Silica and Germanium in Pacific-Ocean Hydrothermal Vents and Plumes

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Earth and Planetary Science Letters
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Dissolved silica (Si) and inorganic germanium (Ge) concentrations were measured in hydrothermal fluids from black smoker vents on the East Pacific Rise (21-degrees-N EPR) and the Southern Juan de Fuca Ridge (45-degrees-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 x 10(-6), more than 10-fold greater than the ratio entering the ocean via rivers (0.54 x 10(-6)) and being recycled in seawater (0.7 x 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-degrees-20-degrees-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-degrees-20-degrees-S). Excess Ge correlates with excess He-3 in plumes at a Ge/He-3 molar ratio of about 1 X 10(4), approximately the same ratio as in black smokers. These observations, combined with low abundances of Ge in Fe-Mn-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 x 10(11) mole yr-1, much lower than that estimated from He-3. 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 He-3 and heat fluxes is associated with MOR hydrothermal convection through the 350-degrees-C isotherm (90% is off-ridge), or (3) the oceanic Ge/Si, He-3 (and Sr-87/Sr-86) balances today are far from steady-state.


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