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Effect of partial melting on seismic velocity and attenuation: Polycrystal anelasticity at near-solidus temperatures

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MIMW03 - From the continuum to the tectonic: the magma/mantle dynamics of planet earth

Co-author: Hatsuki Yamauchi (ERI, Univ. of Tokyo)

Seismic low velocity regions have been detected around the volcanic source regions in the upper mantle, where partial melting is expected to occur. However, temperature of these regions is, for the most part, below the solidus temperature of dry mantle peridotite. This suggests that seismic wave velocity is significantly reduced in the absence of melt or in the presence of a very small amount of melt stabilized by volatiles. Effects of partial melting on the seismic velocity and attenuation have long been studied within the framework of the direct effect of the melt phase, such as poroelastic effect. However, the direct effect is small for small melt fraction, and is difficult to explain the relatively large velocity reduction observed in these regions. Rock anelasticity, which can cause low velocity by grain boundary sliding even without the melt phase, has been considered as a key to solve this problem. However, due to the difficulty of high temperature experiment, we have had a limited understanding of rock anelasticity at the seismic frequencies. We therefore measured anelasticity by using a rock analogue (organic polycrystals). Elasticity, anelasticity, and viscosity were measured continuously from below to above the solidus temperature and the mechanical behavior at near solidus temperatures was clarified over a broad frequency range. The obtained data predict that a steep reduction of seismic shear wave velocity occurs just below the solidus temperature in the absence of melt. Our data also show that the seismic properties are not sensitive to the existence or nonexistence of a very small amount of melt, whereas more than 1 percent melt can cause additional velocity reduction depending on the melt fraction. 

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