The Role of Orthopyroxene in the Deformation of the Upper Mantle

Abstract:

Orthopyroxene is the second most abundant mineral in Earth’s upper mantle and is an important constituent of other planetary lithospheres. Geological evidence suggests that orthopyroxene plays a significant role in the deformation of mantle shear zones. However, in comparison to olivine, relatively little is known about its mechanical behavior. In this contribution, geologic and experimental data on orthopyroxene are assessed, with particular attention to shear localization phenomena in the upper mantle. Orthopyroxene contributes to upper mantle deformation in two ways. First, orthopyroxene acts as the primary pinning phase, suppressing olivine grain-growth, leading to long-lived zones of weakness where olivine deforms by grain-size sensitive mechanisms such as grain boundary sliding. Second, orthopyroxene may contribute to deformation through its own intrinsic rheological properties. Using experiments at high temperature and pressure we have explored the serial processes of dynamic recrystallization and phase mixing in olivine – orthopyroxene aggregates. We show that phase mixing is an inefficient process and that large strains are required to achieve microstructural and rheological steady state. We conclude weakening associated with dynamic recrystallization and phase mixing is a secondary process that contributes to the longevity of shear zones, but not their initial formation. We have also revisited some previous experimental results and derived flow laws and deformation mechanism maps for the grain-size sensitive deformation of orthopyroxene. We show that orthopyroxene deforms by diffusion creep over a wide range of upper mantle conditions.