Mass transport in sandstones around dissolving plagioclase grains

Abstract

The dissolution of plagioclase and precipitation of kaolinite is a common late diagenetic process in sandstones. The common occurrence on a thin-section scale of patchy kaolinite adjacent to a dissolving plagioclase grain is consistent with diffusional transport of dissolved constituents combined with precipitation on a favorable nucleation site. The absence in thin-section view of a diffusionally related halo of kaolinite around a dissolving grain can be explained by a lack of a halo of favorable nucleation sites. Numerical modeling results are presented for the transport of aqueous Al by diffusion coupled with fluid flow in sandstones at 100 °C. Within a few months the Al in the anorthite component of plagioclase grains can be removed by diffusion during diagenesis. Fluid velocities needed to overcome diffusion to prevent precipitation on the upstream side of a dissolving grain depend on the unknown minimum supersaturation needed for precipitation. These velocities are less than those predicted by the Peclet number but are of the order of metres per year at a distance of a few tenths of a millimetre from the dissolving grain. The nonlinear inverse relation of these velocities to the distance from the dissolving grain implies that mass transport to precipitation sites is dominated by fluid flow at distances greater than 1 cm.