Water weakening of clinopyroxenite in diffusion creep

We performed triaxial compressive creep tests to study water weakening of clinopyroxenite in the diffusion creep regime. All tests were carried out on fine-grained samples at confining pressures between 100 and 300 MPa at temperatures of 1321 to 1421 K for water-saturated and 1398 to 1508 K for anhydrous conditions. Samples were prepared by hot-pressing ground powder of Sleaford Bay clinopyroxenite. Water was added to the sample during the run by dehydration of a talc sleeve. Water-saturated aggregates crept ∼30 times faster than aggregates under anhydrous conditions at a temperature of 1400 K and a pressure of 300 MPa. The stress exponent in both cases was ∼1, and the activation energies under water-saturated and anhydrous conditions were 340 ± 30 and 760 ± 20 kJ/mol, respectively. Under water-saturated conditions creep rate increased with increasing water fugacity to the power 1.4 ± 0.2 with an activation volume for creep of 14 ± 6 × 10^-6 m³/mol. Comparison of the water weakening in aggregates of clinopyroxene, olivine, and anorthite deformed in the diffusion creep regime indicates that the creep strength of clinopyroxene has a stronger dependence on the water fugacity than do the creep strengths of olivine and anorthite. In water-rich environments, clinopyroxene will be the weakest component in polycrystalline rocks composed of these phases.