Timescales of porosity and permeability loss by solid-state sintering

The physical properties of granular geologic materials are transient – a variety of densification processes operate throughout Earth's crust. One such process is solid-state sintering, which causes crystalline clasts to coalesce in the absence of fluids or melt. Solid-state sintering operates wherever unconsolidated granular materials are subjected to elevated pressures and temperatures for protracted periods of time. There are, however, few studies that constrain the conditions and timescales for densification and lithification of crystalline geologic materials by solid-state sintering. Here, we present the results of hot-pressing experiments designed to cause a natural glass-free volcanic fault gouge to undergo solid-state sintering. Unconsolidated starting materials subjected to volcanic temperatures (700-900 °C) and pressures (20-70 MPa) are transformed by solid-state sintering into more-coherent porous composites over a period of 4-60 hours. The relative density and competence of experimental products increase and porosity and permeability decrease as sintering pressure, temperature and time increase. We use the experimental results to develop a robust densification model that predicts time-dependent porosity and permeability loss at a pressure-temperature range that includes volcanic and some upper-crustal environments. In these environments, solid-state sintering causes reductions in porosity and permeability of ∼0.35 and ∼10⁴ m², respectively, over days to months depending on pressure-temperature conditions. Applied to volcanic environments, the short timescales of solid-state sintering-driven permeability loss and lithification can dictate the efficiency of outgassing and therefore modulate eruption style (i.e., explosive vs. effusive).