Diffusive properties of fluid-filled grain boundaries measured electrically during active pressure solution

Diffusion through ‘wetted’ grain boundaries is often the rate limiting process during rock deformation by intergranular pressure solution. However, the underlying processes operative within such boundaries are poorly understood. In this contribution we have studied the diffusive properties of wetted grain boundaries by measuring the electrical resistivity of single, annular halite–glass contacts undergoing active pressure solution. Optical observation shows continuous growth (i.e. widening) of the annular contacts by pressure solution. From the resistivity measurements and making use of the Nernst–Einstein equation, it was possible to calculate the apparent grain boundary diffusion coefficient Z=DδC (i.e. the product of grain boundary diffusion coefficient D, grain boundary film thickness δ and the solubility C of the diffusing species in the grain boundary fluid) during the pressure solution process. The Z-values obtained lie in the range 3×10−20–2×10−18 m3/s, show an inverse dependence on normal stress (σn) and agree well with values inferred previously from single contact and polycrystalline compaction experiments.