Stress measurements of deforming olivine at high pressure

Rheological properties of mantle minerals are critical for understanding the dynamics of the Earth’s deep interior. Due to limitations in experimental technique, previous quantitative studies of the rheological properties of mantle minerals are limited to either low pressure or low temperature. The present understanding of mantle flow is mostly inferred from the extrapolation of relatively low-pressure data to mantle high-pressure conditions. However, the effect of pressure (represented by activation volume) on the rheological properties of olivine is still controversial. Therefore, deformation experiments, carried out at mantle pressures, are necessary to understand and model mantle flow. Here we report an experimental study of plastic deformation of San Carlos olivine (Mg, Fe)2SiO4 under upper mantle conditions. Macroscopic differential stress and strain rates have been measured in situ in a large-volume high-pressure apparatus using newly developed techniques. The differential stress at high temperature and high pressure that we measured is significantly lower than that estimated by many currently accepted olivine flow laws. We document the first in situ experimental differential stress results in a multi-anvil press. Our results give direct evidence for a relatively small activation volume (less than 10−5 m3 mol−1). This shows that the effect of pressure on dislocation creep is small.