HR: 0800h
AN: MR11A-0932    [Abstracts]
TI: The rate-controlling processes during pressure solution: insights from a simple contact model
AU: He, W
EM: wenwu_he@hotmail.com
AF: Texas A&M University, Dept. of Geology and Geophysics, TAMU, College Station, TX 77843 United States
AU: * Sparks, D
EM: sparks@seaver.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, TAMU, College Station, TX 77843 United States
AU: Hajash, A
EM: hajash@geo.tamu.edu
AF: Texas A&M University, Dept. of Geology and Geophysics, TAMU, College Station, TX 77843 United States
AB: Intergranular pressure solution in porous grain aggregates may be an important mechanism in deformation of the upper crust and in diagenesis of sedimentary rocks. Three processes may affect the rate of compaction: enhanced dissolution along the highly-stressed grain contacts, diffusion of solute from the contacts into the pore fluid, and low-stress precipitation of the solute within pores. These related processes operate at near the same rate, which is determined by the slowest or rate-controlling process. All three processes are affected by characteristics of the grains and pore fluid (e.g., grain size, reaction rate constants, diffusivity), and by the contact stresses, which change with increasing deformation. We develop a simple mathematical model of a contact between two grains, surrounded by pore fluid. The model is scaled to highlight the important collections of parameters that determine which process is rate-controlling. For simplicity, our model focuses on the early stages of dissolution on a single contact between two grains in a hydrostatic packing of identical grains. Except for fluid expelled during porosity loss, we assume that there is no fluid flow. We model the evolution of the solute concentration in the fluid along the contact and within the pore fluid. The concentration varies between the equilibrium concentration in the pore fluid, and a higher equilibrium concentration on the grain contact (a function of stress on the contact). The degree to which a system is rate-controlled by a particular process depends on the concentrations, relative to these two equilibrium values. The solutions are determined by two dimensionless parameters that are ratios of characteristic times scales for diffusion, contact dissolution and pore precipitation. We calculate how a system evolves through this parameter space with increasing strain. For a cubic packing of spherical grains, we show that dissolution on the contact is always the rate-controlling process at very small strains, with diffusion becoming the rate-controlling process at a strain determined by the system parameters. For example, in coarse (1 mm) quartz sand, assuming a diffusivity of 10$^{-7}$ cm$^{2}$/s, diffusion becomes rate-controlling at volume strains $<$ 1%, while for fine sand (0.1 mm), dissolution remains rate-controlling up to 30% strain. For the quartz aggregates examined, precipitation in the pores is never the rate-controlling mechanism at strains $<$ 30%.
DE: 8045 Role of fluids
DE: 3675 Sedimentary petrology
DE: 3210 Modeling
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting