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 aggregates in the diffusion creep regime. Both nominally melt‐free and melt‐added samples display stress exponents ( n = 1.0 ± 0.1) and grain size 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 activation energy for creep of the nominally melt‐free aggregates is 315±35 kJ/mol. An abrupt change in the rheological behavior of the partially molten aggregates occurs at a melt fraction of 0.05. Below this 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‐added samples is enhanced by a factor of ∼3 relative to that of melt‐free aggregates. This result is consistent with previously published theoretical models for solution‐precipitation enhanced grain boundary diffusion creep in which the melt phase is present along three‐grain junction tubules and four‐grain junction corners. A comparison with published diffusion data indicates that deformation is limited by transport of Si along melt‐free grain boundaries under both melt‐free and melt‐present conditions. At melt fractions above ∼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 ∼25 relative to that of melt‐free aggregates. Micro structural observations of both hot‐pressed and deformed aggregates with melt fractions greater than ∼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. The presence of wetted grain boundaries at melt fractions less than ∼0.19 indicates that the melt topology in the olivine‐basalt system is affected by anisotropic interfacial energies. There is no difference in the strength of partially molten aggregates deformed with or without added water. This result is consistent with the observation that the solubility of water in basalt is ∼3 orders of magnitude greater than that in olivine and supports the conclusion that deformation is limited by transport along melt‐free grain boundaries at all conditions tested.