[S-MP27] 変形岩・変成岩とテクトニクス

[SMP27-16] Olivine morphology and fabric during diffusion creep: Pure shear experiments

*NAHYEON KIM¹、AKIHIRO ANDO¹、Kosuke Yabe¹、Takehiko Hiraga¹ (1.Earthquake Research Institute, University of Tokyo)

キーワード：olivine texture, diffusion creep , grain morphology

We deformed aggregates of Fe-free olivine + 20 vol% diopside in pure shear to study olivine grain morphology and fabric during diffusion creep. Samples were deformed to nearly uniform strains of ~0.7 at different temperatures and experimental durations, which resulted in different grain sizes among the samples due to grain growth. The sample microstructures were observed from each of the three principal stress directions, which revealed various 3D grain morphologies among the samples. Samples with tabular grains displayed a crystallographic preferred orientation (CPO) in olivine, while samples with weakly anisotropic grains had random CPOs. The CPO patterns vary from AG-type (i.e., a uniaxial [010] concentration in the direction of sample shortening) to A-type (i.e., [010] and [100] concentrations in the directions of sample shortening and stretching, respectively), with intermediate types being the most common. The transition from AG- to A-type fabrics is accompanied by a change from oblate to prolate grain morphologies. We explain the direct correlation between the olivine morphology and fabrics based on an inter-granular slip process, which involves preferential grain-boundary sliding at boundaries with fewer steps (ledges). Oblate morphologies with well-developed grain boundaries oriented parallel to (010) indicate slip in the <h0l>{010} system, while relatively prolate morphologies with boundaries parallel to (010) that are elongated along [100] indicates the <100>{010} slip system. Boundaries form parallel to a particular crystallographic plane through anisotropic grain growth that depends on their crystallographic orientation. We propose such grain growth to be the cause of fabric transitions during diffusion creep.