[S-CG42] ハードロック掘削科学：陸上・深海底掘削、そしてオフィオライト

[SCG42-02] オマーン掘削プロジェクトBA3A掘削コアに遺されたマントルファブリック

*道林 克禎^1,2、夏目 樹^1,3 (1.名古屋大学 大学院環境学研究科（岩鉱）、2.海洋研究開発機構 海域地震火山部門 火山・地球内部研究センター、3.神奈川県立 生命の星・地球博物館)

キーワード：カンラン石結晶方位ファブリック、オマーンオフィオライト、リソスフェア、マントル対流

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.