Chemical compaction and fluid segregation in piston cylinder experiments

Fluid segregation is a ubiquitous process in deep-seated rocks, mainly driven by buoyancy and gravitational compaction, which occur in drained systems until permeability decreases and the system become practically undrained. The mechanism and rate of fluid segregation following this stage are poorly constrained, despite the importance of these factors for fluid distribution and the physical and chemical properties in the Earth. To this end, we performed sintering experiments of quartzite, with H₂O–CO₂ fluids of 1.9%–18.0% added volume fraction using a piston-cylinder apparatus at 900°C under nominally isotropic pressure of 1 GPa. The subsequent chemical redistribution of silica and fluids resulted in capsule-scale fluid segregation (CFS) and the formation of dense quartzite (∼0.3%) within 192 h in pure H₂O systems. Comparative experiments showed that dissolution/precipitation was not caused by the temperature gradient. Instead, we considered the fluid pressure difference within the experimental capsule between fluid-rich and fluid-poor domains caused by the different response against small pressure oscillation during experiments. Fluid-rich rock is elastically “soft” (i.e., having larger Skempton's coefficient) compared to fluid-poor rock to increase pore fluid pressure under a compressional stress change. Therefore, more silica dissolves in fluid-rich domains with higher solubility, while it precipitates in fluid-poor domains through diffusive transport, expanding porosity contrast. This chemical compaction in the capsule scale is effective as long as elasticity dominates matrix viscosity because the pressure difference relaxes with time. The calculated relaxation time of quartzite at the experimental condition was comparable to the intervals of the oil pressure addition which produced stress change. A one-dimensional model for porosity evolution showed that time-averaged SiO₂ diffusivity during CFS was much larger than the grain boundary diffusivity of tightly sintered quartz aggregates, but was comparable to the diffusivity of SiO₂ in aqueous fluid with ∼0.1 vol% fluid fraction. During metamorphism where the fluid pressure difference may be maintained for longer duration, the spontaneous silica cementation may play a role for formation of syn-metamorphic veins and control of the amount of pore fluids transported to deep Earth.