KINETICS OF MELT MIGRATION IN UPPER MANTLE-TYPE ROCKS

Experiments have been performed to determine the permeability of an aggregate of olivine plus a silicate melt, as well as the viscosity of the matrix. Melt migration couples were formed between discs composed of olivine with about 12% of a synthetic potassium-aluminum silicate glass and discs of polycrystalline San Carlos olivine. Four melt infiltration experiments were carried out at temperatures between 1050 and 1255-degrees-C at 300 MPa in a gas-medium apparatus; each couple was held at the experimental conditions for 2 h. At temperature, capillary forces cause the molten glass to infiltrate into the dunite along triple junctions because the dihedral angle is less than 60-degrees. In order to analyze the resulting melt migration profiles, the coupled differential equations governing melt migration via "porous flow" driven by capillary forces and resisted by compaction/dilation of the matrix were solved numerically. The effects of dihedral angle, melt fraction exponent and amount of the melt in the source region on the spatial and temporal evolution of the melt distribution were investigated with numerical simulations for the initial and boundary conditions imposed by the experimental geometry. The permeability of the aggregate and the viscosity of the matrix were determined by comparison of the melt migration profiles obtained from the experiments with those generated from simulation. The permeability of the partially molten aggregate increased approximately linearly with increasing melt fraction and, at 1255-degrees-C, the permeability of the rock and viscosity of the olivine are about 8 x 10(-16) m2 and 8 x 10(9) Pa s, respectively, for a grain size of 4.2-mu-m and a melt fraction of 0.145. These results predict a relatively high permeability at low melt fractions, indicating that only very small amounts of melt (approximately 0.1%) could be maintained in a dunitic mantle.