3:45 PM - 4:00 PM
[SCG57-02] High-pressure ultrasonic measurement of annealed antigorite samples
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). Our recent measurement of elastic wave velocities of natural antigorite sample, collected from the Nagasaki metamorphic rocks, shows Vp and Vs values consistent with those reported in Bezacier et al. (2013). In contrast, Wang et al. (2019) reported very low Vp and Vs values, while the reason is not clear. We note that the sample of Wang et al. (2019) is hot-pressed at 3 GPa and 500 °C. In order to clarify possible influence of the high-pressure and high-temperature sintering on Vp and Vs, we investigated Vp and Vs of antigorite samples after high-pressure and high-temperature annealing.
High-pressure and high-temperature annealing experiments were conducted at 500, 600, and 650 °C under 3 GPa pressure condition. We found breakdown of antigorite and formation of olivine at 650 °C, while there are no visible change in the samples annealed at 500 and 600 °C. On the other hand, SEM-EDS analysis of antigorite in the 500 °C and 600 °C samples show smaller value of the total deficiency compared to the original natural sample. In addition, Raman spectroscopy analyses of these samples show new peaks around 1350 cm-1 and 1600 cm-1. The peak around 1600 cm-1 may be interpreted as due to H-O-H bending mode in water, which imply partial release of water from antigorite. High-pressure ultrasonic measurement of the antigorite samples annealed at 500 °C and 600 °C show ~4% lower Vp and Vs compared to those of natural antigorite sample, which may be due to partial release of water from antigorite. On the other hand, the Vp and Vs reported in Wang et al. (2019) is >10 % lower than those of this study and Bezacier et al. (2013), and therefore, the high-pressure and high-temperature annealing cannot explain the marked difference of Vp and Vs reported in previous studies. Although the cause of the discrepancy is still not clear, our observations show Vp and Vs of antigorite similar to those reported in Bezacier et al. (2013).
High-pressure and high-temperature annealing experiments were conducted at 500, 600, and 650 °C under 3 GPa pressure condition. We found breakdown of antigorite and formation of olivine at 650 °C, while there are no visible change in the samples annealed at 500 and 600 °C. On the other hand, SEM-EDS analysis of antigorite in the 500 °C and 600 °C samples show smaller value of the total deficiency compared to the original natural sample. In addition, Raman spectroscopy analyses of these samples show new peaks around 1350 cm-1 and 1600 cm-1. The peak around 1600 cm-1 may be interpreted as due to H-O-H bending mode in water, which imply partial release of water from antigorite. High-pressure ultrasonic measurement of the antigorite samples annealed at 500 °C and 600 °C show ~4% lower Vp and Vs compared to those of natural antigorite sample, which may be due to partial release of water from antigorite. On the other hand, the Vp and Vs reported in Wang et al. (2019) is >10 % lower than those of this study and Bezacier et al. (2013), and therefore, the high-pressure and high-temperature annealing cannot explain the marked difference of Vp and Vs reported in previous studies. Although the cause of the discrepancy is still not clear, our observations show Vp and Vs of antigorite similar to those reported in Bezacier et al. (2013).