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Most of the movement of ions during carbonate diagenesis occurs in aqueous pore systems in which the average diameter is too small to permit mass transfer by fluid flow. In such situations, ionic motion proceeds instead by aqueous diffusion. In pore systems of sufficient diameter, ion transport occurs by fluid flow. Water occupies both habitats in typical carbonate sediments. Sediment-water interactions occur predominantly in the diffusion-controlled system, and flushing of the sediments is achieved in the flow-controlled (aquifer) system.
Three parameters control the diffusion of cations from diagenetic sites to the aquifer (or vice versa): (1) the concentration difference between diagenetic site and aquifer; (2) the diffusion coefficient for the particular cation under ambient conditions; and (3) the length and geometry of the pore path to be traversed. Under the conditions typical of carbonate diagenesis, concentration difference is the dominant variable and pore path plays a secondary role. Concentration differences between diagenetic site and aquifer may be positive or negative; the sign determines the direction of diffusion and the absolute magnitude determines the efficiency of diffusion. At a given moment, cations of different species may travel in different directions and at different rates; hence the concept o differential diffusion. Unlike fluid flow, cation transfer by diffusion is not limited to a single direction and a single velocity. A diagenetic site may thus be relatively open to one cation yet, at the same moment, relatively closed to another. The trace element composition of a limestone is not a simple function of either the aquifer solution or the diagenetic solution, rather it reflects the complex diffusion interaction between the two.
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