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Mantle convection and large‐scale plate motion depend critically on the nature of the lithosphere‐asthenosphere boundary and thus on the viscosity structure of Earth's upper mantle, which is determined by the rheological properties of its constituent minerals. To constrain the flow behavior of orthopyroxene, the second most abundant constituent of the upper mantle, deformation experiments were carried out in triaxial compressive creep on fine‐grained (~6 μm) samples of enstatite at high pressures (3.8–6.3 GPa) and high temperatures (1323–1573 K) using a deformation‐DIA apparatus. Based on results from this study, the deformation behavior of enstatite is quantitatively presented in the form of a flow law that describes the dependence of deformation rate on differential stress, water fugacity, temperature, and pressure. Specifically, the creep rate depends approximately linearly on stress, indicating deformation in the diffusion creep regime. A least squares regression fit to our data yielded a flow law for diffusion creep with an activation energy of ~200 kJ/mol and an activation volume of ~14 × 10−6 m3/mol. The magnitude of the water‐weakening effect is similar to that for olivine with a water fugacity exponent of r ≈ 0.7. This strong dependence of viscosity on water fugacity (concentration) indicates that the viscosity of an orthopyroxene‐bearing mantle varies from one geological setting to another, depending on the large‐scale water distribution. Based on the rheology contrast between olivine and enstatite, we conclude that olivine is weaker than enstatite throughout most of the upper mantle except in some shallow regions in the diffusion creep regime.

To investigate the effect of water on the rheological properties of enstatite, we have conducted triaxial compressive creep experiments on enstatite aggregates using a gas medium apparatus at a confining pressure of 300 MPa and temperatures of 1373–1473 K under both water‐saturated and anhydrous conditions. Samples were mainly deformed in the dislocation creep regime; analyses of mechanical data yield activation energies of 603 and 567 kJ/mol for hydrous and anhydrous conditions, respectively. Under similar differential stress and temperature, the creep rate of enstatite under water‐saturated conditions is a factor of ~50 greater than that under anhydrous conditions. Based on a comparison of creep strength between olivine and enstatite in the dislocation creep regime, our results suggest that, at least under limited pressure and temperature conditions, enstatite is weaker than olivine. The results from this study provide a solid database for modeling geological processes occurring within Earth's interior.

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