Strength of orthoenstatite single crystals at mantle pressure and temperature and comparison with olivine

- Orthoenstatite single-crystal plasticity is investigated at mantle P and T conditions.
- Rheological laws for [001] dislocation slip in (100) and (010) planes are quantified.
- OPx [001](100) slip system is weaker than [001](010) slip system at mantle conditions.
- Olivine slip systems are weaker than OPx slip systems at most mantle conditions.
- Olivine and OPx weak slip systems have comparable strength in the shallow lithosphere.

Oriented single crystals of orthopyroxenes (OPx) were deformed in axisymmetric compression in the D-DIA at pressure and temperature in excess of 3 GPa and 1040 °C. Two crystal orientations were tested with the compression axis parallel to either [101]c crystallographic direction, to investigate [001](100) dislocation slip-system strength, or [011]c direction to investigate [001](010) slip-system strength. These slip systems are the most active in orthopyroxenes. Applied differential stresses and specimen strain rates were measured in situ by synchrotron X-ray diffraction and radiography. We used these data and comparison with previously reported low-pressure flow laws for protoenstatite and orthoenstatite to determine the power law parameters for the deformation of orthoenstatite crystals, which characterize OPx dislocation slip-system strengths. Applying these laws at reasonable mantle stresses along oceanic and continental geotherms indicates that OPx [001](100) slip system is weaker than OPx [001](010) slip system to ∼260 km depth where the strengths converge. It also indicates that both OPx slip systems are significantly stronger than olivine slip systems throughout the upper mantle, except in the upper most mantle, in the lithosphere, were OPx [001](100) slip system may be as weak or even weaker than olivine [100](010) easy slip system.