Calculation of K diffusional rates in bentonite beds

Zoned K-bentonites with illite-rich contacts and smectite-rich interiors are interpreted as products of potassium (K) metasomatism during diagenesis. A mathematical model is developed to describe both K transport in pore water and K fixation in smectite clays ( smectite illitization ). Solution of these coupled equations is done using finite difference techniques. Best-fit modeling of mineralogical and K/Ar data from a 2.5 m thick zoned K-bentonite from western Montana suggests that diffusional exchange of K for Ca is the important transport mechanism in compacted bentonite beds. Diffusion coefficients (D) for K/Ca exchange of about 9 to 13 · 10^-8 cm²/sec best model observed zonations in the 2.5 m bentonite as well as mineralogical zonations observed in bentonites thinner than 2.5 meters. This range in D seems to be reasonable when compared with calculated D values for K/Ca exchange in a highly tortuous medium. Although the precise burial history for these Montana rocks is not well known, geologically reasonable burial histories appear to be best modeled by using fifth- and sixth-order kinetic parameters for illitization. The model suggests that illitization occurred for 7 to 9 my at temperatures of about 80 to 90°C. The model of chemical transport and illitization developed here should have applications in the analysis of the stability of bentonite as a backfill material in a nuclear waste repository.