2:30 PM - 2:45 PM
[SIT28-04] Experimental study of polycrystal anelasticity at near-solidus temperatures and its seismological applications
Keywords:anelasticity, partial melting, seismic attenuation, seismic low velocity, LAB
For a quantitative interpretation of the seismic velocity and attenuation structures in the upper mantle, we need to clarify the rock anelasticity [e.g., Jackson et al. 2002]. In particular, scaling law to extrapolate experimental results to the mantle is necessary. Polycrystal anelasticity follows the Maxwell frequency scaling Q-1(f/fM) with fM = unrelaxed elastic modulus / diffusion creep viscosity [Morris and Jackson 2009; McCarthy et al. 2011]. However, the applicability of this scaling law is limited to f/fM < 104 [Takei et al. 2014], and the scaling law applicable to the seismic frequency range (106 =< f/fM =< 109) has been unknown.
We made an experimental approach to the polycrystal anelasticity at near-solidus temperatures by using a rock analogue (organic polycrystals) and found that the deviation from the Maxwell frequency scaling at high normalized frequencies can be described by using homologous temperature T/Tm, where Tm represents solidus [Yamauchi and Takei 2016]. The most remarkable finding is that polycrystal anelasticity is significantly enhanced just below the solidus temperature (0.94 < T/Tm < 1) in the absence of melt. Viscosity is also reduced in the same temperature range. These changes, which are caused by a solid-state mechanism, were large even for the samples which generate very small melt fraction (< 1%) at T = Tm. In contrast, when melt fraction is small (< 1%), the effects of melt generation at T =Tm on elasticity, anelasticity, and viscosity were negligibly small. We established a new anelasticity model by parameterizing these experimental data.
The applicability of this new model to the mantle was shown by the fitting to the horizontal profiles of seismic shear wave velocity in the Pacific mantle at 50 and 75 km depths, which shows a steep reduction of VS just below the solidus temperature [Priestley and McKenzie 2013]. Then, we applied the new anelasticity model to the vertical profiles of VS showing a discontinuous (steep) reduction at LAB; we used the temperature profiles calculated by the plate-cooling model and the solidus temperature calculated by assuming various distributions of volatile (H2O). The new anelasticity model enables us to interpret these seismological structures, including the seismic discontinuity, by the solid-state mechanism at near-solidus temperatures without invoking melt.
We made an experimental approach to the polycrystal anelasticity at near-solidus temperatures by using a rock analogue (organic polycrystals) and found that the deviation from the Maxwell frequency scaling at high normalized frequencies can be described by using homologous temperature T/Tm, where Tm represents solidus [Yamauchi and Takei 2016]. The most remarkable finding is that polycrystal anelasticity is significantly enhanced just below the solidus temperature (0.94 < T/Tm < 1) in the absence of melt. Viscosity is also reduced in the same temperature range. These changes, which are caused by a solid-state mechanism, were large even for the samples which generate very small melt fraction (< 1%) at T = Tm. In contrast, when melt fraction is small (< 1%), the effects of melt generation at T =Tm on elasticity, anelasticity, and viscosity were negligibly small. We established a new anelasticity model by parameterizing these experimental data.
The applicability of this new model to the mantle was shown by the fitting to the horizontal profiles of seismic shear wave velocity in the Pacific mantle at 50 and 75 km depths, which shows a steep reduction of VS just below the solidus temperature [Priestley and McKenzie 2013]. Then, we applied the new anelasticity model to the vertical profiles of VS showing a discontinuous (steep) reduction at LAB; we used the temperature profiles calculated by the plate-cooling model and the solidus temperature calculated by assuming various distributions of volatile (H2O). The new anelasticity model enables us to interpret these seismological structures, including the seismic discontinuity, by the solid-state mechanism at near-solidus temperatures without invoking melt.