科研之友
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摘要
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 (similar to 6m) samples of enstatite at high pressures (3.8-6.3GPa) and high temperatures (1323-1573K) 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 similar to 200kJ/mol and an activation volume of similar to 14x10(-6)m(3)/mol. The magnitude of the water-weakening effect is similar to that for olivine with a water fugacity exponent of r approximate to 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.
- 出版日期2017-10
- 单位中国科学院; 中国科学院广州地球化学研究所; 同位素地球化学国家重点实验室; 中国科学院大学
