Phengite-mediated fluorine and chlorine fluxes from subduction zones to the deep mantle

The cycling of volatiles through subduction zones fundamentally shapes Earth’s chemical evolution, yet how fluorine (F) and chlorine (Cl) are processed in the deep mantle remains enigmatic. Here, we use high-pressure experiments (5 to 11 gigapascals and 850° to 1200°C) on altered oceanic crust analogs to track deep halogen behavior. We show that phengite remains stable to 11 gigapascals and 1050°C, efficiently transporting F and Cl to ~330-kilometer depth. Upon breakdown or melting, phengite strongly fractionates these halogens: Cl is released into fluids or melts, while F is sequestered in residual phases such as KMgF 3 . The Cl-rich fluids produced (9.6 to 19.9 wt % Cl) closely match high-density fluid inclusions in diamonds, implicating them as key agents of cratonic metasomatism. Our findings establish phengite as a critical carrier mediating postarc fluxes of F (1.7 × 10 12 to 2.6 × 10 12 grams per year) and Cl (0.52 × 10 12 to 1.1 × 10 12 grams per year), providing a major pathway for replenishing deep mantle halogen reservoirs.