Water Weakening Effects on Dislocation Creep of Polycrystalline Diopside Aggregates

To understand the dislocation creep behavior of water-saturated clinopyroxene in the upper mantle, we conducted high-temperature triaxial compression experiments on hot-pressed diopside aggregates under water-saturated conditions at confining pressures of ~300 MPa and temperatures of 1373–1473 K using a Paterson gas-medium apparatus. Fourier transform infrared measurements of the water contents revealed that all experiments were performed under water-saturated conditions. Fitting the mechanical data with a power flow law yielded a stress exponent n of 2.2 ± 0.6, an activation energy Q of 442 ± 33 kJ/mol, and a material-dependent parameter A of 106.9 ± 0.5 MPa−2.2 s−1. For comparison, a single deformation experiment was performed under anhydrous conditions at a temperature of 1473K. The mechanical results show that the water-saturated diopside aggregates deform approximately 1.5–3 orders of magnitude faster than their anhydrous counterpart, indicating a pronounced water-weakening effect. Furthermore, under water-saturated conditions, our mantle-derived diopside aggregates have comparable strengths to that of Fe-rich Sleaford Bay clinopyroxene at 1473 K and laboratory strain rates but significantly weaker than that of olivine aggregates. The results in this study provide key experimental constraints on the flow behavior of mantle-derived clinopyroxene aggregates under water-saturated conditions.