Lithium isotope systematics and water/rock interactions in a shallow-water hydrothermal system at Milos Island, Greece

The active venting fluids of Milos Island, located within the southern Aegean Sea, belong to a shallow-water hydrothermal system (< 200 m depth) that shows chemical compositions and evolution processes comparable to those of mid-ocean ridges (MOR). In this study, we analyze Li and delta Li-7 in 69 vent water samples, grouped into two types based on their salt content. The low-Cl end-member (EM) Cave fluids show a relatively high Li content (0.39-0.54 mM) with MORB-like delta Li-7 (similar to 4.5 parts per thousand, MORB = 3.7 parts per thousand) compared to that of seawater, and the high-Cl brine fluids contain remarkably high Li (6.14-10.6 mM) and variable delta Li-7 (1.4-8.7 parts per thousand). The latter fluids may have derived from metamorphic basement modified by seawater interactions at similar to 300 degrees C. A scenario using a steady-state dissolution/precipitation model can generate consistent Li and delta Li-7 patterns, where linear correlations of Cl and Li suggest phase separation occurred after water/rock interaction at depth. On the contrary, no significant delta Li-7 variation in most Milos fluids suggests limited isotopic fractionation occurred during phase separation. More importantly, the detected Li enrichment in the high-Cl fluids implies a large Li flux, similar to 3.4 x 10(7) mol/yr, to the ocean from the Milos system. Assuming that 10% of the world's shallow-water systems discovered to date have similar Li outputs to those of Milos, this Li flux would represent similar to 1.8% of MOR hydrothermal fluxes which is on the order of similar to 13 x 10(9) mol/yr. These results emphasize the importance of Li flux derived from shallow-water hydrothermal systems, which should not be excluded from the calculation of the marine Li budget and its impact on the global silicate weathering cycles.