JpGU-AGU Joint Meeting 2020

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP38] Deformed rocks, Metamorphic rocks and Tectonics

convener:Yoshihiro Nakamura(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Yumiko Harigane(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST))

[SMP38-01] Pure shear deformation experiments of polycrystalline olivine : Relationship between grain shape and crystallographic preferred orientation during diffusion creep

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

Keywords:Olivine, diffusion creep, grain shape, crystallographic preferred orientation, pure shear

The CPO has long been considered to result uniquely from dislocation processes, while recently, olivine CPO has been found to develop during diffusion creep. Mechanism of the CPO development during diffusion creep has been experimentally explored, which found the ubiquitous occurrence of rigid-body-like grain (lattice) rotation induced by grain-boundary sliding (GBS) during diffusion creep. Preferential GBS on crystallographically controlled low-index-plane grain boundaries promotes the rotation of low-index grain boundaries toward the shear direction, resulting in CPO. In the previous studies, the type of CPO pattern under shear was estimated based on the CPO developed under uniaxial compression and tension where the former and latter could characterize preferential slip (GBS) plane and direction, respectively. Obviously, such results only allow quantitative estimations of the CPO pattern and seismic anisotropy in the upper mantle. Pure shear diffusion experiments were conducted on polycrystalline olivine in this study. Three dimensional olivine grains shapes in the deformed samples are determined and explained based on grain boundary structures, which should allow me to examine the relationship between the grain boundary structures and the observed CPO patterns.

To deform olivine aggregate in pure shear at high temperature, a specially designed carbon die was used. Geometry of the die allowed the square prism shaped specimen with its long dimension in the Z direction, where the specimen was compressed from, to deform only in the X and Z directions. The die with the specimen was installed in the high temperature furnace which was attached to the Instron-type deformation apparatus. We performed experiments at 6 different temperatures (i.e., 1240℃, 1250℃, 1260℃, 1270℃, 1320℃ and 1350℃). I analyzed microstructures of the deformed samples by scanning electron microscopy (SEM), finding the concentration of olivine [100] in the X direction and [010] concentration in the Z direction in the high temperature (1350 and 1320℃) samples. Such CPO pattern corresponds to A-type olivine fabric. In the intermediate temperature (1270 and 1260℃) samples, the olivine [010] concentrates in the Z direction and the [100] and [001] distribute almost uniformly in X-Y plane (i.e., girdle distribution). In the low-temperature (1250 and 1240℃) samples, random CPO or very weak [001] and [100] concentrations in the Z and X directions, respectively, develop. The three-dimensional olivine grain shape was analyzed in the sample XY, YZ and XZ cross sections. I found systematic grain shape changes from prolate-type to oblate-type with decreasing temperature at the intermediate to high temperature conditions. In the low temperature samples, olivine grains have the expected shape of the pure shear geometry. Based on the observed grain shapes, I estimated development of low-index-plane grain boundaries parallel to (010) of olivine with the grain-boundary plane extended in the [100] direction at the high temperature. At the intermediate temperature, (010) grain boundaries with the similar extensions to the [100] and [001] directions develop. The low-index-plane grain boundaries little develop at the low temperature. Such grain boundary structures predict preferential GBS at [100](010) and [h0l](010) at the high and intermediate temperatures, respectively, while GBS occurs evenly among grain boundaries at the low temperature. The grain rotations due to such GBS result in A-type, AG-type and random CPO at the high, intermediate and low temperatures, respectively, during diffusion creep.
I considered the appearance of CPO types based on the temperature normalized by peridotite solidus in the oceanic upper mantle. At the shallow depth, A-type CPO develops, while AG-type develops at the medium depth. Random CPO develops at the deeper portion of the mantle.