EFFECTS OF FLUID COMPOSITION ON PRESSURE SOLUTION AND IMPLICATIONS FOR CRUSTAL RHEOLOGY

It is well established that intergranular pressure solution is an important mechanism of compaction and deformation under diagenetic and low grade metamorphic conditions in the upper and middle crust. It is also believed to play an important role in controlling fault healing, overpressuring and slip in the hydrothermal reaches of crustal fault zones, and in controlling the transition from brittle/frictional to ductile behavior on such faults. However, the extreme slowness of pressure solution has made it difficult to study in the laboratory, with the result that the underlying processes and kinetics have remained poorly understood and inadequately quantified for reliable rheological modeling. Nonetheless, considerable advances have been made in the last few years. This contribution reviews recent work at Utrecht. Criteria will be discussed for both equilibrium and dynamic wetting of grain boundaries, and direct measurements of grain boundary properties will be presented. In addition, experimental data on pressure solution rates in quartz, gypsum and calcite, obtained from high precision compaction experiments, will be compared with microphysical models assuming rate control by dissolution, diffusion and precipitation. The results show that, under chemically closed system conditions, intergranular pressure solution in these materials seems to be controlled by interfacial reaction kinetics rather than by grain boundary diffusion. Under flow-through conditions, the rate and rate-controlling mechanism of pressure solution depend on the concentration of dissolved solid in the pore fluid. Additional experimental data show major rate effects of cation impurities in the pore fluid, which parallel effects seen in geochemical dissolution/growth experiments. The various effects can be incorporated into rate laws for bulk deformation by pressure solution creep or into laws for pressure solution controlled slip on gouge-bearing faults. The results imply that pore fluid composition will play a key role in determining the rheology of crustal rocks and fault zones, and can significantly influence the transition from brittle to ductile behavior. Indeed, pore fluid impurity composition may explain the discrepancies often observed between pressure solution rates estimated from geological constraints versus laboratory data.