DEFORMATION OF FINE-GRAINED AGGREGATES OF OLIVINE PLUS MELT AT HIGH-TEMPERATURES AND PRESSURES

High-temperature, high-pressure constant displacement rate experiments have been performed to investigate the flow behavior of partially molten samples of fine-grained olivine-rich aggregates. Two-phase samples with a grain size of approximately 8 mum and dihedral angles between approximately 45-degrees and 60-degrees were fabricated by hydrostatically hot-pressing powders of San Carlos olivine plus approximately 2 to 9 vol % of four different synthetic silicate melts that contained Al and either Ca or Na as well as Mg and Fe. Single-phase olivine samples with a grain size of approximately 2 mum were prepared from either San Carlos olivine powders or synthetic olivine powders. These samples were deformed at 1100-degrees and/or 1200-degrees-C under a confining pressure of 300 MPa at strain rates between 10(-6) and 10(-4) s-1. At 1100-degrees-C, the two-phase samples were either weaker than or comparable in strength to the single-phase samples. At 1200-degrees-C, the two-phase samples were consistently weaker than the single-phase samples. Stress exponents of n approximately 4, as well as comparison with published creep results, demonstrate that both the single-phase samples and at least two of the two-phase samples deformed predominantly by dislocation creep. In this case, the melt phase would be expected to have only a small effect on flow strength, and the reduced strength probably resulted from a water weakening of the olivine grains. The other two partially molten samples were a factor of 2 to 3 weaker than the single phase samples, suggestive of a change from dislocation to diffusion creep with the addition of wetting melt phases. A comparison of the rheology of single-phase olivine aggregates deformed in Fe capsules with that of aggregates deformed in Ni capsules indicates that the strain rate increases with increasing oxygen fugacity, epsilon is-proportional-to f(O2)1/3 consistent with results published for olivine single crystals. On the basis of these experiments, it seems likely that the presence of a small amount of melt in a partially molten zone in the upper mantle will result in only a relatively minor localized reduction in rock strength.