日時 ： ２０１０年１月１５日（金）※16：30～
場所 ： 愛媛大学総合研究棟Ⅰ 4F 共通会議室（理学部構内）
要 旨
Serpentinite is formed through the hydration of peridotite in subduction zone. Numbers of seismological studies have demonstrated low velocity and high Poisson's ratio anomaly in mantle wedge, which has been considered to represent the existence of serpentinite. However, elastic wave velocity of serpentinite is still poorly understood. A few elastic wave velocity data of serpentinite have been reported by previous studies, but these experimental data is limited to low pressure condition (up to 1 GPa). We need elastic wave velocity data under the pressure conditions of subduction zone (～1 to 5 GPa). In this study, we measured elastic wave velocity of low-temperature type (mixture of chrysotile and lizardite) and high-temperature type (antigorite) serpentinite at high pressures up to 8GPa and room temperature. High-pressure and room temperature experiments were carried out using an Kawai-type multianvil apparatus. We measured P-wave (Vp) and S-wave velocities (Vs) by using ultrasonic technique. The Vp of the high-T type serpentinite increase with increasing pressure to 4 GPa, but it decrease with increasing pressure at 4-6 GPa. The Vp again increase with increasing pressure at higher than 6 GPa. The Vs slightly increase with increasing pressure to 2 GPa, and decrease with increasing pressure at 2-7 GPa. The Vp of the low-T type serpentinite increase with increasing pressure, while the pressure dependence becomes small at higher than 4 GPa. The Vs does not change or slightly decreases with increasing pressure at 3-7 GPa. We estimated the Vp and Vs at 0 GPa by extrapolating the results of this study to compare the result with those of previous studies, and observed Vp=6.70 km/s and Vs=3.96 km/s for the high-T type, which is closed to those of Bezacier et al.(2009) determined by the Brillouin scattering techniques at ambient condition (Vp=6.76 km/s, Vs=3.83 km/s). We also obtained Vp=5.08 km/s and Vs=3.26 km/s for the low-T type, which is markedly higher than those of previous studies (e.g., Vp=4.3 km/s and Vs=2.1 km/s; Christensen, 1966). In contrast, Mookherjee and Stixrude (2009) calculated Vp and Vs of lizardite of 6.36 km/s and 3.63 km/s, respectively, from first principle calculation. The Vp and Vs values are relatively closed to those of antigorite. Our Vp and Vs results locate between those of the first principle calculation and previous ultrasonic measurement of rocks. Because natural rock samples would consists not only of lizardite but also of chrysotile, the variations of Vp and Vs of low-T type serpentinites might be the result of variations of the amount of chrysotile in the rock samples, and chrysotile might have significantly lower Vp and Vs values than antigorite and lizardite.
問い合わせ先：土屋 卓久 TEL （089）927-8198
E-mail takut@sci.ehime-u.ac.jp
