The Effect of Temperature and Physical State of Water on the Frictional Properties of Chlorite‐Altered Basaltic Gouges (Krafla Geothermal Field, Iceland)

Abstract Changes in the physical state of water may alter fault frictional properties in high‐temperature geothermal systems, yet they have been little studied. Here, we perform slide‐hold‐slide experiments to examine the frictional properties (frictional strength μ ss and frictional healing Δ μ ) of chlorite‐altered basalt from Krafla Geothermal Field under realistic hydrothermal conditions (temperature of 100–400°C and pore pressure of 3–30 MPa with water in liquid, vapor, and supercritical states). We show that (a) μ ss is slightly larger in vapor ( μ ss ∼ 0.67–0.73) than in liquid and supercritical water ( μ ss ∼ 0.61–0.64); (b) Δ μ increases with both the logarithm of contact hold time and temperature; (c) stick‐slip occurs at ≥300°C, and the stress drops are largest in vapor water. In the sheared gouges, slip zones are characterized by grain‐size reduction of plagioclase and quartz due to cataclasis (≤300°C) and dissolution (400°C) without evidence of newly‐formed minerals. The temperature‐dependent Δ μ of gouges in the presence of liquid and supercritical water is promoted by the rearrangement of size‐reduced grains and grain dissolution, while the large Δ μ in vapor water may result from the formation of interfacial chemical bonds between grains. Our results agree with the seismological observations in Krafla, where shallow faults cutting chlorite‐altered basalts at temperatures <200°C accommodate deformation mostly by aseismic slip (creep), while deeper faults are seismogenic at ≥300°C and in the presence of boiling fluids.