Terra Rossa Genesis by Replacement of Limestone by Kaolinite. III. Dynamic Quantitative Model

Abstract

We model quantitatively the chemical dynamics of terra rossa formation that we deduced earlier from new field and petrographic evidence. The key phenomenon is that authigenic kaolinite (a proxy for kaolinite solid solutions and other clays) replaces limestone at a downward-moving reaction front at the base of the existing terra rossa. The chemical elements needed to make the authigenic clay are assumed to come from dissolution of eolian clay dust deposited at the surface. The model consists of equations of mass conservation (in one spatial dimension) that incorporate infiltration, diffusion, and replacement and dissolution reactions. Distinctive elements of the model are that (1) the kinetics of the kaolinite-for-calcite replacement is dominated by the crystallization stress generated by the kaolinite growth, in accordance with the view that replacement happens not by dissolution-precipitation but by guest-mineral-growth-driven pressure solution of the host; (2) the field observation that the terra rossa–forming front is narrow indicates that the growth of kaolinite at the front proceeds roughly at the pace at which the appropriate aqueous reactants reach the front, which in turn warrants scaling time, length, and concentrations such that transport and reaction terms are of the same order of magnitude; and (3) the huge contrast between aqueous concentrations and mineral densities, in effect, makes the scaled equation of mass conservation steady state. Numerical simulations correctly reproduce the formation of two adjacent zones in the moving reaction front—replacement and leaching—and of considerable secondary porosity in the leaching zone. This leaching porosity maximum constitutes a solitary wave, or soliton, that moves with the front. The simulations produce a range of predicted front widths that bracket the observed front width. Similarly, model-predicted front velocities of a few meters per million years agree well, within an order of magnitude, with both (1) a paleomagnetically derived rate of terra rossa formation at Bloomington and (2) the rate of authigenic clay formation that could be expected to form—by clay dissolution at the surface, downward infiltration of aqueous solutes, and precipitation of new clay at the front—from average clay dust deposition rates as measured today.