Global halogen flux of subducting oceanic crust

•Altered seafloor basalts preferentially incorporate F relative to altered gabbros, which contain more Cl and Br.•The efficiency of halogen devolatilization from subducting ocean crust is Cl≈Br>I≫F.•At high P-T, lower crustal gabbros can be converted to halogen-enriched eclogites.•Subducted ocean crust represents the largest flux of F, Cl, Br, and I to the mantle.•Approximately 50% of the initial Cl and 95% of initial Br and I is lost before the arc.