|
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 ~8 μm and dihedral angles between ~45¿ and 60¿ were fabricated by hydrostatically hot-pressing powders of San Carlos olivine plus ~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 ~2 μm were prepared from either San Carlos olivine powders of synthetic olivine powders. These samples were deformed at 1100¿ and/or 1200 ¿C under a confining pressure of 300 MPa at strain rates between 10-6 and 10-4 s-1. At 1100 ¿C, the two-phase samples were either weaker than or comparable in strength to the single-phase samples. At 1200 ¿C, the two-phase samples were consistently weaker than the single-phase samples. Stress exponents of n~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, ϵ˙∝fO21/3, 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. ¿ American Geophysical Union 1993 |
