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Research
Atmospheric and hydrospheric sciences
202211202211
Sr isotopic ratios of hydrothermal fluids from the Okinawa Trough and the implications of variation in fluid–sediment interactions
Toki T, Nohara T, Urata Y, Shinjo R, Watanabe SH, Ishibashi J, Kawagucci S
Hydrothermal system, Sr isotopic ratio, Okinawa Trough, Solid phase, Clay mineral
Endmember Sr isotopic ratios of hydrothermal fluids from the Okinawa Trough and sites worldwide. Black represents data from MOR hydrothermal systems, blue represents data from backarc systems, and red represents data from arc volcanoes. Open symbols indicate sediment-starved systems, and closed symbols indicate sediment-associated systems. The Sr isotopic ratios of solids (MORB, volcanic rocks from the Okinawa Trough, and sediments from the East China Sea) are also shown. A light blue star and dash-dotted line indicate the Sr isotopic ratios of seawater.
Schematic diagram showing two types of hydrothermal circulation in the Okinawa Trough (modified after Kawagucci, 2015). Arrows indicate hydrothermal fluid circulation, with color representing the temperature of hydrothermal fluid. a) Hydrothermal circulation in contact with sediments at high temperatures and volcanic rocks at moderate temperatures (> 150°C). Fluid is in contact with sediments at high temperatures, and the sediments supply Sr (e.g., Yonaguni Knoll IV). b) Hydrothermal circulation in contact with sediments at low temperatures and volcanic rocks at high temperatures. Sr from volcanic rocks is supplied to hydrothermal fluids (e.g., Iheya North Knoll and Hatoma Knoll). Thus, CH4 is provided by thermogenic CH4 at high temperatures (a) and microbial sources at low temperatures (b).
Sr isotope ratios of hydrothermal fluids were observed at five sediment-associated sites in the Okinawa Trough to investigate the diversity of subseafloor fluid–rock–sediment interactions. The estimated 87Sr/86Sr ratios of the hydrothermal endmember fluids at the five sites were all higher than those at the sediment-starved sites. The endmember Sr isotopic ratios of hydrothermal fluids were diverse within the Okinawa Trough, ranging from 0.7077 at the Iheya North Knoll site to 0.712 at the Yonaguni Knoll IV site. To our knowledge, 0.712 is the highest value reported to date for seafloor hydrothermal fluids. This variation is likely attributable to the relative contributions of multiple subseafloor Sr reservoirs, which are 87Sr-poor volcanic rock and 87Sr-rich hemipelagic sediments containing clay minerals of terrestrial origin. These data support a model based on the carbon isotope ratio of CH4, which indicates whether volcanic rocks or terrestrial sediments are distributed in the high-temperature reaction zone of the hydrothermal system.
