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

The mechanisms controlling Br/Cl and I/Cl distribution in hydrothermal fluids on seafloor remain ambiguous. In this study, we examined systematically the spatial and temporal distribution of Cl, Br, and I in shallow-water hydrothermal fluids at Milos in the Aegean Sea, collected during two field excursions in 2002 and 2003. These fluids were analyzed for halogens, as well as other major/trace elements by high resolution inductive coupled plasma mass spectrometry. Large temporal variations (56% depletion compared with ambient seawater) in Br/Cl ratios are detected in the vapor enriched cave fluids, discharged through rock fissures near sea level and showed low pH, Cl, Br and Ι. On the other hand, small temporal Br/Cl variability (31% depletion) is characterized in the submarine-brine fluids that show high halogen concentrations. The I/Cl ratios in both cave and submarine-brine fluids are more than 10 times higher than in seawater, but fall into a range between hydrothermal fluids from mid-ocean-ridge (MOR) and sediment-hosted ridges. These results highlight the importance of organic matter degradation. A conceptual model is developed to explain the observed temporal and spatial variation of Br/Cl and I/Cl in the vent fluids at Milos. This model involves geochemical processes of sediment diagenesis, phase separation, and halite formation/dissolution at various stages during vent fluid circulation. The high I/Cl data suggest a scenario in which fluids were influenced by sediment diagenesis prior to phase separation. The latter process boils off vapor with high Br/Cl continuously at deep seated reservoir to create low Br/Cl brines, which was then mixed with seawater to form high Br/Cl variability in vent fluids. In addition tidal regulation at shallow depths may have an effect if halite-coating or dissolution occurs. There was no discernible Br/Cl change reported in MOR fluids. In strong contrast, our results show substantial Br/Cl variations and shed light on the role of phase separation, halite, and sediment diagenesis in shallow hydrothermal systems.