238 Petrology, Metamorphic and Experimental

Wednesday, 5 November 2003
1:30 PM - 5:30 PM

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

Paper number: 238-10
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

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.