Hydrogen, oxygen and sulfur isotope studies of seafloor hydrothermal system at the Desmos caldera, Manus back-arc basin, Papua New Guinea: An analogue of terrestrial acid hot crater-lake

The Onsen site is an active submarine hydrothermal system hosted by the Desmos caldera in the Eastern Manus Basin, Papua New Guinea. The hydrothermal fluid is very acidic (pH=1.5) and abundant native sulfur is deposited around the vent. The δ³⁴S values of native sulfur range from -6.5 to -9.3‰. δ³⁴S values of H²S and SO⁴ in the hydrothermal fluid are -4.3 to -9.9 ‰ and +18.6 to +20.0 ‰, respectively. These δ ³⁴S values are significantly lower than those of the other hydrothermal systems so far reported. These low δ ³⁴S values and the acidic nature of the vent fluids suggest that volcanic SO₂ gas plays an important role on the sulfur isotope systematic of the Onsen hydrothermal system. Relationship among the δ³⁴S values of S-bearing species can be successively explained by the model based on the disproportionation reaction starting from the volcanic SO₂ gas. The predicted δ³⁴S values Of SO₂ agree with the measured whole rock δ³⁴S values. δD and δ ¹⁸O values of clay minerals separated from the altered rock samples also suggest the contribution of the magmatic fluid to the hydrothermal system. Present stable isotopic study strongly suggests that the Onsen hydrothermal site in the Desmos caldera is a magmatic submarine hydrothermal system.