Shear Strength, Polymorphism, and Mechanical Behavior of Olivine, Enstatite, Diopside, Labradorite, and Pyrope Garnet: Tests to 920C and 60 Kb

摘要

Four hundred and seventeen shear and compression tests were completed on five minerals: olivine, enstatite, diopside, labradorite, and pyrope garnet. These minerals are assumed to represent the principal constituents of an upper mantle rock model. Petrographic examination and x-ray analyses indicate that the minerals deform cataclastically at low pressures and temperatures, with intragranular flow beginning at 30 kb normal pressure. Temperature enhances ductility, indicated at high pressures by x-ray line broadening on diffraction patterns and the appearance of kink bands in thin sections. Two phenomena of geophysical importance have been discovered during shear testing: (1) dehydration of less than 5 percent by volume of serpentine weakens dunite by more than 30 percent as the temperature is raised from 300C to 520C; (2) orthorhombic enstatite readily inverts to monoclinic polymorph under the influence of shear stress at all pressures and the temperatures to 920C and 50 kb. The inversion is accompanied by a small, rapid change in volume, sufficient to yield seismic energy for the production of a moderate earthquake. (Author)