ENHANCED SOLUBILITY OF SILICA AND ACCELERATED FLUID-SILICATE REACTION RATES IN AN EXPERIMENTAL SUPERCRITICAL CARBON DIOXIDE-BRINE-ROCK SYSTEM

Multi-phase equilibrium relationships between supercritical CO₂ and brine-rock systems may provide a source of silica cement in sandstones and quartz growth in veins. To investigate, experiments were conducted in a flexible cell hydrothermal apparatus at physical-chemical conditions of the shallow crust. A synthetic arkose (microcline + oligoclase + quartz + biotite) plus argillaceous shale were reacted with 5.5 molal NaCl brine. The system was held at 200 C and 200 bars for 32 days to approach steady state, then injected with CO₂ and allowed to react for an additional 45 days. In a parallel experiment, the system was allowed to react for 77 days without injection of CO₂. Trace ions initially absent from NaCl brine appeared in solution at mM (K, Ca, and SiO₂) to uM (Mg, Al, Fe and Mn) quantities, reflecting reaction of brine with rock. Without CO₂ injection, the SiO₂ concentration (2.4 mM) was stable below calculated quartz solubility (3.9 mM). Injection of CO₂ resulted in decreased pH (1 unit) and increased SiO₂ concentration to a level near calculated chalcedony solubility (5.4 mM).

In the experimental system containing acidic brine and supercritical CO₂, SiO₂ concentrations were doubled by dissolution of silicate minerals and apparent concomitant inhibition of the precipitation of quartz (and other silicates). Evaluation of the mixing of hot hydrothermal solutions with ambient seawater at mid-ocean ridge vents (Janecky and Seyfried, 1984) also noted silica super-saturation and inhibition of quartz precipitation. These phenomena were attributed to kinetics of silica polymerization and precipitation under acid pH conditions, assumptions consistent with recent experimental results (Icopini et al., 2002). Return of silica super-saturated brine into a rock-dominated reaction system buffered to more neutral pH conditions may enhance precipitation of quartz, chalcedony, or amorphous silica as veins or cements, depending on the permeability structure of the host rock. Similarly, loss of CO₂ or phase separation with decreasing pressure can substantially shift pH and result in massive vein or scale formation.