Unique chemistry of the hydrothermal solution in the mid‐Okinawa Trough Backarc Basin

Submarine hydrothermal fluids from JADE and CLAM sites in the mid‐Okinawa Trough Backarc Basin are highly enriched in CO₂, K, Li, NH₄, CH₄ and titration alkalinity compared to MOR fluids so far studied, while their H₂S and ³He contents are similar. The He and C isotopic ratios as well as these chemical features indicate that the hydrothermal systems are controlled by reaction between seawater and CO₂‐rich intermediate to acid volcanic rocks of island‐arc type, with strong influence of organic matter and are consistent with its initial stage of rifting on a continental plate margin. While the endmember JADE fluid is nearly devoid of Mg and SO₄, the CLAM fluid has definite concentrations of Mg (upper limit 22 ± 5 nM) and, most interestingly, of ³⁴S‐enriched SO₄ (upper limit 10 ± 2 mM, δ³⁴S= 27 to 39‰).