The transport and enrichment of rhodium (III) in chloride-rich fluids: Implications for seafloor hydrothermal systems

Hydration and complexation are crucial processes for dissolving metal elements and transporting metal complexes in hydrothermal fluids. However, the impact of hydration and complexation on the transport and enrichment of metal elements, such as those involving rhodium and chloride, has not been thoroughly evaluated. Here, this study employed a hydrolysis experiment of K 3 RhCl 6 at 200–600 °C and 100 MPa to determine the controlling factors and thermodynamics of Rh-Cl complexes in chloride-rich fluids. The results show that the dominant Rh-Cl complex is RhCl 6 3- at 200–400 °C, gradually converting into Rh(III)–OH-Cl complexes over 400 °C. The hydrolysis equilibrium constant (ln K) of RhCl 6 3- at 200–400 °C affected by temperature ( T , kelvin) is calculated as: ln [ M I C K ] = 49.06 ± 7.01 - 48802 ± 3896.0 / T . Accordingly, the Δ r H m Θ and Δ r S m Θ of the hydrolysis reaction were obtained to be 405.8 ± 32.39 kJ·mol −1 and 407.9 ± 58.30 J·mol −1 ·K −1 , respectively. Thermodynamic parameters reveal the dependence of the stability of Rh-Cl complexes on temperature in chloride-rich fluids. For instance, the formation constants (ln β) of RhCl 6 3- vary from 0.0184 ± 0.0022 to −0.0079 ± 0.0012 as the temperature rises from 150 to 400 °C. Geochemical modeling illustrates that low-temperature and acidic fluids can enhance the stability of Rh-Cl complexes, which can be dominated by Cl concentration (over 0.5 wt%). Hydrothermal fluids with low pH and high Cl content, typically occurring in the mid-ocean ridge, promote Rh transport and subsequent enrichment in encrustations and minerals by substituting Mn and Fe for isomorphism, or in the form of alloys, forming a substantial Rh reservoir in the ocean.