The impact of water on slip system activity in olivine and the formation of bimodal crystallographic preferred orientations

• Published in: Earth and planetary science letters 2019-02, Vol.508, p.51-61
• Main Authors: Wallis, D., Hansen, L.N., Tasaka, M., Kumamoto, K.M., Parsons, A.J., Lloyd, G.E., Kohlstedt, D.L., Wilkinson, A.J.
• Format: Article
• Language: English
• Subjects: crystallographic preferred orientation; dislocation slip system; HR-EBSD; olivine; seismic properties; VPSC
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2018.12.007

Crystallographic preferred orientations (CPOs) in olivine are widely used to infer the mechanisms, conditions, and kinematics of deformation of mantle rocks. Recent experiments on water-saturated olivine were the first to produce a complex CPO characterised by bimodal orientation distributions of both [100] and [001] axes and inferred to form by combined activity of (001)[100], (100)[001], and (010)[100] slip. This result potentially provides a new microstructural indicator of deformation in the presence of elevated concentrations of intracrystalline hydrous point defects and has implications for the interpretation of seismic anisotropy. Here, we document a previously unexplained natural example of this CPO type in a xenolith from Lesotho and demonstrate that it too may be explained by elevated concentrations of hydrous point defects. We test and confirm the hypothesis that combined (001)[100], (100)[001], and (010)[100] slip were responsible for formation of this CPO by (1) using high-angular resolution electron backscatter diffraction to precisely characterise the dislocation types present in both the experimental and natural samples and (2) employing visco-plastic self-consistent simulations of CPO evolution to assess the ability of these slip systems to generate the observed CPO. Finally, we utilise calculations based on effective-medium theory to predict the anisotropy of seismic wave velocities arising from the CPO of the xenolith. Maxima in S-wave velocities and anisotropy are parallel to both the shear direction and shear plane normal, whereas maxima in P-wave velocities are oblique to both, adding complexity to interpretation of deformation kinematics from seismic anisotropy. •Unusual bimodal CPO of olivine in experiments and nature explained.•HR-EBSD and VPSC constrain dislocation types and activity in hydrous and anhydrous olivine.•Intracrystalline hydrous point defects increase slip on (001)[100] and (100)[001].•Complex relationship between seismic properties and deformation kinematics.