Hot-pressing experiments have been used to study the effect of a chemically and texturally equilibrated liquid phase on the diffusional creep properties of polycrystalline olivine. Densification (creep) rate data on polycrystalline olivine both with and without the liquid basalt were collected as functions of applied stress/pressure (5-30 MPa), temperature (1300°-1400°C) and grain size (3-13 μm). Olivine-liquid basalt aggregates creep at rates which are persistently a factor of 2-5 greater than similar, melt-free specimens. Microstructural observations of dense olivine-basalt specimens indicate that the creep rate is enhanced by a solution-precipitation (pressure solution) mechanism during the densification of these specimens. Yet, the stress exponent (n = 1), grain-size exponent (m = 3) and average thermal activation energy (Q ≅ 380 kJ/mole) determined for the densification experiments are the same for olivine and olivine-basalt specimens. These observations suggest that because melt does not wet grain boundaries the kinetics of deformation of partially molten olivine-basalt aggregates are rate-limited by diffusive matter transport through melt-free olivine grain boundaries. The creep rate enhancement measured for olivine-basalt specimens occurs because the diffusive path length is effectively reduced by the presence of melt at triple junctions.
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The authors are pleased to acknowledge helpful discussions with Professors R. Raj and R.W. Kay. Mr. G. Chevalier provided technical assistance with the densification experiments. The National Science Foundation is gratefully acknowledged for its support through grant #81-15692.