EXPERIMENTAL SIMULATION OF ALKALINE SEAFLOOR HYDROTHERMAL SYSTEMS

Flow of seawater through hydrothermal systems exhibiting "black smoker" chimneys has previously been shown to alter peridotite to serpentinite (Janecky and Seyfried 1986). The Lost City hydrothermal field (Kelley et al. 2001) shows that certain seafloor hydrothermal systems can also vent alkaline fluids from "white smokers." Experiments were conducted in a flexible cell hydrothermal apparatus on seawater-lherzolite-CO₂ systems to simulate alkaline hydrothermal systems and determine the extent of brine-rock reaction. The synthetic lherzolite was comprised of 71.4% forsteritic olivine, 18.4% diopside, and 10.2% enstatite. The lherzolite was reacted at 300C and 500 bar in a synthetic seawater solution with an ionic strength of 0.69 to approach steady state, then injected with supercritical CO₂ and reacted for ~550 hours.

Brine-rock reaction decreases pH from 7.4 to ~5, consumes ~50 mMol of aqueous magnesium and nearly all of the aqueous sodium and potassium. Approximately 2 to 4 mol percent CO₂ was injected into these experiments after achieving steady-state. Calcium concentrations decrease (~1 to 2 mMol) following CO₂ injection, whereas magnesium concentrations rebound (~1 mMol), as do the silica concentrations (3 to 7 mMol), both likely a result of increased brine acidity. Significant dissolution of olivine and pyroxenes occurred, as shown by surface pits and etching. The powdered solid reactants have been extensively serpentinized, and mineral fragments developed serpentine overgrowths. Needle-like laths of calcium sulfate and rhombs of magnesium carbonate were extensively precipitated on the reactants and the inner surfaces of the reaction cell. The experiments experienced a gradual pressure decrease following CO₂ injection (27 bars); this pressure decrease is a result of dissolution and mineralization of CO₂.

These reactions provide initial constraints as to the extent and rate of reactions occurring in alkaline hydrothermal systems. Additionally, the extensive formation of magnesium carbonate minerals indicates that direct injection of CO₂ into magnesium silicate rich terranes, as such peridotite hosted hydrothermal systems, may be a viable means of sequestering anthropogenic CO₂.