Osmium Transport and Enrichment From the Lithosphere to the Hydrosphere: New Perspectives From Hydrothermal Experiments and Geochemical Modeling

Abstract

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°C and pressure of 100 MPa through hydrolysis experiments. The results show that the dominant species of Os is OsCl₆²⁻ at temperatures between 150 and 450°C and 100 MPa, gradually converting into Os(IV)-OH-Cl and Os(II)-Cl complexes over 450°C. The equilibrium constant (ln K) (K = [HCl]⁴ ⨯ [Cl⁻]²/[OsCl₆²⁻]) between OsCl₆²⁻ 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 · mol⁻¹ and (419.3 ± 38.52) J · mol⁻¹ · K⁻¹. Furthermore, the formation constant (ln β) of OsCl₆²⁻ exhibits a change from −0.097 to −0.104 as temperatures increase from 150 to 400°C, 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 OsCl₆²⁻. Geochemical modeling further demonstrates that high solubility of OsCl₆²⁻ could exist in low-temperature and acidic fluids (≤300°C and pH < 5), or relatively high-temperature and acidic-neutral fluids (>300°C 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.