The potential role of ridge-flank hydrothermal systems on oceanic germanium and silicon balances

Sediment pore-water samples and warm spring fluids from three ridge flank hydrothermal systems (40–75°C) were analyzed for Ge, Si, and Mg to quantify the contribution of these systems to oceanic Ge and Si balances. These warm hydrothermal fluids have elevated concentrations of Ge and Si relative to bottom seawater, indicating that these systems are net sources of Ge and Si to the oceans. End-member Ge:Si molar ratios range from 12 to 34 μmol/mol, which is similar to, but elevated relative to, the average ratio from high temperature (350°C) midocean ridge hydrothermal systems. The source for this Ge and Si is suggested to be a combination of seawater reaction with basalt and diffusive exchange with overlying basal pore waters, which are in pseudo equilibrium with amorphous silica. Because the Ge:Si molar ratio in both basalt and sediment is much less than those measured in the dissolved phase, a net preferential removal of Si relative to Ge must occur within basaltic basement. As suggested for high temperature hydrothermal systems, we suggest that this removal mechanism is likely a consequence of quartz precipitation and the formation of secondary clays (i.e., smectite). We estimate that ridge flank hydrothermal systems likely provide ∼10% of the Ge and Si fluxes to the oceans.