Deformation experiments on orthoenstatite aggregate at upper mantle pressures and temperatures under hydrous conditions

Water weakening of nominally anhydrous minerals of upper mantle is important for understanding the rheological structure of Earth’s interior. Enstatite is the 2nd dominant phase in the upper mantle, next to olivine. The partition coefficient for water between olivine and enstatite aggregates C^ol_OH/C^en_OH is ~0.5 at 3.8‒6.3 GPa and 1323‒1573 K (Zhang et al., 2017; JGR), indicating that water weakening of enstatite effectively proceeds in the olivine-enstatite system. The water weakening of enstatite would be accelerated at high pressures, since the amount of water dissolved into enstatite drastically increase with pressure.

We hereby experimentally evaluated the creep strength of wet orthoenstatite aggregates under pressure and temperature conditions at 1.9‒5.3 GPa and 1200‒1380 K using a deformation DIA apparatus combined with synchrotron X-ray radiation. At a constant strain rate ranging from 6.7 × 10^-6 to 9.4 × 10^-5 s^-1, steady-state creep strength of wet orthoenstatite followed the power-law flow law with the stress exponent of ~3, indicating deformation in the dislocation creep regime. Our results show dislocation creep rate of wet orthoenstatite is ~1 order of magnitude faster than dry orthoenstatite under the same P-T conditions. FTIR spectra from the recovered samples indicate that the amount of dissolved water in orthoenstatite is up to 1370 ppm wt.%. The dependence of strain rate on water fugacity was determined with the water fugacity exponent of ~1. Depending on the water content in the upper mantle, dislocation creep of wet orthoenstatite could lead to strain localization in the lithosphere.