Seismogenic Consequences of COH‐Fluid Rock Interactions in Ultramafic Fault Zones Insights From Carbonation‐Decarbonation Friction Experiments

Abstract Fluid‐rock interactions have major implications on the frictional behavior of fault rocks. Infiltration of CO 2 ‐bearing fluids in ultramafic fault zones causes the progressive formation of serpentinite, talc‐magnesite (soapstone) and quartz‐magnesite (listvenite) assemblages. Reversed reactions occur when CO 2 ‐poor fluids interact with carbonated fault zones. To constrain the effect of such COH‐fluid‐rock interactions on fault strength and stability, we performed rotary‐shear experiments at conditions approaching those in natural fault zones (300°C, 250 MPa normal stress, 100 MPa pore pressure, velocities from 0.002 to 10 μm/s). Serpentinite gouges show high frictional strength ( μ = 0.42–0.45) and cataclasis. Coupled compaction‐dilation potentially caused unstable frictional sliding. Soapstone gouges are weak under wet conditions ( μ = 0.23–0.32). Velocity‐weakening behavior at v = 0.002–0.1 μm/s transitions to velocity‐strengthening behavior at v = 1–10 μm/s. Listvenite gouges are strong ( μ ≈ 0.6) and velocity‐strengthening over the complete tested velocity range. Chemical disequilibrium between pore fluid and mineral assemblage invoked decarbonation reactions in quartz‐bearing assemblages during sliding, causing weakening and slip instabilities at 0.002 μm/s. We infer that natural serpentinized fault zones have seismogenic potential at elevated temperature and pressure conditions, while talc‐magnesite fault rocks are expected to induce transient aseismic creep. Intense carbonation to listvenite locks the fault and increases the seismogenic potential. Our experiments demonstrate that fluid‐assisted deformation is highly sensitive to fluid chemistry, with important consequences for engineered CO 2 mineralization.