17:15 〜 18:45
[SIT14-P01] P- and S-wave velocities of lunar pyroxene composition under high pressure and high temperature conditions
キーワード：弾性波速度、高圧、月マントル、直方輝石
Understanding the structure and composition of the lunar mantle is important for discussing the formation and evolution of the Moon. Seismic velocity structure data of the lunar interior (e.g., Weber et al., 2011) is one of the most useful information for understanding the structure and composition of the lunar mantle. According to Weber et al. (2011), in the upper mantle region of the Moon at the depth of 40-738 km, the Primary wave velocity (Vp) 7.6-7.7 km/s, and Secondary wave velocity (Vs) is 4.4 km/s, while in the lower mantle of the Moon at 738-1257 km depth, Vp is 8.5 km/s and Vs is 4.5 km/s. On the other hand, previous petrological studies (Lin et al., 2017) have proposed crystallization of olivine and orthopyroxene from the lunar magma ocean, and therefore knowledge of the Vp and Vs of lunar olivine and pyroxene compositions is fundamental for discussing the constituent of the lunar mantle. Elastic wave velocity measurements of olivine have been reported by Bejina et al. (2021) and Kono et al. (2023) at high pressure and high temperature conditions. However, Vp and Vs of pyroxene has not been well understood.
In this study, we investigate Vp and Vs of a lunar pyroxene composition proposed by Lin et al. (2017) at high pressure and high temperature conditions of the lunar mantle. First we prepared the lunar pyroxene composition (Mg0.84Fe0.13Ca0.03SiO3) glass by using a laser heated levitation furnace. The polycrystalline pyroxene sample was synthesized from that glass at 3 GPa and 1573 K by using a Paris-Edinburgh press. We used pyroxene composition glass prepared by a laser heated levitation furnace as the starting material. The polycrystalline sample shows consistent density values between XRD measurement (3.330±0.002/cm3) and Archimedes' method (3.38±0.21 g/cm3), indicating that the sample is of good quality without porosity. We carried out ultrasonic measurement, X-ray radiography, and X-ray diffraction measurements at 0.9-6.6 GPa and 300-1273 K at the BL04B1 beamline of SPring-8. We obtained unit cell volume of the pyroxene and it is fit into the third-order Birch-Murnaghan equation of state as V0 = 839.7±1.2 ų, K0,300 = 117.5±6.8 GPa, K'0,300 = 4 (fixed), (∂K0,T/∂T)P = -1.18×10-2±1.51×10-2 GPa K-1, and the thermal expansivity parameters: a0 = 1.44×10-5±0.19×10-5 K-1,b0 = 1.96×10-8±0.67×10-8 K-2. In addition, Vp and Vs results are expressed by Vp = 7.909+0.130×P-0.00062×(T-300) and Vs = 4.699+0.036×P-0.00031×(T-300). Vp and Vs of the lunar pyroxene composition under lunar pressure and temperature conditions (Khan et al., (2014)) are 5.5-5.8% and 2.0-2.7% slower than those of San Carlos olivine, respectively (Kono et al., 2023). Our observation indicates that the seismic velocity structure of the lunar upper mantle (Weber et al., 2011) can be explained by a mineral proportion of 75 % olivine and 25 % orthopyroxene.
In this study, we investigate Vp and Vs of a lunar pyroxene composition proposed by Lin et al. (2017) at high pressure and high temperature conditions of the lunar mantle. First we prepared the lunar pyroxene composition (Mg0.84Fe0.13Ca0.03SiO3) glass by using a laser heated levitation furnace. The polycrystalline pyroxene sample was synthesized from that glass at 3 GPa and 1573 K by using a Paris-Edinburgh press. We used pyroxene composition glass prepared by a laser heated levitation furnace as the starting material. The polycrystalline sample shows consistent density values between XRD measurement (3.330±0.002/cm3) and Archimedes' method (3.38±0.21 g/cm3), indicating that the sample is of good quality without porosity. We carried out ultrasonic measurement, X-ray radiography, and X-ray diffraction measurements at 0.9-6.6 GPa and 300-1273 K at the BL04B1 beamline of SPring-8. We obtained unit cell volume of the pyroxene and it is fit into the third-order Birch-Murnaghan equation of state as V0 = 839.7±1.2 ų, K0,300 = 117.5±6.8 GPa, K'0,300 = 4 (fixed), (∂K0,T/∂T)P = -1.18×10-2±1.51×10-2 GPa K-1, and the thermal expansivity parameters: a0 = 1.44×10-5±0.19×10-5 K-1,b0 = 1.96×10-8±0.67×10-8 K-2. In addition, Vp and Vs results are expressed by Vp = 7.909+0.130×P-0.00062×(T-300) and Vs = 4.699+0.036×P-0.00031×(T-300). Vp and Vs of the lunar pyroxene composition under lunar pressure and temperature conditions (Khan et al., (2014)) are 5.5-5.8% and 2.0-2.7% slower than those of San Carlos olivine, respectively (Kono et al., 2023). Our observation indicates that the seismic velocity structure of the lunar upper mantle (Weber et al., 2011) can be explained by a mineral proportion of 75 % olivine and 25 % orthopyroxene.