Dislocation Interactions in Olivine Revealed by HR-EBSD

Abstract

Interactions between dislocations potentially provide a control on strain rates produced bydislocation motion during creep of rocks at high temperatures. However, it has been difficult to establishthe dominant types of interactions and their influence on the rheological properties of creeping rocks due toa lack of suitable observational techniques. We apply high-angular resolution electron backscatter diffraction tomap geometrically necessary dislocation (GND) density, elastic strain, and residual stress in experimentallydeformed single crystals of olivine. Short-range interactions are revealed by cross correlation of GND densitymaps. Spatial correlations between dislocation types indicate that noncollinear interactions may impedemotion of proximal dislocations at temperatures of 1000°C and 1200°C. Long-range interactions are revealedby autocorrelation of GND density maps. These analyses reveal periodic variations in GND density and sign,with characteristic length scales on the order of 1–10 μm. These structures are spatially associated withvariations in elastic strain and residual stress on the order of 103and 100 MPa, respectively. Therefore,short-range interactions generate local accumulations of dislocations, leading to heterogeneous internalstress fields that influence dislocation motion over longer length scales. The impacts of these short- and/orlong-range interactions on dislocation velocities may therefore influence the strain rate of the bulk materialand are an important consideration for future models of dislocation-mediated deformation mechanisms inolivine. Establishing the types and impacts of dislocation interactions that occur across a range of laboratoryand natural deformation conditions will help to establish the reliability of extrapolating laboratory-derivedflow laws to real Earth conditions.