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The abundance and isotopic compositions of volatile elements trapped in fluid inclusions of submarine hydrothermal mineral deposits in Western Pacific subduction zones (Okinawa Trough, Izu‐Bonin arc, Mariana Trough, and Lau Basin) and in Kuroko ores in northeastern Japan are presented. The helium isotopic compositions corrected for air contribution of the Okinawa and Mariana troughs, ranging 4.49–7.68 Ra are lower than those of the Izu‐Bonin and Lau Basin, 7.62–8.91 Ra. This characteristic might reflect the differences in regional tectonic setting. The Okinawa and Mariana troughs are related to back‐arc spreading with strong graben sedimentary signature, whereas the Izu‐Bonin arc is associated with island arc magmatism. The arc contribution to the Lau Basin volcanism is significantly strong, even though it is assigned to back‐arc spreading. Nitrogen isotopes can also be explained by a similar hypothesis, whereas argon and carbon isotopes cannot be used to discriminate tectonic setting. δ13C–CO2/3He and δ15N–N2/36Ar diagrams elucidate the source of carbon and nitrogen. The MOR‐type mantle contributions to carbon are mostly smaller in the Okinawa and Mariana troughs (ranging 0.06–8.9% with the average of 2.4%) than in the Izu‐Bonin and Lau Basin (2.1–25% with the average of 7.7%). The sedimentary contributions to nitrogen are larger in the Okinawa and Mariana troughs (11–65% with the average of 35%) than in the Izu‐Bonin and Lau Basin (4–24% with the average of 15%), and the Kuroko samples agree well with the latter. Carbon and nitrogen fluxes are again higher in Okinawa trough than in Izu‐Bonin arc.

Mineralogical and geochemical features of hydrothermal alteration minerals in the sediment cores from the Jade hydrothermal field in the Izena Hole, mid-Okinawa Trough, were studied by XRD, EPMA and TEM-EDS analyses. A core sample 1186MBL collected from the surface sediment near the sulfide chimney venting high temperature fluid up to 320 C was characterized by occurrence of kaolinite, with sulfide minerals such as sphalerite and galena. The kaolinite would be related to be formed under acidic condition caused by oxidation and dissolution of the sulfide minerals by penetrating seawater. Core samples (1188MB, 1193MB) were collected from the surface sediment in the vicinity of clear hydrothermal fluid venting of~100 C, which is located in 400 m distant from the sulfide chimney. In these cores, occurrence of chlorite and smectite was identified. The chlorite in the core 1188MB had chemical composition close to Al-rich chlorite which is classified as sudoite, although chlorite found in other hydrothermal fields in the Okinawa Trough is characterized as significantly Mg-rich chlorite. Core samples of up to 4-6 m length were also collected near the low temperature fluid venting to study alteration in deep layers. One of two long core samples (BMS-J-2) was characterized by chlorite and illite assemblage below 380 cmbsf, while the other (LC-J-2) was characterized by abundant occurrence of K-feldspar below 300 cmbsf. Occurrence of euhedral crystals of K-feldspar in size up to several tens μm suggests the formation by precipitation from high temperature fluid.