[S-CG51] 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

[SCG51-11] 海洋プレート形成論〜オマーンオフィオライトを例として〜

*道林 克禎^1,2 (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)

キーワード：アセノスフェア、リソスフェア、レオロジー、カンラン石、オマーンオフィオライト

Crystallographic preferred orientations (CPOs) of olivine within natural peridotites are commonly expressed by pole figures for the [100], [010], and [001] axes, and can be categorized into six well-known olivine crystal fabric types: A, B, C, D, E, and AG. In this study, we examined olivine crystal fabrics preserved within serpentinized peridotites in mantle sections of the Oman ophiolite^[1]. The olivine CPOs in the Samail and Hilti mantle sections have coarse granular textures, interpreted as retaining primary fabrics developed during asthenospheric mantle flow. The olivine CPOs in the Maqsad mantle diapir in the Samail mantle section are dominated by the AG–A type. The olivine CPOs in the Hilti horizontal mantle section were dominated by the AG type. Thus, in general, olivine CPOs in the high-T mantle fabrics of the Oman ophiolite are dominated by the AG type, with few occurrences of A and D types. In contrast, olivine CPOs in the lower-T shear zones of the Oman ophiolite represent secondary fabrics modified later by shear deformation. Three large-scale shear zones developed in the Fizh and Hilti mantle sections of the northern Oman ophiolite. The olivine CPOs within the northernmost shear zone outcropped in the Fizh mantle section varied from A to D as the textures varied from coarse granular to porphyroclastic. Olivine CPOs within the southernmost shear zone in the Hilti mantle section were of the E type, as the textures varied from coarse elongated to porphyroclastic to mylonitic. In summary, olivine CPOs in secondary, lower-temperature mantle fabrics widely range from D to E to C types. Consequently, there is a distinct difference in the olivine CPOs between the high-T and low-T mantle textures: the high-T olivine CPOs are dominated by the AG type, whereas the low-T olivine CPOs vary from the A to D to E to C types. We propose that olivine fabric transition could have occurred during continued deformation at progressively lower temperatures until mantle flow ceased within the lithospheric mantle. In the Oman ophiolite, this may have occurred during the flow away from the spreading center and/or during the initial stage of the obduction process.

[1] Michibayashi et al., 2025, Legacy of mantle fabrics preserved within heavily serpentinized peridotites in Hole BA3A cores of the Oman Drilling Project. Lithos, 496–497, 107970.