The genesis of oxidized-type (magnetite series) and reduced-type (ilmenite series) granites is discussed on the basis of thermodynamic considerations. We show that SO2 in the magma can oxidize augitc to form magnetite with decreasing temperature because the SO.•-H.•S buffer line intersects with the augitc-magnetitequartz buffer line on a 1/T -fo• diagram. When the magma contains SO.• above 250 ppm and the f% is constrained by the SO2-H.•S buffer, the magma forms oxidized-type granites (>0.2 modal % magnetite). On the other hand, •vhen the SO.• content is less than 250 ppm, the magma solidifies as a reduced-type granite (<0.2 modal % magnetite). If the f% of the magnaas is buffered by H.•O-H2, CO.•-CO, or (•H4-(•O2 no magnetite will crystallize and reduced-type granites will be formed. As a result, the genesis of oxidized-type and reduced-typ• granites can be attributed to the difference in volatile components in magnaas.

Crystal structures of magnetite ( Fe3O4) and Ni-ferrites ( Ni0.4Fe2.6O4 and Ni0.6Fe2.4O4) were determined using X-ray single-crystal diffraction methods. The single crystals were grown hydrothermally at 673–743 K, and some of the crystals were heated at 1303 K. After the positional and thermal parameters were initially determined using the MoKα radiation data set, the cation distributions were determined using the synchrotron radiation data set. The synchrotron X-rays were tuned at a wavelength 0.015 Å longer than the Fe K-absorption edge. This allowed us to precisely determine the cation distributions with a large difference in scattering factors. Our structural analyses with the synchrotron radiation measurements show that all the Ni ions in the ferrites are involved in the octahedral B sites.

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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 dioctahedral 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.