17:15 〜 18:45
[SCG46-P22] 弾性波速度と電気比抵抗の測定に基づくかんらん岩に形成するサーマルクラックのネットワーク
キーワード：かんらん岩、弾性波速度、電気比抵抗、異方性、クラック
Recently, the thermal cracks formed during the cooling process of the oceanic mantle has been focused (e.g., Korenaga, 2020). Because cracks play the role of water channels, evaluating the crack network is important for understanding water distribution in the mantle. However, there are few experimental studies on the orientation and the connectivity of the thermal cracks formed in peridotite. In this study, I measured the elastic wave velocity, the electrical resistivity and the porosity of peridotite containing thermal cracks formed by heat-treated, and investigated the orientation and the connectivity of the thermal cracks based on the anisotropy of the physical properties.
The cube specimen of peridotite collected from the Horoman Peridotite Complex in the Hidaka metamorphic belt, Hokkaido, was used in the experiments. To form thermal cracks, the experimental samples were thermally treated up to 600℃ at a rate of 5℃ /min using a nitrogen gas purged oven. The compressional and shear wave velocities were measured from the pulse transition method under dry and wet conditions. The electrical resistivity was measured from an alternative current impedance method using LCR meter. The elastic wave velocity and the electrical resistivity were measured in three directions, which defined parallel to the lineation (X direction), normal to the lineation in the foliation (Y direction) and normal to the foliation plane (Z direction). The porosity was calculated from the bulk volume measured using a caliper and the pore volume measured from the mass of specimen between dry and wet conditions.
Porosity before thermally treatment were 0.1-0.2%. The compressional velocity and the electrical resistivity in X direction showed the highest velocity and the lowest resistivity in the three directions. Porosity after thermally treatment increased to 0.7-0.8%, indicating that thermal cracks are formed. In all directions, elastic wave velocity and electrical resistivity decreased with increasing porosity related to thermal induced crack. Especially, the compressional wave velocities and electrical resistivity in Y and Z directions markedly decreased, resulting that the azimuthal anisotropy of the compressional wave velocity increased and the anisotropy of the electrical resistivity decreased. These experimental results suggest that the opening of grain boundary cracks along X direction elongated minerals causes the velocity reduction in the Y and Z direction, and the anisotropy of electrical resistivity decreased by increasing the crack connectivity in the Y and Z directions due to the intragranular cracks perpendicular to the X direction. My experimental results are expected to contribute to the understanding of thermal crack networks not only in isotropic medium but also in anisotropic medium such as mantle.
The cube specimen of peridotite collected from the Horoman Peridotite Complex in the Hidaka metamorphic belt, Hokkaido, was used in the experiments. To form thermal cracks, the experimental samples were thermally treated up to 600℃ at a rate of 5℃ /min using a nitrogen gas purged oven. The compressional and shear wave velocities were measured from the pulse transition method under dry and wet conditions. The electrical resistivity was measured from an alternative current impedance method using LCR meter. The elastic wave velocity and the electrical resistivity were measured in three directions, which defined parallel to the lineation (X direction), normal to the lineation in the foliation (Y direction) and normal to the foliation plane (Z direction). The porosity was calculated from the bulk volume measured using a caliper and the pore volume measured from the mass of specimen between dry and wet conditions.
Porosity before thermally treatment were 0.1-0.2%. The compressional velocity and the electrical resistivity in X direction showed the highest velocity and the lowest resistivity in the three directions. Porosity after thermally treatment increased to 0.7-0.8%, indicating that thermal cracks are formed. In all directions, elastic wave velocity and electrical resistivity decreased with increasing porosity related to thermal induced crack. Especially, the compressional wave velocities and electrical resistivity in Y and Z directions markedly decreased, resulting that the azimuthal anisotropy of the compressional wave velocity increased and the anisotropy of the electrical resistivity decreased. These experimental results suggest that the opening of grain boundary cracks along X direction elongated minerals causes the velocity reduction in the Y and Z direction, and the anisotropy of electrical resistivity decreased by increasing the crack connectivity in the Y and Z directions due to the intragranular cracks perpendicular to the X direction. My experimental results are expected to contribute to the understanding of thermal crack networks not only in isotropic medium but also in anisotropic medium such as mantle.