5:15 PM - 6:45 PM
[SCG46-P03] Fluid movement in subduction zones based on measurements of elastic wave velocities and electrical resistivity of serpentinite and peridotite
Keywords:Ultramafic rocks, physical properties, Anisotorpy
Water is involved in various phenomena that occur in subduction zones. The formation of serpentinite is one of them. When a subducting plate releases water into the mantle wedge, the water reacts with the plate surface and mantle wedge to form serpentinite. In particular, serpentinites formed on the plate surface are strongly deformed and have well-developed foliation. It is also well known that cracks develop along the foliation. In this study, we investigate the azimuthal dependence of elastic wave velocity and electrical resistivity of schistose serpentinite and peridotite, and discuss the migration path of fluid in subduction zones and the members of seismic anisotropy.
In this study, deformed antigorite serpentinite from the Nomo Peninsula, Nagasaki Prefecture, and deformed peridotite from Hodonotani, Besshi Village, Ehime Prefecture, were used as experimental samples. These rocks show strong mineral orientation under the microscope. The samples were formed parallel and perpendicular to the foliation. Simultaneous measurements of elastic wave velocities (Vp and Vs) and electrical resistivity were performed under hydrostatic pressure to evaluate the orientation dependence of the rocks: P-wave velocity was measured for three orthogonal components, S-wave velocity for two vibration directions (six components in total) for the three orthogonal components, and electrical resistivity for two orthogonal components. The experiments were conducted using an intravessel deformation and fluid flow apparatus, and the confining pressure was increased in steps from 5 MPa to 200 MPa to investigate the pressure effect. The fluid was a 0.5 mol/L NaCl solution, and the fluid pressure was controlled to 1 MPa at room temperature.
In both serpentinite and peridotite, the P-wave velocity tended to be faster in the direction of propagation parallel to the foliation than in the direction of propagation perpendicular to the foliation. In peridotite, the P-wave velocity tended to be the fastest in the direction parallel to the lineation structure. The S-wave velocity tended to be faster in the direction of propagation parallel to the foliation than in the direction of propagation across the foliation or in the direction of oscillation. These results can be explained by the existence of many cracks parallel to the foliation. From these results, the anisotropy of P- and S-waves was calculated and compared for the two samples, and both P- and S-wave anisotropy tended to be greater in serpentinite than in peridotite. The anisotropy decreased rapidly with increasing pressure up to 60 MPa, and then changed slowly, suggesting that most of the cracks were closed at 60 MPa. The anisotropy of elastic wave velocity remains about 10% to 20% even after pressurization up to 200 MPa, but if many cracks were closed at 200 MPa, this is considered to be due to the anisotropy of the mineral. The electrical resistivity of serpentinite was about one order of magnitude lower when measured parallel to the foliation than perpendicular to it. On the other hand, the electrical resistivity of peridotite is almost the same in the parallel and perpendicular directions to the foliation, and is almost the same as that of the perpendicular component of serpentinite.
From the analysis of multi-component elastic wave velocities, the elastic constant tensor was calculated, and a seismic wave velocity pole figure was generated. The pole figure shows that seismic velocities in anisotropic rocks strongly depend on the direction of propagation and oscillation. The electrical resistivity results suggest that serpentinite is more water-transparent than peridotite due to the higher connectivity of cracks parallel to the foliation. Based on these results and seismic tomography observations, we discuss the distribution of serpentinites and the moving process of fluids near subducting plates.
In this study, deformed antigorite serpentinite from the Nomo Peninsula, Nagasaki Prefecture, and deformed peridotite from Hodonotani, Besshi Village, Ehime Prefecture, were used as experimental samples. These rocks show strong mineral orientation under the microscope. The samples were formed parallel and perpendicular to the foliation. Simultaneous measurements of elastic wave velocities (Vp and Vs) and electrical resistivity were performed under hydrostatic pressure to evaluate the orientation dependence of the rocks: P-wave velocity was measured for three orthogonal components, S-wave velocity for two vibration directions (six components in total) for the three orthogonal components, and electrical resistivity for two orthogonal components. The experiments were conducted using an intravessel deformation and fluid flow apparatus, and the confining pressure was increased in steps from 5 MPa to 200 MPa to investigate the pressure effect. The fluid was a 0.5 mol/L NaCl solution, and the fluid pressure was controlled to 1 MPa at room temperature.
In both serpentinite and peridotite, the P-wave velocity tended to be faster in the direction of propagation parallel to the foliation than in the direction of propagation perpendicular to the foliation. In peridotite, the P-wave velocity tended to be the fastest in the direction parallel to the lineation structure. The S-wave velocity tended to be faster in the direction of propagation parallel to the foliation than in the direction of propagation across the foliation or in the direction of oscillation. These results can be explained by the existence of many cracks parallel to the foliation. From these results, the anisotropy of P- and S-waves was calculated and compared for the two samples, and both P- and S-wave anisotropy tended to be greater in serpentinite than in peridotite. The anisotropy decreased rapidly with increasing pressure up to 60 MPa, and then changed slowly, suggesting that most of the cracks were closed at 60 MPa. The anisotropy of elastic wave velocity remains about 10% to 20% even after pressurization up to 200 MPa, but if many cracks were closed at 200 MPa, this is considered to be due to the anisotropy of the mineral. The electrical resistivity of serpentinite was about one order of magnitude lower when measured parallel to the foliation than perpendicular to it. On the other hand, the electrical resistivity of peridotite is almost the same in the parallel and perpendicular directions to the foliation, and is almost the same as that of the perpendicular component of serpentinite.
From the analysis of multi-component elastic wave velocities, the elastic constant tensor was calculated, and a seismic wave velocity pole figure was generated. The pole figure shows that seismic velocities in anisotropic rocks strongly depend on the direction of propagation and oscillation. The electrical resistivity results suggest that serpentinite is more water-transparent than peridotite due to the higher connectivity of cracks parallel to the foliation. Based on these results and seismic tomography observations, we discuss the distribution of serpentinites and the moving process of fluids near subducting plates.