DILATATIONAL ANELASTICITY IN PARTIAL MELTS - VISCOSITY, ATTENUATION, AND VELOCITY DISPERSION

Thermodynamic constraints on the geometry of melt/solid interfaces req uire that a texturally equilibrated partial melt respond to changes in mean normal stress through adjustment of the sizes of melt-bearing tr iple-grain junctions. This dilatational response is well-modeled as th at of a standard anelastic solid provided that the response rate is li mited by solid-state deformation processes attending the dilatation ra ther than by melt flow itself. Using this model, we have analyzed flex ural creep data obtained for a fine-grained partial melt comprising a MgSiO3 Polycrystalline solid phase in equilibrium with a sodium alumin osilicate melt. The anelastic dilatational (or ''bulk'') viscosity for this two-phase material is nearly an order of magnitude less than its shear viscosity, the latter being determined by diffusional creep. Th e corresponding modulus that controls the extent of dilatation is seve ral orders of magnitude smaller than the elastic bulk modulus of the t wo-phase aggregate. Applied to the case of harmonic loading, the model predicts a substantial band-limited dissipation spectrum for dilatati on (Q(K)-1) that would be absent but for the presence of the melt. Thi s creates a large and strong P wave absorption (Q(P)-1) band for the e nstatite material, accompanied by substantial P wave velocity dispersi on. Through this enhanced P wave attenuation, the presence of the melt phase suppresses P-to-S velocity ratios and produces equivalent Q val ues for the two modes.