5:15 PM - 7:15 PM
[SCG54-P06] Fluid-rock reaction of olivine and granite gouges by frictional heating associated with fault slip
Keywords:Frictional heating, Hydrothermal condition, High-velocity friction, Earthquakes
Earthquakes within the oceanic mantle occur at outer-rise faults and transform faults along which seismological observations suggest that water infilters into the mantle. Therefore, serpentinization of the mantle peridotite can be promoted along these faults due to frictional heating during earthquakes. Fluids causing intraslab earthquakes and arc volcanisms can be supplied by dehydration reactions of hydrous minerals formed by such fluid-rock reactions. In addition, major slip zones of inland active faults commonly contain clay minerals which may have been produced by hydration of granite during fault slip. Frictional heating associated with coseismic faulting may accelerate the fluid-rock reaction in a fault zone, but how the reaction is accelerated remains unclear. In this study, we conducted high-velocity friction experiments under hydrothermal conditions on gouges of San Carlos olivine and Inada granite using the hydrothermal rotary shear apparatus (HDR) at Kochi Institute for Core Sample Research, JAMSTEC to investigate how the fault slip enhances the fluid-rock reactions within the fault zone.
Thirteen experiments were conducted at slip velocity of 1.12 m/s, fluid pressure of 50 MPa, effective normal stress of 2.5 or 5 MPa, and temperature from room temperature to 400℃. For the experiments conducted at low temperatures up to 200℃, friction coefficient increased immediately after the beginning of friction experiment and decreased with slip. For the experiments at high temperatures of 300℃ and 400℃, friction coefficient increased with slip, and in some cases friction coefficient exceeded 1. This may be induced by high friction between gunmetal or brass jackets and the sample holder made by Inconel superalloy due to a difference in thermal expansivities of these materials, in addition to leakage of gouges during experiment. Future experiments at high temperatures will be performed with a carbon jacket with a low thermal expansivity. Microstructural observations indicate that grains smaller than 1 µm were produced in the principal slip zone, which were considerably finer than the starting material (100-500 µm). Mineral composition of the recovered samples was analyzed using XRD, SEM-EDS, and Raman spectroscopy. These results showed that no serpentinite and clay mineral peaks were found in the principal slip zone; even olivine and quartz/feldspar peaks were not detected in the recovered samples of the olivine and the granite gouges, respectively. This indicates that amorphization has occurred during the experiments. To investigate dynamic fluid-rock reactions during earthquakes, future experiments performed under further higher normal stresses and with larger slips will be required.
Thirteen experiments were conducted at slip velocity of 1.12 m/s, fluid pressure of 50 MPa, effective normal stress of 2.5 or 5 MPa, and temperature from room temperature to 400℃. For the experiments conducted at low temperatures up to 200℃, friction coefficient increased immediately after the beginning of friction experiment and decreased with slip. For the experiments at high temperatures of 300℃ and 400℃, friction coefficient increased with slip, and in some cases friction coefficient exceeded 1. This may be induced by high friction between gunmetal or brass jackets and the sample holder made by Inconel superalloy due to a difference in thermal expansivities of these materials, in addition to leakage of gouges during experiment. Future experiments at high temperatures will be performed with a carbon jacket with a low thermal expansivity. Microstructural observations indicate that grains smaller than 1 µm were produced in the principal slip zone, which were considerably finer than the starting material (100-500 µm). Mineral composition of the recovered samples was analyzed using XRD, SEM-EDS, and Raman spectroscopy. These results showed that no serpentinite and clay mineral peaks were found in the principal slip zone; even olivine and quartz/feldspar peaks were not detected in the recovered samples of the olivine and the granite gouges, respectively. This indicates that amorphization has occurred during the experiments. To investigate dynamic fluid-rock reactions during earthquakes, future experiments performed under further higher normal stresses and with larger slips will be required.