科研之友
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摘要
Eclogite rheology, an essential control on the mechanics of subduction zones, is probably governed by the mechanical properties of the volumetrically dominant constituent, omphacite. Triaxial compression tests were performed in a solid-medium apparatus on synthetic aggregates with five compositions along the solid-solution series jadeite-diopside. The bulk water content of the synthetic material is of the order of 10(-2) H/Si. The deformation experiments were performed at a confining pressure of 2 center dot 5 GPa and temperatures between 900 and 1100 degrees C (i.e. conditions in the stability field of jadeite). Strain rates ranged from 10(-6) to 10(-4) s(-1). For a given composition, strength decreases with increasing temperature and decreasing strain rate. The relation between strength and composition changes with increasing temperature. Whereas jadeite is the weaker endmember at all explored conditions, intermediate compositions are stronger than either endmember at 900 degrees C and 1000 degrees C. Samples with calcic compositions deformed at differential stresses above 500 MPa have abundant mechanical (100)[001] twins. In contrast, almost all deformation steps on jadeite aggregates yield strengths below 500 MPa and the resulting microstructures are dominated by well-developed subgrain boundaries and locally recrystallized grains. A weak crystallographic preferred orientation is developed in the deformed samples. Combining mechanical data and observed microstructures we hypothesize that the rate-controlling deformation mechanism differs for the two compositional endmembers. Deformation of diopside aggregates is controlled by dislocation glide, whereas the kinetics of dislocation climb controls the deformation of jadeite aggregates at the explored conditions. Reasonable parameters are found when fitting corresponding, micromechanically motivated flow laws to our data. The variation in strength with composition obeys scaling based on the dependence of melting temperature and shear modulus on composition. Extrapolation of the constrained flow laws to geological strain rates indicates considerable strength for omphacite aggregates at temperatures up to 1000 degrees C. Continuous deformation of eclogites by dislocation creep therefore appears unlikely in subduction zones. We suspect that exhumed eclogites exhibiting microstructures indicative of crystal plastic deformation experienced high-stress episodes related to seismic events.
- 出版日期2010-4
