Deformation of fine‐grained aggregates of olivine plus melt at high temperatures and pressures

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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 or 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 1200deg;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, , 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.