Fate of sulfate in seafloor hydrothermal systems: Insights from in situ observation of the liquid–liquid phase separation in hydrothermal fluids

Abstract

Abstract The ubiquitous hydrothermal circulation within the newly formed oceanic crust plays an important role in the transport of heat, water and chemical constituents in the earth-ocean-atmosphere system. One enigma during this process is whether seawater-derived sulfate participates in the deep hydrothermal circulation. Our results showed that homogeneous sulfate-bearing fluid separated to a sulfate-rich dense liquid phase and a sulfate-poor light liquid phase. The liquid–liquid phase separation (LLPS) temperature increased with rising fluid pressure but decreased with the addition of low dielectric constant (D) components. In situ Raman spectroscopic characterization revealed that the Zn2+-SO42- association increased with the addition of low D components. All these observations confirmed our previous conclusions that strong ion complexation triggered the LLPS. The formation of a sulfate-rich dense liquid phase could postpone the precipitation of sulfate, resulting in the participation of seawater-derived sulfate in the deep hydrothermal circulation. Sulfate would be reduced to S2− and mixed with that leached from the basalt, leading to the elevated δ34S value of the sulfide precipitated on the seafloor.