10:45 AM - 12:15 PM
[SCG58-P10] Effects of thermal cracks in peridotite on anisotropy of elastic wave velocity
Keywords:Elastic wave velocity, Anisotropy, Mantle, Peridotite, Crack
Geophysical surveys of the mantle have observed seismic anisotropy, which are interpreted by the preferred orientation of olivine crystals due to mantle flow (e.g., Kodaira et al., 2014). Recently, the formation of thermal cracks during the cooling process of the oceanic plate has been focused (e.g., Korenaga, 2020), but the effects of thermal cracks in the mantle on the seismic wave velocities and its anisotropy have not been investigated. In this study, we investigated the effects of the thermal cracks by measuring the elastic wave velocities and porosity of heat-treated peridotites.
Peridotites collected from the Horoman Peridotite Complex in the Hidaka metamorphic belt, Hokkaido, were used as the experimental samples. We defined the three structural directions (X, Y, Z), which refer lineation, normal to the lineation in the foliation and normal to foliation plane, respectively. In order to prevent mineral oxidation, the samples were heat treated up to 600℃ at a rate of 5℃/min in the oven filled with nitrogen gas. After the samples kept in 600℃ for 2 hours, the samples were slowly cooled to lower than 100℃. Compressional and shear wave velocities in each direction were measured from the pulse transmission method under dry and wet conditions. Porosity was calculated from solid and pore volume measured from the gas pycnometer method and the impregnation method, respectively (Shimamoto et al., 2006; Nagase et al., 2020).
Porosity of samples before heat-treatment were 0.18%−0.21%, and that after heat-treatment increased to 0.56%−0.72%. The elastic wave velocity was the fastest in the X direction and showed similar velocities in the Y and Z direction. Compressional wave velocities under dry condition after heat treatment tended to decrease markedly in the Y and Z directions, and the azimuthal anisotropy of compressional wave velocities increased, suggesting that the cracks are formed anisotropically. The azimuthal anisotropy of compressional wave velocity under wet condition tends to be lower than that under dry conditions. Such change of the anisotropy with the type of pore fluid are similar to the result of the effective medium theory in which the penny-shaped cracks are oriented (Anderson et al., 1974). Our experimental results indicate the possibility that the thermal cracks in the mantle are oriented and may contribute to the understanding of the distribution of pore fluid in the mantle.
Peridotites collected from the Horoman Peridotite Complex in the Hidaka metamorphic belt, Hokkaido, were used as the experimental samples. We defined the three structural directions (X, Y, Z), which refer lineation, normal to the lineation in the foliation and normal to foliation plane, respectively. In order to prevent mineral oxidation, the samples were heat treated up to 600℃ at a rate of 5℃/min in the oven filled with nitrogen gas. After the samples kept in 600℃ for 2 hours, the samples were slowly cooled to lower than 100℃. Compressional and shear wave velocities in each direction were measured from the pulse transmission method under dry and wet conditions. Porosity was calculated from solid and pore volume measured from the gas pycnometer method and the impregnation method, respectively (Shimamoto et al., 2006; Nagase et al., 2020).
Porosity of samples before heat-treatment were 0.18%−0.21%, and that after heat-treatment increased to 0.56%−0.72%. The elastic wave velocity was the fastest in the X direction and showed similar velocities in the Y and Z direction. Compressional wave velocities under dry condition after heat treatment tended to decrease markedly in the Y and Z directions, and the azimuthal anisotropy of compressional wave velocities increased, suggesting that the cracks are formed anisotropically. The azimuthal anisotropy of compressional wave velocity under wet condition tends to be lower than that under dry conditions. Such change of the anisotropy with the type of pore fluid are similar to the result of the effective medium theory in which the penny-shaped cracks are oriented (Anderson et al., 1974). Our experimental results indicate the possibility that the thermal cracks in the mantle are oriented and may contribute to the understanding of the distribution of pore fluid in the mantle.