite-bearing carbonated eclogite undergoes silicate melting at 3–4 GPa and temperatures ∼850 °C, leaving carbonate as a refractory phase in equilibrium with eclogitic residues. At high pressures (5–8 GPa), the near-solidus liquid is replaced by carbonatitic liquid at temperatures of ∼900–950 °C. Dehydration of phengite is responsible for the formation of both silicate and carbonatitic liquids in subducted eclogite. The intersection of the carbonate melting curve with slab geotherms at depths greater than ∼150 km can produce 5–7 wt% potassium-rich carbonatitic liquids for mantle metamorphism at temperatures achievable in warm/hot subduction zones. We suggest that phengite dehydration-induced carbonate melting could be an important C-cycling mechanism in subduction zones, effectively removing most of the C from subducted eclogites.