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.
Bibliographical noteFunding Information:
This research was supported by Strategic Priority Research Program (B) of Chinese Academy of Sciences (grants XDB18000000 and XDB06060300), National Natural Science Foundation of China (grants 41574079, 41174072, 41421062, and 41674097), and NASA grant NNX15AL53G. Experiments were carried out at the X17B2 beamline of the National Synchrotron Light Source and M6B at Advanced Photon Source. We thank W.B. Durham and N. Dixon for their help in preparing experiments, J. L. Mosenfelder for help on collecting FTIR spectra, W.L. Liu and D.P. Wen for help on conducting SEM-EBSD analyses, and H.Y. Chen for technical support at both beamlines. Importantly, thorough reviews by two anonymous reviewers greatly enhanced the quality of this contribution. Data used in this study can be found in Table 1. For more details, please refer to the corresponding author via e-mail contact. This is contribution No.IS-2442 from GIGCAS.
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- diffusion creep
- mantle viscosity
- water fugacity