The Role of Diffusion During Carbonate Diagenesis

ABSTRACT The dissolution-reprecipitation process which characterizes the early mineralogical stabilization of carbonate sediments takes place in a somewhat open chemical system. Studies of the trace element chemistries and isotopic compositions of carbonate grains before and after stabilization have documented the exchange of ions between the diagenetic site and the aquifer solution. If the pores at (reaction zone) and in the immediate neighborhood of the diagenetic site are of insufficient diameter to allow effective solution flow, the exchange of ions between the diagenetic site and the aquifer solution (the large volume of water which flows through the macropores of the carbonate sediment) takes place by aqueous diffusion. This paper examines diffusion applied specifically to the conditions of carbonate diagenesis. The diffusion equation, modified to these circumstances, serves as a guide to understanding the diffusion process. At a given temperature, three variables determine the efficacy of diffusion: the diffusion coefficient, the pore path and geometry, and the concentration gradient. The diffusion coefficient may be considered a measure of the inherent velocity at which a given ion travels under standard conditions. The double positive cations common in carbonates (Ca2+, Sr2+, Ba2+, Zn2+, Mg2+, etc.) and the carbonate anion all have similar diffusion coefficients. Thus this term of the equation exerts little influence during normal carbo ate diagenesis. The pore path and geometry refer to the distance from aquifer to reaction zone and the diameter, shape, and directness of the pore passage. This term is difficult to quantify in diagenetic systems but is constant for different ions at a single diagenetic site. The longer and the more complex the path, the less effective diffusion is. There is a general facies-dependence to this term: grainstones have short paths from reaction zone to aquifer due to their permeability whereas mudstones and wackestones are characterized by long and torturous paths through the mud grains to the aquifer solution. Exchange of ions in the fast situation is more effective than in the second. The concentration gradient, the difference between the concentration of an ion in th reaction zone and in the proximal aquifer solution, is the most variable parameter in diffusion. The sign of the gradient, which may be positive or negative, indicates the direction of diffusion (transport of the ions toward the region of lower concentration). One cation may diffuse from the reaction zone while, simultaneously, a second cation diffuses toward the reaction zone. This "two-way traffic" has been documented in diagenetic calcite corals; Sr2+ diffuses away from and Mg2+ diffuses toward the diagenetic site. Additionally, one cation may diffuse effectively whereas a second diffuses ineffectively, the latter species being characterized by a weaker concentration gradient. Thus, a given diagenetic site may be chemically open for one cation and simultaneously somewhat closed for another. Application of diffusion principles to the aragonite/calcite transformation during early diagenesis indicates the compositions of solutions and diagenetic calcites undergo temporal changes. Early on, with few active diagenetic sites, Sr2+ diffuses effectively from the reaction zone to the aquifer since there is a steep concentration gradient. Diagenetic calcites are relatively low in Sr2+ because of its effective removal from the diagenetic site. As the number of active diagenetic sites increases, so much Sr2+ may be delivered to the aquifer that Sr2+ builds up in the aquifer solution and the concentration gradient shallows. Diffusion from individual sites becomes less effective, Sr2+ concentrates in the reaction zone, and diagene ic calcites enriched in Sr2+ are produced. As more and more aragonite is consumed a final phase is reached in which less total Sr2+ is delivered to the aquifer and the system returns to the conditions characteristic of the early phase.