DENSIFICATION OF QUARTZ POWDER AND REDUCTION OF CONDUCTIVITY AT 700-DEGREES-C

We have measured axial strain, volumetric strain, and electrical condu ctivity during the densification at 700 degrees C of ultra-fine quartz powder (5-10 mu m diameter) saturated with distilled water. Individua l experiments were run at confining pressures ranging from 200 to 370 MPa and pore pressures of 30, 100, and 200 MPa. During the experiments , which lasted from 10 hours to 8 days, the porosity decreased from an initial value of about 40% to final porosities ranging from 19% to as little as 8 +/- 1%. In all experiments, initial volumetric compaction rates were rapid (10(-5) to 10(-6) s(-1)) but decreased to between 10 (-7) and 10(-8) s(-1) after approximately 1 day. Electrical conductivi ty also decreased monotonically from 10(-2) to 10(-4) S/m during the e xperiments. We present a model in which changes in conductivity are co ntrolled by constrictions in interconnecting channels, while porosity is controlled primarily by deposition of quartz in the pores. Both exp erimental and model results suggest a densification process in which c onductivity reduces to matrix conductivity while leaving a residual po rosity of 3-5%. In the Earth, the porosity at a given instant will be the net resultant of porosity reducing processes and porosity producin g processes including fracturing. The rapid loss of conductivity and, by inference, permeability suggests that in the absence of processes w hich increase permeability, both of these properties should have very small values in the lower crust. Furthermore, these experiments lend s upport to arguments that time-dependent compaction of fault gouge can play an important role in modifying fluid pressure and fault strength during the interseismic portion of the earthquake cycle for large eart hquakes.