Br/Cl and I/Cl systematics in the shallow-water hydrothermal system at Milos Island, Hellenic Arc

► Large Br/Cl changes in vent fluids have occurred at Milos in contrast to MOR systems. ► Halite contributes the Br/Cl changes in segregated vapors as pore pressure varied. ► Continuously boiling off the vapors induces residual brines has low Br/Cl at deep. ► High I/Cl in cave and brine fluids imply sediment contribution have occurred at deep. ► The I/Cl in brines and cave fluids fall in a similar range as those in MOR systems.