Shear Deformation of Balsam Gap Dunite: Preliminary Results of High Pressure-Temperature Experiments in Large Volume Torsion Apparatuses

The Large Volume Torsion apparatuses (LVT1 and LVT2) are newly developed by and housed in the Rock Deformation Laboratory at Washington University in St. Louis. LVT1 and LVT2 are solid medium torsional deformation apparatuses capable of deforming specimens up to ~4.2 mm in diameter at pressure-temperature conditions relevant to Earth's upper mantle. These apparatuses are also capable of deforming specimens to extremely large strains (γ > 50). Here we showcase preliminary results from recent experiments on Balsam Gap dunite deformed to approximately 2.9 GPa and 1280°C with a shear strain rate of 5·10-6 s-1.

In addition to demonstrating the capabilities of these new rock deformation apparatuses, the purpose of these experiments is to characterize the crystallographic preferred orientation (CPO) of deformed olivine at a critical range of conditions. Simplified relationships between the CPO of olivine and mantle flow have been widely used to argue that the seismically fastest crystallographic axis (i.e. [100]) becomes aligned with the flow direction during simple shear. Hence, seismic anisotropy may be used to infer patterns of mantle flow. However, in tectonic settings where deformation conditions can change over relatively short timescales, such as at plate boundaries, the CPO may lag behind changes in kinematics, generating an apparent misalignment with the mantle flow direction. Understanding the relationships between seismic anisotropy and mantle flow near plate boundaries necessitates systematic investigations of the effects of pressure, temperature, and strain rate on the evolution of olivine CPOs from moderate to large strains (γ ~ 2-20+).