Maisy Waech

Deforming pyroxene and olivine in compression to establish the mechanism of mineral mixing at plate boundaries

The theory of plate tectonics is a paradigm of Earth science, and it provides a theoretical link among many of Earth’s processes. There is some consensus that a key process in the formation of plate-boundary shear zones is the production of a fine-grained mixture of multiple phases, which leads to macroscopic weakening and strain localization. Several models exist to explain how this mixing occurs. Previous work involved in testing and developing these models has focused on phase mixing in samples deformed in torsion. We examined the initiation of phase mixing in a simpler deformation geometry, uniaxial compression normal to an interface between olivine and orthopyroxene. We made two samples: one sample used natural San Carlos olivine, and the other used olivine synthesized from oxide powders, and both samples used synthesized orthopyroxene. Samples were deformed in a gas-medium Paterson apparatus by triaxial compression to 80% strain at 1250°C and 300 MPa confining pressure. After deformation experiments, backscatter-electron images were taken of the interface between the phases to investigate the boundary for mixing. We found no evidence of phase mixing in our compression experiments. However, there is some evidence for the formation of the intrusions of one phase into the other, consistent with the initial stages of one of the primary models for phase mixing. We can conclude that compression alone, normal to an interface, does not lead to mixing and therefore weakening.