Hydrothermal systems located in the Okinawa Trough Back Arc Basin are considered to be related to rifting activities in the continental margin. Hydrothermal fluids sampled from two sites (JADE and CLAM) are significantly enriched in gas species compared to those from mid-oceanic ridges. A high-temperature fluid (320°C) from the JADE site shows a CO2 concentration of 200 mmol kg-1, which is more than one order of magnitude higher than that of sediment-starved ridge crest hydrothermal systems. Both helium R RA = 6.1 -6.5 and carbon (δ13CCO2 = -5.0 to -4.7‰ PDB) isotopic compositions are typical of island arc magmatic gases. Also the abundance ratio of CO2/3He (= 3·1010) is within the range of values found in island arcs. Chemical and isotopic composition of the JADE fluid are considered to indicate that both CO2 and helium are derived from a magmatic source beneath the hydrothermal system without significant contamination. Contrary to these species, CH4 is attributed to a thermogenic origin based on its isotopic composition δ13C(CH4) = -41 to -36%. PDB) and high concentration ( = 7.1 mmol kg-1), which values are similar to those observed in sediment-hosted hydrothermal systems. The CLAM fluids show gas geochemistry much more affected by fluid-sediment interaction. Lower 3He/4He ratios R RA=3.7-3.8) and higher CO2 3He ratios ( >1011) may reflect involvement of gas species introduced during secondary fluid circulation within the sedimentary pile. Magmatic activity is the main contributor of gas species to the hydrothermal fluids in both the JADE and CLAM sites, although fluid-sediment interaction modifies their composition. The relatively high abundance of CO2 in the Mid-Okinawa Trough hydrothermal systems reflects the geochemical nature of island arc magma.
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