REACTIVE TRANSPORT INVOLVING ROCK-BUFFERED FLUIDS OF VARYING SALINITY

Abstract

There is a large body of evidence which indicates that the concentration of many aqueous components in shallow crustal waters is controlled at least in part by mineral-buffering, even at relatively low temperatures. Variations in temperature, pressure, and the total concentration of anionic charge, as reflected by variations in chlorinity and salinity, influence the relative proportions of dissolved cations in saline subsurface fluids. As salinity increases, the concentrations of rock-buffered divalent cations increase at a much greater rate than the concentration of monovalent cations, giving rise to: (1) the observed progression from Na-Cl to Na-Ca-Cl waters with increasing salinity in shallow crustal waters, and (2) nonlinear relations between the concentrations of individual cations and the concentration of chloride. This nonlinear behavior has the potential to induce water-rock reaction and mass transfer.The results of theoretical mass balance calculations performed to simulate the effects of advective-dispersive mixing of fluids of differing chlorinity on mass transfer in rock-buffered systems indicate that reactive transport should occur in any porous medium where there is a chlorinity gradient, even where molecular diffusion is the sole solute transport mechanism. In the hypothetical systems investigated, the calculated rates of fluid-mineral reaction are low during advection and dispersion when cation concentrations are a continuously fixed function of chloride concentration as a result of rock-buffering. A significantly greater degree of mass transfer could occur where mixing of waters of diverse salinity is conservative and cation concentrations are initially a function of physical mixing without reaction. In theory, large volumes of individual mineral phases could be dissolved or precipitated as a result of such mixing, without the need for the flow-through of large volumes of fluid, as fluid-mineral reactions attempt to rebuffer fluid compositions. Rates of mass transfer here would be dependent both on reaction kinetics and on rates of solute transport between reacting phases.