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Rare Earth elements (REE) are gaining importance due to their increasing industrial applications and usefulness as petrogenetic indicators. REE-sulfate complexes are some of the most stable REE aqueous species in hydrothermal fluids, and may be responsible for REE transport and deposition in a wide variety of geological environments, ranging from sedimentary basins to magmatic hydrothermal settings. However, the thermodynamic properties of most REE-sulfate complexes are derived from extrapolation of ambient temperature data, since direct information on REE-sulfate complexing under hydrothermal conditions is only available for Nd, Sm and Er to 250 °C (Migdisov and William-Jones, 2008, 2016). We employed ab initio molecular dynamics (MD) simulations to calculate the speciation and thermodynamic properties of yttrium(III) in sulfate and sulfate-chloride solutions at temperatures and pressures up to 500 °C and 800 bar. The MD results were complemented by in situ X-Ray Absorption Spectroscopy (XAS) measurements. Both MD and XAS show that yttrium(III) sulfate complexes form and become increasingly stable with increasing temperature (≥200 °C). The MD results also suggest that mixed yttrium-sulfate-chloride complexes (that cannot be distinguished from mixtures of chloride and sulfate complexes in XAS experiments) form at ≥ 350 °C. Two structures with two different Y(III)-S distances (monodentate and bidentate) are observed for Y(III)-sulfate bonding. The formation constants, derived via thermodynamic integration, for the Y(III) mono- and di-sulfate complexes parallel the trends for those of Nd, Sm and Er determined experimentally to 250 °C. The derived formation constants were used to fit revised Helgeson-Kirkham-Flowers equation-of-state parameters that enabled calculation of formation constants for Y(SO 4)+ and Y(SO 4) 2 − over a wide range of temperatures and pressures. The presence of sulfate increases the solubility of Y(III) under specific conditions. Since the stability of sulfate is redox sensitive, Y(III) solubility becomes highly redox-sensitive, with rapid precipitation of Y minerals upon destabilisation of aqueous sulfate. [ABSTRACT FROM AUTHOR]

The solubility of PdO(cr) was measured in NaOH (to 0.1m, mol/kg H2O) solutions at 400°C, 1kbar, and the solubility of Pd(cr) was determined at 400–500°C, 1kbar in acidic chloride solutions (to 1.5m NaCl) buffered with respect to hydrogen. The Pd electrode potential for the reaction + 2e− =Pd(cr) + 4Cl− was determined at 50 and 70°C in 1m chloride solutions. These data, together with reliable literature values, were used for calculation of the standard thermodynamic properties and the formation constants for Pd–OH, Pd–Cl, and Pd–S–HS complexes within the framework of the revised Helgeson–Kirkham–Flowers model. It was found that and become the most important Pd complexes in high temperature (t >300°C), chloride-rich fluids, and predominates at m(Cl)>0.1. The stability of Pd–Cl complexes increases sharply with increase in temperature. The near-neutral chloride–sulfide solutions (1m NaCl, <0.1m Stot) can transport Pd at ppm concentration levels at t ⩾600°C, whereas decrease in temperature and increase in pH can lead to effective deposition of Pd minerals. The stability of Pd–S–HS complexes ( , and PdS(HS)2−) decreases with increase in temperature. Therefore, the role of these complexes in hydrothermal transport of palladium is restricted to the low temperature solutions (t <100°C) and sulfur can be considered an efficient depositing agent for Pd. The calculated HKF Equation of State parameters were used to predict thermodynamic properties of Pd2+, Pd–OH, Pd–Cl, and Pd–S–HS complexes to 700°C, 2kbar. These parameters are incorporated into the FreeGs web-enabled database (http://www-b.ga.gov.au/minerals/research/methodology/geofluids/thermo/calculator/search.jsp) that can be used for geochemical application of thermodynamic data obtained in the present study. [ABSTRACT FROM AUTHOR]