Volatile element isotopes of submarine hydrothermal mineral deposits in the Western Pacific

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.