[SIT24-P05] Intrinsic attenuation factor Q of partially molten Fe–S–O system
Keywords:attenuation, partially melting, Young Modulus, inner core
We investigated Fe-S-O system. Sulfur and oxygen are regarded as the representative light element in the Earth’s core. Fe samples with 1, 3 and 14 wt. % S were synthesized in piston cylinder apparatus at 900oC and 1 GPa. a small amount of oxygen was included accidentally. In-situ X-ray radiographic observations were performed at the bending magnet beam line BL04B1 at SPring-8. Deformation-DIA press with a facility of cyclic loading was used. This system could produce sinusoidal displacement in a wide range of oscillation periods from 0.2 to 100 s and generating variable amplitudes (Yoshino et al., 2016). Q value characterizing intrinsic attenuation was determined by phase lag of sample strain against the reference material at 1 GPa and up to 1200oC.
The experimental results show that frequency dependence of attenuation is not remarkable except for a fine grained-sample, whereas decrease of attenuation was found at 1100 and 920oC for Fe- 1 wt. %S and Fe- 14 wt. %S samples, respectively. The attenuation drop would occur with increasing melt fraction. Experiments of Fe-14 wt. %S sample along heating and cooling paths were done to check the influence of grain growth during experiments. Data obtained from both heating and cooling paths show that abrupt change of the attenuation occurred at around 925oC. The Young’s modulus calculated from the strain ratio of sample and reference shows strong temperature dependence. An obvious drop occurred between 900 and 980oC in association with the occurrence of partial melting. Relationship between velocity and attenuation anomalies of the Fe–S–O system shows a positive correlation when melt fraction reaches a certain value.
Partial melting could decrease Q-1, which could explain the relative low attenuation on uppermost 30 km layer throughout the top of the inner core. Combing with the positive correlation between attenuation and velocity, a large amount of partial melting could occur on the uppermost 30 km layer of the rest area except the Pacific hemisphere in the inner core.