Silica and germanium in Pacific Ocean hydrothermal vents and plumes

Dissolved silica (Si) and inorganic germanium (Ge) concentrations were measured in hydrothermal fluids from black smoker vents on the East Pacific Rise (21°N EPR) and the Southern Juan de Fuca Ridge (45°N SJdFR: North and South Cleft Sites, Axial Volcano). These typically display end-member concentrations ranging from 16 to 23 mM (Si) and 150 to 280 nM (Ge), and end-member Ge Si ratios clustering between 8 and 14 × 10^-6, more than 10-fold greater than the ratio entering the ocean via rivers (0.54 × 10^-6) and being recycled in seawater (0.7 × 10^-6). 'Excess' concentrations of dissolved Si and Ge above oceanic background are observed in mid-water hydrothermal plumes over mid-ocean ridge (MOR) spreading centers on the Southern EPR (SEPR) (10°-20°S) and the SJdFR. The largest Si and Ge concentration anomalies occur over the North Cleft Segment of the SJdFR. These are a factor of three greater than anomalies over the SEPR (10°-20°S). Excess Ge correlates with excess ³He in plumes at a Ge ³He molar ratio of about 1 × 10⁴, approximately the same ratio as in black smokers. These observations, combined with low abundances of Ge in FeMn-rich metalliferous sediments, suggest that Ge (and Si) behave conservatively in mid-ocean ridge hydrothermal plumes. A simple ocean Si and Ge balance, constrained by the global river silica flux and Ge Si ratios in hydrothermal vents, rivers and biogenic silica, suggests that the global hydrothermal silica flux is about 1-4 × 10¹¹ mole yr^-1, much lower than that estimated from ³He. Either (1) 70-80% of the Ge flux to the ocean is removed in as-yet undiscovered sinks (not opal), or (2) only 10% of the mantle to ocean ³He and heat fluxes is associated with MOR hydrothermal convection through the 350°C isotherm (90% is off-ridge), or (3) the oceanic Ge Si, ³He (and ⁸⁷Sr ⁸⁶Sr) balances today are far from steady-state.