9:15 AM - 9:30 AM
[SCG42-02] Legacy of mantle fabrics preserved within heavily serpentinized peridotites in BA3A cores of Oman Drilling Project
Keywords:olivine crystal-fabrics, Oman ophiolite, lithosphere, mantle flow
We studied structural and petrophysical characteristics of olivine aggregates preserved within serpentinites in BA3A cores drilled by the ICDP Oman Drilling Project. The BA3A cores consist mostly of serpentinized harzburgites with small amount of thin serpentinized dunitic layers and mafic layers. Foliations in the cores were manually identified by elongated orthopyroxene grains, dipping approximately 30°. Quarter slabs were obtained from the BA3A cores where the degrees of serpentinization were relatively lower based on X-ray CT images. Mineral phases and crystal-orientations in each polished thin section were analyzed simultaneously by SEM-EBSD. Then, EBSD mapping and data processing were performed to reveal olivine crystal-preferred orientations (CPOs) as the legacy of mantle fabrics. Since the drilled cores had been rotated horizontally, we rotated all olivine CPOs horizontally so that maxima of olivine [100] axes in all samples have the same azimuth (i.e., 90°). As a result, we found that the maxima of olivine [100] axes were aligned in a range of ca. 10° within 300 meters. In contrast, the maxima of [010] axes varied from top to bottom, whereas those of [001] axes have bimodal distributions in the total core length. It shows that the olivine fabrics had been changed from [100] axial pattern (AG type) to {0kl}[100] pattern (D type) or (001)[100] pattern (E type), although their P-wave seismic anisotropies have only small differences within 8–10%. It shows that olivine fabrics preserved in the BA3A cores were modified during a single mantle flow, possibly during progressive retrogression until the mantle flow has been ceased to be a part of lithospheric mantle.