17:15 〜 18:30
[SIT21-P05] Elastic properties of fine-grained polycrystalline clinopyroxene under high P-T conditions
キーワード：単斜輝石、サブミクロン、弾性波速度、ピストンシリンダー型高圧発生装置
Grain size may play an important role in determining the physical properties of rock-forming minerals under some conditions. Even though the occurrences of minerals with grain size of < 1 μm are limited, they might have a significant effect on geophysical processes due to its high surface area to volume ratio; for example, nanocrystalline forsterite is more compressible than forsterite with micron-scale grains due to the weakening of elastic properties of grain boundary and triple junction (Couvy et al., 2010; PCM). In this study, we demonstrated ultrasonic wave velocity measurements of polycrystalline clinopyroxene with submicron grain sizes under high P-T conditions, and compared its elastic properties with those reported from samples with much larger grain size.
Fine-grained polycrystalline clinopyroxene with average grain size of ~400 nm were sintered from pulverized clinopyroxene crystals (Ca0.92Na0.07Mg0.93 Fe0.01Al0.06Mn0.01Si2O6). The Vp and Vs values of the fine-grained clinopyroxene were measured up to 1.0 GPa and 640 °C for Vp and 750 °C for Vs with a piston-cylinder type high-pressure apparatus. At room temperature, Vp increases rapidly with increasing pressure up to 0.5 GPa, whereas it exhibit a gradual increase between 0.5 and 1.0 GPa, suggesting closure of porosity at > 0.5 GPa. The velocity increase at pressures from 0.5 to 1.0 GPa is ~1 %, and the velocity reduction at temperatures from 25 to 640 °C at 1.0 GPa is ~3-5 %. At 1.0 GPa and 640 °C, Vp of fine-grained polycrystalline is ~7.6 km/s, which is smaller than those of polycrystalline clinopyroxene with grain size of < 100 µm (Liebermann and Mayson, 1976; PEPI). The calculated bulk modulus for fine-grained clinopyroxene is relatively small, and this Vp difference may be attributed to this small value of bulk modulus in this study. This decrease of bulk modulus for fine-grained clinopyroxene is consistent with the grain-size-sensitive elastic behavior of nanocrystalline forsterite. According to our results, we found that fine-grained polycrystalline clinopyroxene exhibit low bulk modulus, causing the decrease in Vp and Vp/Vs, and this might be associated with the decrease of grain size to submicron scale.
Fine-grained polycrystalline clinopyroxene with average grain size of ~400 nm were sintered from pulverized clinopyroxene crystals (Ca0.92Na0.07Mg0.93 Fe0.01Al0.06Mn0.01Si2O6). The Vp and Vs values of the fine-grained clinopyroxene were measured up to 1.0 GPa and 640 °C for Vp and 750 °C for Vs with a piston-cylinder type high-pressure apparatus. At room temperature, Vp increases rapidly with increasing pressure up to 0.5 GPa, whereas it exhibit a gradual increase between 0.5 and 1.0 GPa, suggesting closure of porosity at > 0.5 GPa. The velocity increase at pressures from 0.5 to 1.0 GPa is ~1 %, and the velocity reduction at temperatures from 25 to 640 °C at 1.0 GPa is ~3-5 %. At 1.0 GPa and 640 °C, Vp of fine-grained polycrystalline is ~7.6 km/s, which is smaller than those of polycrystalline clinopyroxene with grain size of < 100 µm (Liebermann and Mayson, 1976; PEPI). The calculated bulk modulus for fine-grained clinopyroxene is relatively small, and this Vp difference may be attributed to this small value of bulk modulus in this study. This decrease of bulk modulus for fine-grained clinopyroxene is consistent with the grain-size-sensitive elastic behavior of nanocrystalline forsterite. According to our results, we found that fine-grained polycrystalline clinopyroxene exhibit low bulk modulus, causing the decrease in Vp and Vp/Vs, and this might be associated with the decrease of grain size to submicron scale.