5:15 PM - 6:45 PM
[SIT14-P15] Deformation-induced crystallographic-preferred orientation of ε-FeOOH
Keywords:Seismic anisotropy, Crystallographic-preferred orientation, Dense hydrous magnesium silicates (DHMSs)
Ubiquitous presence of seismic anisotropy near subducted slab in the upper part of the Earth's lower mantle has been reported (e.g. Ferreira et al., 2019; Lynner and Long, 2015). Some of these anisotropy is not well explained by crystallographic-preferred orientation (CPO) of anhydrous major lower mantle minerals. Phase H (MgSiO2(OH)2), one of the dense hydrous magnesium silicates, is a candidate mineral which may produce the observed anisotropy, because hydrous minerals can be produced by reaction with water transported by subducted slab and phase H is known to have strong elastic anisotropy (Tsuchiya and Mookherjee, 2015). In this study, we have conducted high-pressure and high-temperature deformation experiments on ε-FeOOH which has same crystal structure as phase H and is stable at relatively lower pressures. Based on the results, we discuss the origin of the seismic anisotropy in the upper part of the lower mantle.
Deformation experiments were conducted using D111-type apparatus (Nishihara et al., 2020) installed at BL04B1, SPring-8. Starting materials for deformation runs were the pelletized powder of α-FeOOH (low pressure phase) or ε-FeOOH pre-synthesized at 12 GPa and 973 K. Uniaxial compression and tensile tests were carried out at ~12 GPa and 673-973 K using anvils with 5 mm truncation edge length and octahedral pressure medium with 10 mm edge length. CPO was determined by analyzing recovered samples using SEM-EBSD. In a few runs, in-situ X-ray observation was carried out using monochromatized synchrotron X-ray with energy of ~60 keV in which CPO was determined by analyzing two-dimensional diffraction images using a software MAUD.
The recovered samples from uniaxial compression tests consistently showed CPO pattern with [010] aligned parallel to compressional direction. Development of similar CPO pattern was observed in the in-situ observation of uniaxial compression. In an in-situ uniaxial tensile test, [010] was aligned parallel to the axis during pressurization, and this texture was gradually erased during tensile deformation. These results indicate that dominant slip plane in ε-FeOOH is (010) under the studied conditions. This suggests that phase H deformed in horizontal shear in the Earth's lower mantle may yield shear wave polarization anisotropy of VSV > VSH.
Deformation experiments were conducted using D111-type apparatus (Nishihara et al., 2020) installed at BL04B1, SPring-8. Starting materials for deformation runs were the pelletized powder of α-FeOOH (low pressure phase) or ε-FeOOH pre-synthesized at 12 GPa and 973 K. Uniaxial compression and tensile tests were carried out at ~12 GPa and 673-973 K using anvils with 5 mm truncation edge length and octahedral pressure medium with 10 mm edge length. CPO was determined by analyzing recovered samples using SEM-EBSD. In a few runs, in-situ X-ray observation was carried out using monochromatized synchrotron X-ray with energy of ~60 keV in which CPO was determined by analyzing two-dimensional diffraction images using a software MAUD.
The recovered samples from uniaxial compression tests consistently showed CPO pattern with [010] aligned parallel to compressional direction. Development of similar CPO pattern was observed in the in-situ observation of uniaxial compression. In an in-situ uniaxial tensile test, [010] was aligned parallel to the axis during pressurization, and this texture was gradually erased during tensile deformation. These results indicate that dominant slip plane in ε-FeOOH is (010) under the studied conditions. This suggests that phase H deformed in horizontal shear in the Earth's lower mantle may yield shear wave polarization anisotropy of VSV > VSH.