DEFORMATION MICROSTRUCTURES OF OLIVINE AND PYROXENE IN MANTLE XENOLITHS AND IMPLICATIONS FOR SEISMIC ANISOTROPY

We studied lattice preferred orientations (LPOs) of olivine, orthopyroxene (Opx; enstatite), and clinopyroxene (Cpx; diopside) in mantle xenoliths at Shanwang, eastern China to understand deformation microstructures and seismic anisotropy in the upper mantle. The Shanwang area is located across the Tan-Lu fault that was formed due to the collision between the northern and southern China blocks. Petrologically, all of the samples are spinel lherzolite and wehrlite, which consist of mainly olivine (58 – 83 %), Cpx (13 – 37 %), Opx (16 – 27 %), and minor minerals such as magnetite and spinel. LPOs of minerals were determined by using electron backscattered diffraction (EBSD) in SEM. Two types of LPOs of olivine were found: type-E and type-B. Enstatite showed two types of LPOs (type-BC and type-AC), and diopside showed three different types of LPOs. Since LPO of olivine is influenced by water, water content was determined by using the Nicolet 6700 Fourier transformation infrared (FTIR) spectroscopy. The FTIR data showed that all of the olivine in samples contained some amount of water (20 – 570 ppm H/Si). Enstatite and diopside, however, contained much more water than olivine (50 – 3600 ppm H/Si and 530 – 14000 ppm H/Si, respectively). Water is considered to be escaped from olivine after formation of type-B and -E fabrics because of fast diffusion of H in olivine.

Seismic anisotropy was calculated using the LPO of minerals. The seismic anisotropy of P-wave (V_P) was in the range of 2.2 – 11.6 % for olivine, 1.2 – 2.3 % for enstatite, and 2.1 – 6.4 % for diopside. The maximum anisotropy of the shear wave (AV_S) was in the range of 1.93 – 7.53 % for olivine, 1.53 – 2.46 % for enstatite, and 1.81 – 6.57 % for diopside. As a result, the seismic anisotropy of olivine was stronger than both enstatite and diopside. Furthermore, the thickness of anisotropic layer was calculated for three geodynamic models to understand the origin of seismic anisotropy in the study area by using delay time from shear wave splitting and S-wave velocity & anisotropy from LPOs of minerals. Finding the type-B LPO of olivine in nature is important because trench-parallel seismic anisotropy of fast S-wave found in many subduction zones can be attributed to the type-B LPO.