[S-IT16] Structure and Dynamics of Earth and Planetary Mantles

[SIT16-03] Olivine fabric development during the corner flow in diffusion creep

*NAHYEON KIM¹, Takehiko Hiraga¹ (1.Earthquake Research Institute, University of Tokyo)

Keywords:Corner flow, MOR, CPO, Diffusion creep, Polycrystalline rock deformation , Laboratory experiments

Seismic anisotropy observed in the upper mantle is often explained by crystallographic preferred orientation (CPO) of olivine which is the most abundant mineral in the upper mantle. The CPO has been observed in laboratory rock experiments under both dislocation creep and diffusion creep conditions (e.g., Karato, 2008; Hansen et al., 2014; Miyazaki et al., 2013), and it is crucial to distinguish the dominant creep mechanism in understanding the mantle rheology. Many laboratory experiments with various geometry of deformation (e.g., triaxial compression or tension, simple shear, and torsion test) have been conducted and they reproduced the high strain zone in nature. However, rock deformation with corner-flow geometry has not performed yet. The corner-flow geometry is associated with continuous change in flow direction during the flow and it is often assumed to appear beneath mid-ocean ridges (MORs) or subducting plates. In this study, we did the corner-flow deformation experiments with fine-grained forsterite+diopside(10%) sample under diffusion creep condition to see the evolution of olivine CPO during the corner flow in diffusion creep. The corner-flow experiment was produced using ECAP (equal channel angular pressing) die that consist of two equidimensional channels intersecting at 90°. Shear occurs where flow direction changes along the channels and the strain accumulates until the change in flow ceases. In order to see the flow pattern within the sample, we also put strain markers (Au sphere (~5um), 3vol%) when synthesizing the forsterite+diopside(10%) sample. After experiments, we could identify streamlines defined by the arrangement of Au spheres on the sample surface and we analyzed marker strain or olivine CPO along the streamlines (especially along the most central streamline). The analysis of the strain markers suggests, after full corner-flow, the shear strain (γ) is 2.4. In the samples deformed at T>0.92Tm, AG- or A- type CPO has developed. The development of the CPO is related to the alignment of forsterites that show anisotropy in grain shapes at T>0.92Tm. The CPO rotates counterclockwise along the streamlines and the rotation of CPO shows good agreement with the rotation of the strain marker (S1). The CPO becomes stronger along the streamline and finally shows moderate to strong fabric strength (J~5.5) after the corner-flow is fully achieved. This is different from the dislocation-dominated creep mechanism where a much larger strain is required to produce the moderate to strong fabric strength and to produce a relatively good agreement between the rotation of CPO and strain marker. The theoretical shear strain for 90° corner-flow geometry (e.g., MORs) does not exceed γ=2 and we propose that if the olivine deforms in diffusion creep condition it is large enough strain to produce moderate to strong CPO in the upper mantle (A- and AG- type).