5:15 PM - 7:15 PM
[SCG54-P12] Crystal Orientation Analysis and Multicomponent Velocity Analysis of naturally deformed Serpentinite and Peridotite
Keywords:Ultramafic rocks, physical properties, Anisotorpy, Crystal-preferred orientations
Water is involved in several phenomena that occur in subduction zones. The formation of serpentinite is one of them. Geophysical observations such as seismic velocity have recently revealed the presence of water in the deep interior of the Earth. The interpretation of these results is based on isotropic rock properties and does not consider the anisotropy. The elastic wave velocity anisotropy of peridotite and serpentinite has been evaluated by SEM-EBSD, but the anisotropy does not consider the effect of cracks. Therefore, this study aims to clarify the effect of cracks on elastic wave velocity anisotropy by measuring multicomponent elastic wave velocity and CPO by SEM-EBSD for a well-developed peridotite and serpentinite.
The rock samples used in this study are peridotite from Mt. Higashi-Akaishi, Ehime Prefecture, and serpentinite from Nomo Peninsula, Nagasaki Prefecture, which have strong mineral orientation under a polarized microscope. The main constituent minerals of peridotite are olivine and antigorite, and those of serpentinite are antigorite. To determine the direction of measurement, the X-axis is parallel to the lineation and the Z-axis is perpendicular to the foliation. Laboratory elastic wave velocity measurements were performed on these rocks to evaluate the bulk elastic wave velocity anisotropy. Elastic wave velocity was measured using an intravessel deformation and fluid flow apparatus. Experimental conditions were controlled by increasing the confining pressure from 5 MPa to 200 MPa to investigate pressure effects. A 0.5 mol/L NaCl solution was used as the fluid, and the fluid pressure was controlled at 1 MPa at room temperature. The anisotropy of the elastic wave velocities of the minerals was evaluated by measuring the CPO of olivine and antigorite by SEM-EBSD using a scanning electron microscope at Nagoya University. The CPO of olivine was quantitatively typed using a Vp-Flinn diagram.
The experimental results show that the P-wave velocities of serpentinite and olivine tend to be faster along the X- and Y-axes than along the Z-axis on the foliation. The S-wave velocities of serpentinite were 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. The anisotropy of the elastic wave velocity at 200 MPa was about 15%, which was attributed to the crystalline-selective orientation of the mineral. From the multicomponent elastic wave velocities obtained in this experiment, the elastic constant tensor was calculated to produce an elastic wave velocity polar map of the bulk rock. As a result of the SEM-EBSD of the peridotite, the olivine CPO was classified as B-type based on the Vp-Flinn diagram. The antigorite CPO shows a strong concentration of the b-axis on the X-axis and a weak concentration of the c-axis on the Z- and Y-axis. In serpentinite, the antigorite CPO has a weak concentration of the b-axis on the X-axis and a strong concentration of the c-axis on the Z-axis. Based on these CPOs and the elastic wave constants of each mineral, elastic wave velocity polar maps were generated to evaluate the elastic wave velocity anisotropy by mineral. The elastic wave velocity of the peridotite was found to behave like that of the A-type. This may be due to the influence of antigorite. Based on these results, the effect of cracks on elastic wave velocity anisotropy is discussed.
The rock samples used in this study are peridotite from Mt. Higashi-Akaishi, Ehime Prefecture, and serpentinite from Nomo Peninsula, Nagasaki Prefecture, which have strong mineral orientation under a polarized microscope. The main constituent minerals of peridotite are olivine and antigorite, and those of serpentinite are antigorite. To determine the direction of measurement, the X-axis is parallel to the lineation and the Z-axis is perpendicular to the foliation. Laboratory elastic wave velocity measurements were performed on these rocks to evaluate the bulk elastic wave velocity anisotropy. Elastic wave velocity was measured using an intravessel deformation and fluid flow apparatus. Experimental conditions were controlled by increasing the confining pressure from 5 MPa to 200 MPa to investigate pressure effects. A 0.5 mol/L NaCl solution was used as the fluid, and the fluid pressure was controlled at 1 MPa at room temperature. The anisotropy of the elastic wave velocities of the minerals was evaluated by measuring the CPO of olivine and antigorite by SEM-EBSD using a scanning electron microscope at Nagoya University. The CPO of olivine was quantitatively typed using a Vp-Flinn diagram.
The experimental results show that the P-wave velocities of serpentinite and olivine tend to be faster along the X- and Y-axes than along the Z-axis on the foliation. The S-wave velocities of serpentinite were 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. The anisotropy of the elastic wave velocity at 200 MPa was about 15%, which was attributed to the crystalline-selective orientation of the mineral. From the multicomponent elastic wave velocities obtained in this experiment, the elastic constant tensor was calculated to produce an elastic wave velocity polar map of the bulk rock. As a result of the SEM-EBSD of the peridotite, the olivine CPO was classified as B-type based on the Vp-Flinn diagram. The antigorite CPO shows a strong concentration of the b-axis on the X-axis and a weak concentration of the c-axis on the Z- and Y-axis. In serpentinite, the antigorite CPO has a weak concentration of the b-axis on the X-axis and a strong concentration of the c-axis on the Z-axis. Based on these CPOs and the elastic wave constants of each mineral, elastic wave velocity polar maps were generated to evaluate the elastic wave velocity anisotropy by mineral. The elastic wave velocity of the peridotite was found to behave like that of the A-type. This may be due to the influence of antigorite. Based on these results, the effect of cracks on elastic wave velocity anisotropy is discussed.