Intergranular Pressure Solution in Nacl: Grain-To-Grain Contact Experiments under the Optical Microscope

Abstract

Intergranular Pressure Solution (IPS) is an important geologic lithification, compaction and deformation mechanism in a wide variety of crustal rocks. Experimental studies of IPS in quartz aggregates have not been very successful due to the low rate of IPS, and IPS experiments performed using wet halite as a rock analogue (Spiers and Schutjens, 1990; Hickman and Evans, 1991) have left uncertainty about the detailed IPS mechanism and grain contact structure/wetting in this material. The present study reports four contact dissolution experiments performed under the optical microscope to study the mechanism and kinetics of IPS at single halite/halite and halite/glass contacts loaded under brine (room temperature). Normal constant contact forces in the range 1. 0 to 2. 6 N were applied in the presence of NaCl-saturated brine, exerting stresses of 0. 8 to 7. 4 MPa. Time-dependent mass removal and convergence were observed at all contacts. In all cases, loading of the contact (or increasing the load on the contact) led to instantaneous formation of a rough contact morphology, composed of a crystallographically-controlled pattern of islands and channels with a length scale of several micrometers. This nonequilibrium microstructure evolved with time to an optically flat contact face while contact broadening and convergence continued. The smoothing/convergence process must therefore have involved diffusion of mass out of the contact, and expulsion of brine, through a connected brine phase within the contact. Whether a fine-scale rough structure persisted in contacts which evolved to optical flatness is not known, though post-test SEM (Scanning Electron Microscopy) observations suggest that it may have. If so, its amplitude was less than 500 nm. Measurements of dissolution rates enabled comparison with a model for IPS. The analysis suggests that solute diffusion through the contact boundary was probably rate-controlling, with the contact structure and effective diffusivity varying with contact force and on-going convergence. The results agree broadly with those of previous compaction creep experiments performed using wet halite powder. Discrepancies with other workers results for single-contact dissolution experiments can be explained in terms of differences in experimental configuration and competition between driving forces.