FY 1993

Silica and germanium in Pacific Ocean hydrothermal vents and plumes

Mortlock, R.A., P.N. Froelich, R.A. Feely, G.J. Massoth, D.A. Butterfield, and J.E. Lupton

Earth Planet. Sci. Lett., 119(3), doi: 10.1016/0012-821X(93)90144-X, 365–378 (1993)

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 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 × 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.