Geodynamic models of deformation in the Earthís mantle

The two-phase flow equations that describe fluid flow through a porous medium require knowledge of both a shear and bulk viscosity of the matrix. A principle difficulty in applying these equations rests in the difficulty of separating these two viscosities. In order to approximate the bulk viscosity of the olivine + basalt system, we have deformed synthetic peridotites samples around rigid zirconia inclusions in triaxial compression and simple shear. After McKenzie and Holness (2000), if the ratio of bulk to shear viscosity is high, then compaction will not occur and the matrix material will deflect off the sphere. However, if this ratio is reversed, compaction will occur. Melt will then be expelled that then flows down the pressure gradient created by the rigid sphere. This forms a pressure shadow in the plane orthogonal to s1. Our samples are composed of 95% fine-grained San Carlos olivine and 5% MORB with an average of 30 small zirconia inclusions. Compression samples were deformed to strains between 10-20% strain, and shear samples were deformed to 250% strain. All experiments occurred at 1523 K and 300 MPa confining pressure in a Paterson type gas-medium pressure apparatus with differential stresses ranging from 8.5 to 100 MPa. Our results indicate that significant migration of melt does occur under these conditions, implying that the ratio of bulk to shear viscosity is high. Efforts to decrease compaction length by reducing permeability with the addition of 2% FeS show decreased melt migration.  Samples were deformed to varying strains in order to determine the strain effect on pressure shadow equilibrium. Samples that underwent the most strain showed the most melt migration, although it is unclear whether the largest strain samples represent a steady state.