CHEMICAL-WEATHERING RATE LAWS AND GLOBAL GEOCHEMICAL CYCLES

In this paper, we discuss the recent kinetic work on water-rock intera ctions. Standard activity-activity diagrams are reinterpreted, using a mass transfer kinetic model and recent data on the relative rates of mineral reactions. The development of a fully integrated rate law is d iscussed, with special attention to the important effects of deviation from equilibrium on the rates of mineral-water reactions. The combine d effects of temperature, pH, ionic strength, and saturation condition s on the overall dissolution and precipitation rates of minerals must be properly described before any seriously quantitative model of coupl ed fluid flow and chemical reaction can be undertaken. A rate law that integrates these effects is proposed. The functional dependence of th e rate on DELTAG(r), the free energy change for the mineral reaction, is discussed, based on recent experimental work. An important result i s the presence of a surface transition in the reaction mechanism leadi ng to a very strong nonlinear dependence of the dissolution rates on D ELTAG(r). The possible role of dislocation defects in this surface tra nsition is discussed. The relation of global weathering rates and geoc hemical cycles to the recent experimental and theoretical water-rock k inetic work is explored. The temperature effect on the silica content of streams is reevaluated. The variation of silica concentration with runoff in the rivers of the world is quantified, using a coupled fluid flow and reaction model and the full rate law developed for a proto-g ranite system by the kinetic experiments. Implications of the water-ro ck kinetic data for the current geochemical cycles models are discusse d with especial emphasis on the link between physical weathering and c hemical weathering.