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

Abstract:

This report describes abundances and isotopic compositions of volatile element trapped in fluid inclusions of submarine hydrothermal ore deposits collected in Western Pacific subduction zones (Okinawa Trough, Izu-Bonin arc, Mariana Trough, and Lau Basin) together with those in Kuroko ore deposits in northeastern Japan. The helium isotopic composition of Okinawa Trough, 6.6±1.0 Ra is much smaller than that of Izu-Bonin samples, 8.4±0.5 Ra. Those of Mariana Trough samples are similar to those of the Okinawa Trough, whereas Lau Basin data are consistent with those of Izu-Bonin. These characteristics might reflect the tectonic setting of regions: the former is related to back-arc spreading or rifting with a sediment signature in a graben, although the latter is attributable to island-arc type magmatism and/or its influence to back-arc volcanism. Argon and nitrogen isotopes are also explainable according to a similar hypothesis, although carbon isotopes are not the discriminator of tectonics. Origins of carbon and nitrogen are estimated respectively by the δ¹³C-CO₂/³He and δ¹⁵N-N₂/³⁶Ar diagrams. The sedimentary contributions of both carbon and nitrogen are larger in Okinawa and Mariana Troughs than in Izu-Bonin and Lau Basin, whereas Kuroko samples agree well with the latter. Carbon and nitrogen fluxes are again larger in the former than in the latter. The CO₂/N₂ flux ratio at Okinawa and Mariana Troughs is larger than that at Izu-Bonin and Lau Basin, although both are considerably larger than that at MOR, suggesting that the additional sedimentary component has a higher C/N ratio than the upper mantle value.