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).
Bibliographical noteFunding Information:
This study was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants program ( RGPIN-2018-03841 ; JKR) and The Geological Society of America (GSA) Graduate Student Research Grants program (AGR). We thank Laura Gardner (Department of Materials Science and Engineering; University of Sheffield) for running the HIP experiments, and Jacob Kabel (Electron Microbeam and X-Ray Diffraction Facility; University of British Columbia) and Gethin Owen (Centre for High-Throughput Phenogenomics; University of British Columbia) for their help during imaging. We thank David Kohlstedt and Lori Kennedy for discussions on creep and sintering in geologic materials. Finally, we would like to thank Heather Wright and one anonymous reviewer for their detailed, constructive comments.