Experimental determination of the solubility of ReO 2 and the dominant oxidation state of rhenium in hydrothermal solutions

Abstract

The dominant oxidation state of rhenium in high-temperature, chloride-bearing hydrothermal solutions at geologically reasonable oxygen fugacities has been experimentally determined to be +4, although a small percentage of dissolved rhenium (≤10%) may be present in the +5 or higher valence state. This conclusion is based on a series of solubility experiments in 1.0 molal KCl solutions at 500°C using ReO 2, the K–feldspar–muscovite–quartz assemblage as a pH buffer, and Co–CoO, Ni–NiO, Re–ReO 2, MnO–Mn 3O 4, and MoO 2–MoO 3 pairs as oxygen fugacity buffers. Experiments were also conducted on the solubility of the Re–ReO 2 oxygen fugacity buffer in 0.5 and 1.5 m KCl solutions in equilibrium with the K–feldspar–muscovite–quartz pH buffer at 500°C. The solubility of Re increases dramatically with increasing KCl concentration under these conditions, implying the formation of a Cl-bearing Re species. Because Re +4 is a hard acid owing to its high ionic potential, relatively hard ligands such as Cl −, F −, O 2− and OH − should be important in the transport of Re during geological processes at elevated temperatures. Mixing/dilution, which may induce changes in pH and/or chloride concentration, should be important for the deposition of Re because the solubility of rhenium has a strong dependence on pH and the concentration of Cl −. Our experimental results may have implications for the hydrothermal remobilization and enrichment of rhenium in high-temperature environments, for the Re–Os isotopic system, and for the rhenium budget in the continental crust. Our study may also have bearing on the aqueous geochemistry of fissiogenic 99 Tc , for which Re is an analogue. These implications are discussed in detail.