Fate of Sulfate in Seafloor Hydrothermal Systems: Insights from In Situ Observation of the Liquid-Liquid Phase Separation in Hydrothermal Fluids - 2021

Mining & Minerals - Evaluations / Deep Sea Mining

Wang and two co-authors reported in 2020 that 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. The authors' 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⁺-SO₄^2- 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 S^2- and mixed with that leached from the basalt, leading to the elevated δ³⁴S value of the sulfide precipitated on the seafloor.

The authors concluded that LLPS in aqueous ZnSO₄ + C₂H₅OH solutions was investigated at elevated temperatures up to 300 °C and at pressures from 5.10 to 44.34 MPa. The fluid composition and structure were characterized by using in situ Raman spectroscopy.

1. LLPS tends to occur in low pressure hydrothermal fluids. In 0.5 m ZnSO₄ °C/MPa. Then, the starting temperature for LLPS increased at a rate of 0.685 °C/MPa. The presence of low D components can decrease the starting temperature of LLPS.
2. Complex ion association triggers the LLPS in sulfate-bearing solutions. The RPM modeling indicates that LLPS occurs at a near constant value of the product of D of water and absolute temperature (T ). D of water increases with rising pressure or decreasing temperature. The addition of low D components lowers the D of the solution. Decrease in D of water can promote the Zn²⁺-SO₄^2- interaction, favoring the occurrence LLPS. In situ Raman spectroscopic characterization of the v1(SO₄^2-) band also indicates that the addition of C₂H₅OH strengthens the Zn²⁺-SO₄^2- association.
   
   
3. The formation of a sulfate-rich dense liquid phase can interpret the elevated δ³⁴S value of seafloor venting sulfide. The occurrence of LLPS hinders the precipitation of sulfate during seafloor hydrothermal circulation, favoring the involvement of seawater-derived sulfate in deep hydrothermal cycle. At higher temperatures, sulfate was reduced to S^2-, which mixed with reduced S from basalt and ascended to the seafloor.