[3ch301-12] R1 [Regular Session]Plutonic rocks, volcanic rocks and magmatic processes

[R1-O-1] Microstructural analyses of coarse-grained peridotite in the vicinity of mantle diapir in Oman opholite: Implication for seismic anisotropy in lithospheric mantle

*K Michibayashi^1,2,3, Kakihata Yuki¹, Onoue Ayaka², Ohya Syoma², Odashima Norihiro² (1. Nagoya University, 2. Shizuoka University, 3. JAMSTEC)

Keywords:Mantle, diapir, Mid-ocean ridge, olivine, fabric

Petrophysical characteristics of coarse-granular peridotites in a mantle diapir beneath the mid-ocean ridge have been quantitatively compared with those in a common lithospheric mantle peridotites outcropped in the Oman ophiolite. Maqsad mantle section has subvertical structures known to be a mantle diapir, whereas Hilti mantle section is characterized by the subhorizontal structure subparallel to the crust-mantle boundary, presumably a fragment of typical oceanic lithosphere at some distance from the ridge. Despite of the fact that the attitude of foliation and lineation are very different each other, both peridotites have similar textures and grain sizes but slightly different features in olivine crystal-preferred orientations (CPOs). Maqsad peridotites vary from axial [010] patterns (AG type) to (010)[100] patterns (A type), whereas Hilti peridotites are dominated by axial [010] patterns (AG type). It may suggest that the olivine CPOs could have been formed during vertical plastic flow in the mantle diapir and were somehow modified during subsequent horizontal plastic flow under subsolidus conditions. Furthermore, the azimuthal anisotropy would vary in dependent on the structural orientations as well as olivine CPOs, even though actual Vp anisotropies are nearly constant in both peridotites.