PH buffering by metastable mineral-fluid equilibria and evolution of carbon dioxide fugacity during burial diagenesis

Abstract

Numerous potential pH buffers including reactions among aqueous organic acid and carbonate species carbonate and silicate minerals are typically present during burial diagenesis. Buffering of pH in natural systems is a function of mass action, mass balance kinetic constraints. In most sedimentary basins, carbonate and silicate minerals are present in amounts sufficient to buffer pH the activities of aqueous species are consistent with metastable equilibrium among observed diagenetic minerals. These observations indicate that mass balance and kinetic constraints are relatively less important than mass action constraints measured by the buffer index, β, here defined as follows: β = −dξ dpH The buffer index ultimately dictates which buffer reaction controls pH under diagenetic conditions; buffer reactions with high β values are favoured over those with low values. Buffer indices for a number of potential diagenetic buffer reactions have been calculated by reaction path modeling. Heterogeneous equilibria among carbonate and silicate minerals and an aqueous phase have greater β values than those for homogeneous reactions among aqueous carbonate and organic acid species. This implies that pH, calcite dissolution ƒ CO 2 are strongly dependent on carbonate-silicate-fluid interactions during diagenesis. The role of carbonate-silicate reactions in controlling pH is tested by examining the evolution of CO 2 fugacities with temperature during burial diagenesis. Carbon dioxide fugacities have been calculated by reaction path modeling of diagenetic carbonate-silicate equilibria for sedimentary and geothermal systems. Calculated CO 2 fugacities are in general agreement with observed CO 2 fugacities. The combination of high buffer index, apparent metastable equilibrium between diagenetic minerals and waters the relatively accurate prediction of z. hfl; CO 2 trends with temperature suggest that carbonate-silicate reactions are important in determining the evolution of fluid compositions in sedimentary basins and influence the course of dissolution events in burial diagenesis.