Factors controlling reaction pathways during fluid–rock interactions

Abstract

Potential fluid pathways for fluid–rock interactions and the factors controlling these pathways have been investigated experimentally by simulating hydrothermal conditions, using sample cubes of Carrara Marble (calcite) and an anorthosite (plagioclase) rock in different solutions (pure water, sodium chloride, artificial seawater, sodium phosphate and sodium silicate) at 200 °C. Analytical techniques including SEM, Raman Spectroscopy, atomic force microscopy, and Electron Microprobe Analysis were used to characterize fluid-induced reactions. Results show aqueous fluids can penetrate grain boundaries within rocks and, dependent on fluid and solid compositions, coupled replacement reactions can occur. The available fluid volume for the reaction in a grain boundary versus the bulk fluid can influence replacement reaction pathways. When 0.1 M Na2HPO4 was used with Carrara Marble, or a Na-silicate solution was used with anorthosite, the replacement of calcite by hydroxylapatite or labradorite by albite, respectively, occurred along the grain boundaries of both rock types. In the experiments using seawater, the replacement of calcite by Mg-carbonates occurred predominantly from the sides of the cube samples and the grain boundaries were minimally affected within the timescale of the experiments (1–3 months). With 1 M Na2HPO4, hydroxylapatite precipitated both along the marble grain boundaries and the sample sides. Models based on experimental observations and PhreeqC simulations highlight the importance of grain boundaries and interconnected porosity in fluid-induced reactions. Such factors play an important role in the kinetics and relative solubilities of rock systems by changing the conditions at the interfacial fluid–mineral boundary layer that will determine initial dissolution or precipitation and whether the supersaturation of a product phase is reached.