Keywords:Oman Drilling Project, mantle peridotite, Crystallographic Preferred Orientation (CPO)
Oman Ophiolite, which is the biggest ophiolite located in the eastern part of the Arabian Peninsula, preserves sequential structures of oceanic crust and upper mantle. The Oman Drilling Project has obtained the cores penetrating from the lower crust to the uppermost mantle including the crust–mantle transition zone. Structural characteristics of ultramafic cores in three holes at BA site have been studied in this study. We have selected 61 core samples in BA1B, BA3A and BA4A, and performed electron backscatter diffraction (EBSD) analyses in order to understand nature of the uppermost mantle flow. The ultramafic core samples consist of intensely serpentinized harzburgites and dunites. Deformation structures vary ranging from granular to protomylonite exhibited by elongated and aligned pyroxene grains. Measured dip angles of elongated pyroxene are dominantly subhorizontal in the overall core. Pyroxene and olivine grains in the protomylonitic peridotites with distinct foliations are intensely elongated and show kink bands and undulatory extinction in thin sections. In the granular peridotites, pyroxene grains have rounded shapes. In processing EBSD data, the primary textures have been reconstructed by extrapolation, when they were obscured by the intense serpentinization. Based on the reconstructed EBSD maps, grain sizes of olivine were in the range from 0.4 to 1.1 mm, suggesting that the grain size reduction occurred locally. Crystallographic preferred orientations (CPOs) of olivine were analyzed by the EBSD maps. J-index, which is the index of strength of olivine CPOs, ranges from 2 to 4. In BA1B, J-index and mean grain sizes slightly decreased, whereas olivine CPOs gradually changed from A to D type in the deeper cores. In BA3A, J-index and mean grain size decreased at around 200 m in depth, whereas olivine CPOs were fundamentally D type. In BA4A, harzburgites have constant value of J-index around 2.3 and A to D type. These results show that the ultramafic cores in the BA site would have been modified their primary peridotite textures by secondary lithospheric deformation before serpentinization.