Metal complexation and speciation is the primary process responsible for metal transport and circulation in hydrothermal systems, during which stable and soluble metal complexes play a pivotal role. Here, we investigate the speciation of Os and the thermodynamic stability of Os(IV)-Cl complexes in chloride-bearing solutions at temperatures ranging from 150 to 600 ℃ and pressure of 100 MPa through hydrolysis experiments. The results show that the dominant species of Os is OsCl₆^2- at temperatures between 150 and 450 ℃ and 100 MPa, gradually converting into Os(IV)-OH-Cl and Os(II)-Cl complexes over 450 ℃. The equilibrium constant (ln K) (K = [HCl]⁴ x [Cl^-]²/[OsCl₆^2-]) between OsCl₆^2- and water molecule is determined as ln K = (50.43 +/- 4.633) - (54223 +/- 2525.6)/T, and Δ_rH_m^Θ and Δ_rS_m^Θ are inferred to be (450.8 +/- 21.00) kJ center dot mol^-1 and (419.3 +/- 38.52) J · mol^-1 · K^-1. Furthermore, the formation constant (ln β) of OsCl₆^2- exhibits a change from -0.097 to -0.104 as temperatures increase from 150 to 400 ℃, while the change values in standard Gibbs free energy (Δ_rG_m^Θ ) for the hydrolysis reactions decrease with rising temperature, suggesting a temperature-dependent thermodynamic stability of OsCl₆^2-. Geochemical modeling further demonstrates that high solubility of OsCl₆^2- could exist in low-temperature and acidic fluids (<= 300℃and pH < 5), or relatively high-temperature and acidic-neutral fluids (>300℃ and pH < 7), primarily influenced by the Cl concentration. Acidic and near-neutral fluids with high Cl concentration venting in the mid-ocean ridge, back-arc, and sediment-hosted systems contribute more to dissolving and transporting Os from the lithosphere to the hydrosphere, thereby impacting the global ocean dissolved Os budget.
Plain Language Summary The rare and precious metal osmium is typically enriched in the deep Earth and associated mantle-derived rocks. However, some submarine sediments and hydrothermal fluids may contain elevated levels of Os, suggesting a potential pathway for Os transport from the lithosphere to the hydrosphere. The key point is to clarify in what form and how Os transports from the deep Earth to the seawater. To achieve this goal, hydrolysis experiments were conducted to determine the speciation and stability of Os in chloride-bearing solutions. The results revealed that OsCl₆^2- can remain stable in low-temperature, then convert into Os(IV)-OH-Cl and Os(II)-Cl complexes over 450℃ at similar to NNO (Ni-NiO) oxygen fugacity conditions. Quantitative relationship among Os solubility, chloride concentration, pH, and temperature was established, indicating that high Os solubility could exist in the low-temperature and acidic fluids (<= 300℃ and pH < 5) or relatively high-temperature and acidic-neutral fluids (>300℃ and pH < 7), such as those found in submarine vent fluids in mid-ocean ridge, back-arc, and sediment-hosted systems. During ascent, these Os-rich hydrothermal fluids can mix with seawater, thereby dominating Os transport and distribution in submarine hydrothermal systems. This study provides new evidence and insights into Os transport and enrichment mechanisms responsible for transporting Os from the lithosphere to the hydrosphere.