Dynamic weathering model: Constraints required by coupled dissolution and pseudomorphic replacement

Abstract

In lateritic weathering parent mineral grains are partly replaced by oxides and oxyhydroxides and partly congruently dissolved. The replacement of a parent grain by an oxide at a site results from the physical coupling of oxide growth and parent-grain dissolution: the stress between the growing oxide and an adjacent parent grain makes the volumetric rates of the two “half-reactions” equal to each other, which yields the constant volume characteristic of pseudomorphic replacement. The congruent dissolution at a site and the replacement at another site a few grain lengths below also are coupled, by aqueous ions carried from the former to the latter. The reaction-transport model herein takes account of this coupling and of the constant volume characteristic to replacement, plus continuity, diffusion, and flow. Numerical solutions for anorthite weathering predict the (petrographically) observed spatial association of textures—that is, from top to bottom, partial congruent dissolution of plagioclase, partial plagioclase replacement by gibbsite, and fresh rock. Influx of H + in rainwater drives partial dissolution of a grain. Dissolution releases aqueous Al, which infiltrates downward and drives partial replacement of a second grain by gibbsite. About 1 cm of anorthite is partly dissolved and partly replaced in a few hundred years, depending on boundary conditions such as rainfall and rain pH, and initial conditions such as plagioclase grain size. The role of grain/grain stress in deciding when and where in the system replacement takes place is carried out in the model by artificial kinetic switches.