[S-CG46] Hard-Rock Drilling Science: From Continental to Deep Sea Drilling, and Oman Project

[SCG46-08] Surpentinization of both oceanic upper mantle and mantle wedge in subduction zone: a case study of the Oman mantle section

*Eiichi TAKAZAWA^1,3, Yohki YOSHIBA², Main TOKINAGA² (1.Department of Geology, Faculty of Science, Niigata University, 2.Graduate School of Sceince and Technology, Niigata University, 3.Japan Agency for Marine Earth Science and Technology)

Keywords:Oman ophiolite, serpentine, alteration, upper mantle, peridotite

How deeply water penetrates into the oceanic crust and mantle, how much water is stored in each rocks, and how much water penetrates into the mantle wedge from the subducted slab. These remain as unresolved issues. Therefore, our research group is investigating the alteration process of peridotite in the mantle section of the Oman ophiolite. The target area is the mantle section of the Fizh and Salahi blocks located in the northern part of the Oman ophiolite. These mantle sections are composed of residual peridotites formed beneath the north-south-trending oceanic ridge axis. It is presumed that the segment boundary of the paleo-ridge axis was located in the northern part of the Fizh block and the southern part of the Salahi block, and the center of the segment was located in the southern part of the Fizh body (Adachi et al., 2003; Miyashita et al., 2003; Umino et al., 2003). Oman ophiolite formed at spreading ridge about 95 Ma and thrusted over the subducting plate, and finally reached the eastern end of the Arabian Peninsula about 80 Ma. During the subduction and thrusting process, the oceanic crust subducting under the ophiolite undergoes dehydration and metamorphosed into metamorphic soles (mainly amphibolite and greenschist), and the mantle section on the hanging wall became the the mantle wedge of incipient stage of subduction zone (MacLeod, et al., 2013; Kusano et al., 2017). In this way, Oman ophiolite is the only place in the world where the oceanic crust formed on the mid-ocean ridge and the early process of island arc formation can be observed simultaneously on land. The Oman ophiolite is also the only place where both hydrothermal circulation from above to the uppermost mantle and hydrothermal fluid from below the mantle wedge can be detected simultaneously. Studies of the alteration of the mantle section of the Omani ophiolite have made it possible to unravel the process of serpentinization in the subduction zone from those in oceanic plate, but at the same time have the complexity of having to decipher the duplication of multiple processes. In this study we are investigating the spatial distribution of alteration minerals in the mantle sections of Fizh and Salahi blocks. As a result of the observations so far, it has been clarified that while the mesh structure of lizardite is universally present, some antigolite, which is a high-temperature serpentine, is also present. It was also clarified that the distribution of magnetite is biased and is universally present near the basal thrust, but is limited to the inside of a relatively thick serpentine vein (several millimeters in width) inside the each blocks. Talc is heterogeneous in distribution and may depend on local sources of silica (such as Opx). We will discuss these characteristics in terms of the relationship with the segment structure described above, the depth from the Moho, which is the boundary between the crust and the mantle, and the distance from the basal thrust. The characteristics of the distribution of altered minerals in the mantle sections of the Fizh and Salahi bodies are reported in this session by Tokinaga et al. (2022) and Yoshiba et al. (2022), respectively.