MINERAL REPLACEMENT: KEY TO GEOCHEMICAL DYNAMICS AND TO ITS MODELING

P.K. Weyl (JGR 1959), said: “In studying ‘solution alteration’ in rocks, … we are not interested in what is now, but in how it became that way. We must therefore not base our classification [of ‘dissolution-precipitation’ mechanisms] on the present appearance [of a rock].” By assigning it to dissolution-precipitation, Weyl missed the unique phenomenon of replacement, whereby mineral A occupies the space formerly occupied by mineral B, but preserving B’s volume and morphological details. The modeling of lateritization, diagenesis, burial dolomitization, ore deposits, and metamorphism – in all of which replacement is common – is in trouble today (1) because modeleres, following Weyl, in effect adjust mineral reactions arbitrarily, assuming local conservation of a component or assuming that minerals react “mole for mole”; and (2) because the dynamic reaction-transport equations do not incorporate the kinetic feedback that keeps mineral volume constant.

The brilliant idea that replacement is produced by crystallization-stress-driven pressure-solution was proposed by Maliva & Siever (Geology, 1988). The much older mechanism of dissolution-precipitation was discarded already by Bastin, Lindgren et al. (Econ Geol, 1931); then it was unwittingly revived by Weyl (1959), and continues to be defended by many geochemists today.

I will discuss three examples: the replacement of periclase by brucite in marbles; the replacement of feldspar by gibbsite in lateritization; and the self-induced rheological change of replacive to displacive dolomite growth in burial dolomitization. In the first two, just adjusting the respective replacements on volume leads to a deeper understanding of how the process works. In the third, because the displacive dolomite growth is brought about by the prior self-accelerating replacive growth combined with the strain-rate-softening nature of crystalline carbonates, the crystallization stress driving the displacement must have existed already during the replacive growth to be able to lower the dolostone’s viscosity. This constitutes outstanding new evidence in support of Maliva & Siever’s (1988) fundamental new insight into the mechanism of replacement.