Abstract
Interpretation of seismic velocity and attenuation in partially molten
rocks has been limited, with few exceptions, to models that assume
the melt to be distributed either as spheres or as thin films. However,
other melt phase geometries, such as interconnected tubes along grain
edges, might equally well account for seismic observations if there
is a much larger fraction of melt. Seismic velocity and attenuation
are estimated in rocks in which the melt phase has the tube geometry,
and the results are compared with results expected for the more familiar
film model under similar conditions. For a given melt fraction, tubes
are found to give moduli intermediate between moduli for rigid spherical
inclusions and compliant films. For example, in polycrystalline olivine
at 20 kbar the model predicts a decrease in Vs of 10\% and a decrease
in Vp of 5\% at 0.05 melt fraction, without considering inelastic
relaxation. Shear attenuation appears to be dominated by viscous
flow of melt between the tubes and/or films. For olivine the tube
model predicts the increment of relaxation due to melt, (Delta mu)/mu,
to be 0.01 at 0.05 melt fraction. Relaxation of the bulk modulus
is dominated by flow between melt pockets of different shape, heat
flow, and solid-melt phase change. If melt is present, considerable
bulk attenuation is expected, although the relaxation may be observable
only at long periods, outside the seismic body wave band.
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