EXPERIMENTAL CONSTRAINTS ON THE DYNAMICS OF THE PARTIALLY MOLTEN UPPER-MANTLE - DEFORMATION IN THE DIFFUSION CREEP REGIME

Deformation experiments have been conducted to investigate the effect of melt fraction and grain size on the creep behavior of olivine aggre gates in the diffusion creep regime. Both nominally melt-free and melt -added samples display stress exponents (n = 1.0 +/- 0.1) and grain si ze exponents (p = -3.0 +/- 0.5 for nominally melt-free, p = -3.2 +/- 1 .2 for melt-added) indicative of grain boundary diffusion creep. The a ctivation energy for creep of the nominally melt-free aggregates is 31 5 +/- 35 kJ/mol. An abrupt change in the theological behavior of the p artially molten aggregates occurs at a melt fraction of 0.05. Below th is melt fraction the influence of melt on strain rate is rather modest . For example, at a melt fraction of 0.04 the strain rate of melt-adde d samples is enhanced by a factor of similar to 3 relative to that of melt-free aggregates. This result is consistent with previously publis hed theoretical models for solution-precipitation enhanced grain bound ary diffusion creep in which the melt phase is present along three-gra in junction tubules and four-grain junction corners. A comparison with published diffusion data indicates that deformation is limited by tra nsport of Si along melt-free grain boundaries under both melt-free and melt-present conditions. At melt fractions above similar to 0.05, the strain rate enhancement is significantly greater than that predicted by the theoretical models. For example, at a melt fraction of 0.07 the strain rate of melt-added samples is enhanced by a factor of similar to 25 relative to that of melt-free aggregates. Microstructural observ ations of both hot-pressed and deformed aggregates with melt fractions greater than similar to 0.05 demonstrate that a significant number of two-grain boundaries are ''wetted'' by melt. These boundaries provide rapid transport paths not accounted for in the theoretical models. Th e presence of wetted grain boundaries at melt fractions less than simi lar to 0.19 indicates that the melt topology in the olivine-basalt sys tem is affected by anisotropic interfacial energies. There is no diffe rence in the strength of partially molten aggregates deformed with or without added water. This result is consistent with the observation th at the solubility of water in basalt is similar to 3 orders of magnitu de greater than that in olivine and supports the conclusion that defor mation is limited by transport along melt-free grain boundaries at all conditions tested.