Implications of replacement for reaction-transport modeling

Mineral replacement in rocks consists of growth of guest mineral and dissolution of host. The two reactions are coupled by the grain-grain stress generated by growth of the guest grain in a rigid or very viscous rock in which it has initially no available room. This stress self-adjusts to make the volumetric rates of guest growth and host dissolution equal to each other, which accounts for the volume conservation typical of replacement. Volume preservation during replacement is required by conservation of mass and momentum as well. The two reactions are also simultaneous and proceed by a sequence of many tiny alternating increments; where these increments are small enough, internal textural details of the host grain or grain aggregate are morphologically preserved (as ghost textures) by the replacement. The strong variation of mineral reaction rates with stress required by replacement is evidenced also experimentally. Reported widespread replacement in many rocks (laterites, diagenesis of siliciclastics and carbonates, metasomatic ores, metamorphic rocks, and hydrothermal alteration) warrants creating water-rock reactive transport models that predict when and where in a system replacement should take place. Current geochemicaI models implicitly view replacement as sequential, uncoupled dissolution of host and growth of guest; this sequence of reactions may also occur in rocks, but crucially differs from replacement. Implications of replacement for water-rock reaction modeling include: 1. balancing of replacement reactions on volume; 2. use of Helmholtz (rather than Gibbs) free energies of minerals involved in replacement; 3. taking account of the strong stress-dependence of mineral growth and dissolution rates in rocks; and 4. taking account of possible accommodation of growth by local deformation (as well as pressure solution) of the host rock, and of the effect of strain rate ( = growth rate) on local rock viscosity. (C) 1998 Elsevier Science B.V. All rights reserved.