Pervasive pressure solution transfer in a quartz sand

The kinetics of deformation of a quartz aggregate by pervasive pressure sol ution can be, under certain conditions of temperature and grain size, stron gly dependent upon the diffusivity of silica into the grain-to-grain contac ts. An analysis of the factors affecting this key parameter (and less well constrained in the analysis of the problem of rock deformation by pressure solution) is presented. This analysis is based on recent advances on studyi ng silica surfaces and particularly on the existence of a silica gel layer on the silica surfaces undergoing dissolution. By reinvestigating the elect roviscous effect occurring at the grain-to-grain contact the present analys is shows that the diffusivity of silica at the grain-to-grain contacts is l ikely to be relatively similar to that in the bulk pore water (maybe 1 orde r of magnitude smaller but not more). This contradicts the previous work by Rutter [1976], which has been the key reference used in many subsequent pa pers to justify an extremely low value for the diffusivity of silica at the grain-to-grain contacts (5 orders of magnitude smaller than the diffusivit y of silica in free water). This finding has dramatic implications concerni ng the deformation rate of quartz sands and sandstones by pressure solution in sedimentary basins with regard to (1) the limiting step affecting the k inetics of the process (diffusion of the solute or dissolution/precipitatio n chemistry) and (2) the existence of a thermodynamic equilibrium state whe n deformation by pressure solution occurs over geological timescales. A por oviscoplastic model is used to describe deformation associated with pervasi ve pressure solution transfer in quartz sands. This model is shown to be co nsistent with the current state of knowledge of the surface chemistry of si lica. In addition, the comparison between this model and both laboratory an d field data is rather good.