Iheya-North-Knoll is one of the small knolls covered with thick sediments in the Okinawa Trough back-arc basin. At the east slope of Iheya-North-Knoll, nine hydrothermal vents with sulfide mounds are present. The Integrated Ocean Drilling Program (IODP) Expedition 331 studied Iheya-North-Knoll in September 2010. The expedition provided us with the opportunity to study clay minerals in deep sediments in Iheya-North-Knoll. To reveal characteristics of clay minerals in the deep sediments, samples from the drilling cores at three sites close to the most active hydrothermal vent were analyzed by X-ray diffraction, scanning electron microscope and transmission electron microscope. The sediments are classified into Layer 0 (shallow), Layer 1 (deep), Layer 2 (deeper) and Layer 3 (deepest) on the basis of the assemblage of clay minerals. Layer 0 contains no clay minerals. Layer 1 contains smectite, kaolinite and illite/smectite mixed-layer mineral. Layer 2 contains chlorite, corrensite and chlorite/smectite mixed-layer mineral. Layer 3 is grouped into three sub-layers, 3A, 3B and 3C; Sub-layer 3A contains chlorite and illite/smectite mixed-layer mineral, sub-layer 3B contains chlorite/smectite and illite/smectite mixed-layer minerals, and sub-layer 3C contains chlorite and illite. Large amounts of di-octahedral clay minerals such as smectite, kaolinite, illite and illite/smectite mixed-layer mineral are found in Iheya-North-Knoll, which is rarely observed in hydrothermal fields in mid-ocean ridges. Tri-octahedral clay minerals such as chlorite, corrensite and chlorite/smectite mixed-layer mineral in Iheya-North-Knoll have low Fe/(Fe+Mg) ratios compared with those in mid-ocean ridges. In conclusion, the characteristics of clay minerals in Iheya-North-Knoll differ from those in mid-ocean ridges; di-octahedral clay minerals and Fe-poor tri-octahedral clay minerals occur in Iheya-North-Knoll but not in mid-ocean ridges.
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