Experimental Evidence for Reaction-Induced Weakening of (Carbonated) Serpentinite Fault Gouges

Abstract

The seismic potential of faults depends on the local mineralogy and can change upon mineral reactions. We conducted friction experiments on serpentinite, carbonate and carbonated serpentinite fault gouges at temperatures from 400°C to 630°C, under 100 MPa effective normal stress and fluid saturated conditions. Pure serpentinite fault gouges exhibited unstable slip with significant strain-hardening. Carbonate-bearing serpentinite fault gouges showed stable sliding at temperatures <500°C, but displayed unstable stick-slip behavior and strong strain weakening at temperatures ≥500°C. Microstructural analyses revealed localization and the formation of olivine and pyroxene from devolatilization reactions at temperatures ≥500°C. The degree of devolatilization increased near major slip planes and was enhanced by higher temperature and carbonate content, as shown by three-dimensional micro-computer tomography analyses. Nano-scale transmission electron microscopy analyses revealed the absence of hydrous and carbonate phases along major slip planes. We attribute the strong weakening and unstable slip behavior in carbonated serpentinite fault gouges to the formation of nano-sized anhydrous phases of olivine and pyroxene along the slip plane. Our results indicate that serpentinized fault zones may experience seismic event nucleation at temperatures approaching the thermodynamic stability limit of serpentine. This suggests that the absence of seismic events cannot exclusively be attributed to serpentinization. The formation of carbonates, through replacive and additive carbonation, can explain aseismic deformation in transform faults, but at elevated temperatures, devolatilization reactions in carbonated serpentinites cause strong localization and strain weakening, accompanied by laboratory seismicity.