[S-CG44] Rheology, fracture and friction in Earth and planetary sciences

[SCG44-P02] Crystallographic preferred orientation properties of (Mg, Fe)O polycrystals obtained from large strain deformation experiments under lower mantle pressures

*Bunrin Natsui¹, Shintaro Azuma¹, Keishi Okazaki², Kentaro Uesugi³, Masahiro Yasutake³, Saori Kawaguchi³, Ryuichi Nomura, Kenji Ohta¹ (1.Tokyo Institute of Technology, 2.Hiroshima University, 3.JASRI)

Keywords:Rheology, Deformation experiment, Lower mantle

Seismic tomography has shown that Large Low Shear Velocity Provinces (LLSVPs) with seismic anisotropy extend from the mid-lower to the lowest mantle beneath the African and Pacific. Seismic anisotropy possibly infers the development of crystallographic preferred orientation (CPO), and thus it is important to investigate the relationship between deformation and CPO development in constituent minerals of LLSVPs. However, few studies have discussed the relationship from experimental approaches due to the difficulty of deformation experiments under lower mantle pressure conditions. In addition, whether LLSVPs is thermal or compositional feature is still unclear. Previous studies suggest that the composition of LLSVPs must be 10% denser than that of the surrounding mantle in order to withstand stirring by mantle convection. Therefore, Fe-rich periclase and bridgmanite are candidate constituent minerals for LLSVPs. In this study, we performed large strain deformation experiments on (Mg, Fe)O polycrystals under lower mantle pressure conditions. The aim of this study is to clarify the relationship between the dominant slip system associated with the deformation of LLSVPs and its seismic anisotropy.
We performed torsional deformation experiments on (Mg, Fe)O polycrystals using rotational diamond anvil cells (rDAC). Pt markers for pre- and post-experimental strain measurements were placed in the sample parallel to the sample rotation axis by focused ion beam (FIB) deposition. All deformation experiments were conducted at BL47XU, SPring-8. Strain of deformed samples were determined from the reconstructed cross-sectional images of Pt markers obtained using X-ray laminography imaging techniques. The strain of the samples were determined from the reconstructed cross-sectional images obtained from the X-ray laminography method, and CPO determination of (Mg,Fe)O during deformation experiments were attempted from single angle (60° direction to the axis of rotation) XRD. We used Rietveld analysis with a crystal orientation distribution function (ODF) incorporated by MAUD to determine the CPO. In addition, we recovered samples after deformation experiments under reduced pressure and performed multi-angle (-35° to 35°) XRD measurements at SPring-8 BL10XU to determine the CPO as well.
We have deformed (Mg_0.8, Fe_0.2)O , (Mg_0.6, Fe_0.4)O , (Mg_0.1, Fe_0.9)Oat pressures of 5-75 GPa, temperatures of 300-700 K, and constant strain rates. The CPO results calculated from XRD of (Mg_0.6, Fe_0.4)O during deformation experiments and from decompression recovered samples are harmonic. Therefore, the effects of sample deformation and XRD pole figure coverage due to decompression are considered to be minor. The slip system during shear deformation under lower mantle pressure conditions is {100}<011>, which is harmonic with the results for high temperature and high pressure side of MgO. It is possible that the inclusion of Fe could activate the {100}<011> slip system in (Mg, Fe)O under lower pressure conditions than in MgO.