Significant effect of grain size distribution on compaction rates in granular aggregates

Abstract

We investigate the role of pressure solution in deformation of upper- to mid-crustal rocks using aggregates of halite as a room temperature analog for fluid-assisted deformation processes in the Earth's crust. Experiments evaluate the effects of initial grain size distribution on macroscopic pressure solution rate of the aggregate and compare the results to theoretical models for pressure solution. We find that the grain size exponent deviates significantly from the theoretical value of 3 for diffusion-controlled pressure solution. Models typically assume mono-dispersed spherical particles in pseudo-regular packing. We infer that the discrepancy between experimentally determined grain size exponents and the theoretical values are a result of deviation of experimental (and natural) samples from regular packs of mono-dispersed spherical particles. Moreover, we find that compaction rates can vary by up to one order of magnitude as a function of the width of the grain size distribution for a given mean grain size. Wider size distributions allow for higher initial compaction rates, increasing the macroscopic compaction rate with respect to more narrow grain size distributions. Grain sizes in rocks, fault gouges, and hydrocarbon reservoirs are typically log-normal or power law distributed and therefore pressure solution rates may significantly exceed theoretical predictions. Spatiotemporal variations in pressure solution rates due to variations in grain size may cause the formation of low porosity zones, which could potentially focus deformation in these zones and produce pockets of high pore pressures, promoting nucleation of frictional instability and earthquake rupture.