The Effect of Melt on the Creep Strength of Polycrystalline Olivine

Currently deformation experiments suggest that small amounts of melt have only a minor effect on the rheology of olivine-dominated upper mantle rocks. One problem with the interpretation of these experiments is that the reference material for melt-free behavior is itself only nominally melt-free, containing up to 1 % melt. In order to better isolate the possible effects of melt, deformation experiments were conducted on fine-grained (3 - 6 micron), fully synthetic Fo90 olivine aggregates in a gas medium (Paterson) apparatus at 300 MPa confining pressure. The strain rates of the solution-gelation-derived and therefore genuinely melt-free, dry samples are about two orders of magnitude lower than the strain rates of nominally melt-free aggregates at the same pressure and temperature conditions and grain size. Benchmark deformation tests with Anita Bay dunite and mild steel reproduce published data. The creep strength of melt-added sol-gel olivine is similar to the published creep strength of dry, melt-bearing olivine derived from natural rocks. Non- linear least squares fits to the melt-free deformation data give an activation energy of 484 kJ/mol, a stress exponent of 1.4 and a grain size exponent of 3 over a temperature range from 1150 - 1360 deg. C and stresses from 15 - 210 MPa. These results suggest that small amounts of melt may be similarly effective in reducing the creep strength of upper mantle rocks as small amounts of water.