10:15 〜 10:30
[SIT20-12] The effect of water on energy dispersion of olivine and its implications for the origin of the sharp contrast of seismic observation at the lithosphere-asthenosphere boundary
キーワード：energy dispersion, water, upper mantle, attenuation, velocity, lithosphere and asthenosphere boundary
Seismic observations have shown a sharp drop of shear wave velocity at the lithosphere-asthenosphere boundary (LAB) (Kawakatsu et al., 2010) and strong attenuation in the asthenosphere. The temperature difference itself was difficult to explain such sharp change (Karato et al., 2015). Such as presence of partial melt, deduced grain size, or increased water content in asthenosphere have been considered as the possible origins for those anomalies (e.g., Anderson and Sammis, 1970; Hirth and Kohlstedt, 1996; Faul and Jackson, 2005; Yoshino et al., 2006; Jackson et al., 2010; Karato, 2012). Considering the thermal structure and sharp drop of velocity in the old oceanic upper mantle (100 Myr), those anomalies could not be explained by partial melting (Hirschmann, 2010) or reduced grain size (Karato, 2018). Seismic attenuation is a useful tool to figure out this origin. Although recently Cline et al. (2018) showed a small effect of water content on energy dispersion of olivine aggregates, they measured attenuated properties for Ti-related H defects, which is stable at the low pressure they conducted. Because the actual H substitution mechanism in the upper mantle is related to Mg or Si vacancies, thus we need to measure anelastic properties at relatively higher pressure at which these substitution mechanisms become dominant.
To investigate the effect of water on anelastic property of mantle materials, we measured the energy dispersion and Young’s modulus of olivine aggregates, which is the most dominant mineral in the upper mantle, in various oscillation periods (0.5 -1000 s) at 1373 K and 3 GPa (70 km depth in the Earth). The newly built short-period cyclic loading system in situ X-ray observation was used in our experimental procedures (Yoshino et al., 2016).
The experimental results show that the energy dispersion increases with increasing period of oscillation, decreasing grain size, and increasing the water content of olivine, while the energy dispersion reduces its Young’s modulus. When water content increases, an attenuation peak appears at the short period (5~10 s). The modified generalized Burger’s model was used to acquire the effect of water content and grain size on attenuation and velocity of seismic wave quantitatively (Jackson et al., 2005). The fitting results show that there are two contributions from the high-temperature background (diffusion accommodated grain boundary sliding, DGBS) and attenuation peak to the energy dispersion. The characteristic relaxation time for DGBS showed a mild frequency dependence (0.33), which is consistent with the results of the previous study. The water could enhance the relaxation time with an exponent factor of 2.5(0.1) for DGBS. For the attenuation peak, water could enhance the peak height with an exponent factor of 1.2(1).
Takeuchi et al. (2017) found little frequency-dependent attenuation beneath the old oceanic floor, which is consistent with the attenuation behavior of hydrous samples showing an attenuation peak at a short period. The present results demonstrate that the old lithosphere with less than 1 wt.ppm H2O overlying hydrous asthenosphere (100 wt. ppm H2O) can produce the observed velocity drop (5-10 %). The difference of seismic properties between the asthenosphere and the old oceanic lithosphere can be explained by a remarkable difference in water content between them.
To investigate the effect of water on anelastic property of mantle materials, we measured the energy dispersion and Young’s modulus of olivine aggregates, which is the most dominant mineral in the upper mantle, in various oscillation periods (0.5 -1000 s) at 1373 K and 3 GPa (70 km depth in the Earth). The newly built short-period cyclic loading system in situ X-ray observation was used in our experimental procedures (Yoshino et al., 2016).
The experimental results show that the energy dispersion increases with increasing period of oscillation, decreasing grain size, and increasing the water content of olivine, while the energy dispersion reduces its Young’s modulus. When water content increases, an attenuation peak appears at the short period (5~10 s). The modified generalized Burger’s model was used to acquire the effect of water content and grain size on attenuation and velocity of seismic wave quantitatively (Jackson et al., 2005). The fitting results show that there are two contributions from the high-temperature background (diffusion accommodated grain boundary sliding, DGBS) and attenuation peak to the energy dispersion. The characteristic relaxation time for DGBS showed a mild frequency dependence (0.33), which is consistent with the results of the previous study. The water could enhance the relaxation time with an exponent factor of 2.5(0.1) for DGBS. For the attenuation peak, water could enhance the peak height with an exponent factor of 1.2(1).
Takeuchi et al. (2017) found little frequency-dependent attenuation beneath the old oceanic floor, which is consistent with the attenuation behavior of hydrous samples showing an attenuation peak at a short period. The present results demonstrate that the old lithosphere with less than 1 wt.ppm H2O overlying hydrous asthenosphere (100 wt. ppm H2O) can produce the observed velocity drop (5-10 %). The difference of seismic properties between the asthenosphere and the old oceanic lithosphere can be explained by a remarkable difference in water content between them.