[S-CG51] New perspectives of subduction zone earthquake dynamics through experiments across-scales

[SCG51-02] Ultrasonic elastic wave velocity measurement of antigorite at high pressures to 10 GPa

*Yoshio Kono¹, Riseno Nose¹, Nozomi Kondo¹, Ken-ichi Hirauchi² (1.Geodynamics Research Center, Ehime University, 2.Shizuoka University)

Keywords:Serpentine, Elastic wave velocity, High pressure

Knowledge of P- (Vp) and S-wave (Vs) velocities of antigorite is important to understand low seismic wave velocity anomalies in subduction zones. Efforts have been made to understand Vp and Vs of antigorite at high pressure conditions of subduction zones, while there are serious discrepancies in the Vp and Vs results at high pressures (>1 GPa) in previous studies (Bezacier et al., 2013; Wang et al., 2019). Here we investigated elastic wave velocities of natural antigorite sample up to ~10 GPa. The natural serpentinite sample, collected from the Nagasaki metamorphic rocks, is mainly composed of antigorite with minor amounts (<3.5 %) of other minerals. The sample does not show any clear planar and/or linear structures. Four samples were cored from orthogonal directions to check possible inhomogeneity of the rock sample. Sample size is 4.5 mm in diameter and 1.5-1.9 mm in length. Ultrasonic Vp and Vs measurements were conducted up to 8.5-10.3 GPa under room temperature condition in 1000 ton multi-anvil high-pressure apparatus.

Vp of the antigorite increases with increasing pressure below 4-5 GPa, while it starts to decrease at higher pressures. Vs also increases with increasing pressure at low pressures of <2.5-3.5 GPa, while it decreases at higher pressures. Vp/Vs ratio shows increase with increasing pressure. There are some variations in the Vp, Vs, and Vp/Vs ratio of antigorite obtained in four experiments, which are interpreted as due to inhomogeneous proportion of other mineral constituents. Vp, Vs, and Vp/Vs ratio of pure antigorite is calculated by linear fitting of four experimental results as a function of proportion of other mineral constituents. Our obtained Vp, Vs, and Vp/Vs ratio of antigorite is consistent with the results of Bezacier et al. (2013). In contrast, Vp, Vs, and Vp/Vs ratio reported in Wang et al. (2019) shows markedly different values compared to those of this study and Bezacier et al. (2013).