A Global Compilation of Olivine and Orthopyroxene Crystallographic Preferred Orientations and Their Relationship to Deformation Conditions in the Lithospheric Mantle

Rachel Bernard, Amherst College
Whitney Behr, ETH Zürich

Abstract

Olivine lattice or crystallographic preferred orientation (CPO) forms through viscous deformation and is thought to be the primary cause of upper mantle seismic anisotropy. Laboratory experiments suggest that deformation conditions (specifically water content, stress magnitude, pressure, and temperature) play key roles in the development of olivine CPO "types," which themselves directly influence how we infer mantle flow directions from observations of seismic anisotropy. Complicating matters, orthopyroxene -- the second most abundant mineral in the upper mantle -- also forms CPO types that may influence seismic anisotropy. While much less research has been devoted to the types of CPO formed in orthopyroxene, experiments have pointed to water content and modal percentage as key factors in their development.

This talk highlights results of a study utilizing 65 peridotites from a wide range of tectonic settings to investigate whether naturally deformed rocks agree with these experiments. The samples investigated here were also combined with an extensive literature review of over 500 published samples from 52 localities. They reveal that water content, stress magnitude, pressure, temperature, and modal percentage may affect olivine and orthopyroxene CPO development less in nature than deformation history and geometry. Additionally, through calculations of seismic velocities and anisotropy using electron backscattered diffraction (EBSD) data, we find that while orthopyroxene decreases the anisotropic signal overall, the relationship between seismic anisotropy and flow direction is determined by olivine CPO alone (in most circumstances). The complex relationships found between olivine CPO and deformation conditions may explain the lack of agreement between geophysical models and seismic anisotropy patterns in the continental mantle lithosphere. This study highlights the difficulty in using olivine or orthopyroxene CPO in natural peridotites to infer deformation conditions as well as the need for further rock deformation experiments.

Session

Session 2: Rheology of the Lithosphere