An Evaluation of Secondary Mineral Formation/Dissolution and Phase Separation Based on Mg Isotopic Fractionation: The Shallow-Water Hydrothermal System in Milos, Greece

This study investigates Mg isotopes (δ²⁶Mg) in vent fluids from Milos, Aegean Sea, to evaluate phase separation and secondary mineral formation. The δ²⁶Mg vary significantly in Milos, exceeding 0.66‰, allowing for the classification of the fluids into three sub-groups based on chemical characteristics: seawater-like, cave fluids, and submarine-brines. The seawater-like fluids exhibit large δ²⁶Mg variation, −0.64 to −1.18‰, and mostly follow a Rayleigh fractionation trend, with a fractionation factor α = 1.00020 ± 0.00011. The cave fluids are highly acidic, have low Cl, are vapor-rich, and display heavy δ²⁶Mg compositions (−0.52 to −0.63‰). The submarine-brines are characterized by high Cl, high non-volatile metals, and light δ²⁶Mg (−0.65 to −1.00‰). The latter two fluid types represent vapors and brines, respectively, which underwent phase separation at depth in Milos. These δ²⁶Mg values were combined with major/trace elements, as well as Li and B isotopes, to explore possible controlling mechanisms. We report for the first time a shallow submarine hydrothermal system that has a vapor component enriched in heavy δ²⁶Mg, but with no detectable isotopic changes in the brines. It is evident that δ²⁶Mg in vent fluids is unique for separating effects of water/rock interaction and secondary mineral and phase separation at shallow-water systems.