THE AQUEOUS GEOCHEMISTRY OF THE RARE EARTH ELEMENTS AND YTTRIUM: VI. STABILITY OF NEODYMIUM CHLORIDE COMPLEXES FROM 25 TO 300°C

Abstract

The stability and stoichiometry of Nd(III) chloride complexes have been experimentally determined in the temperature range 40 to 300°C, P = Psat. The solubility of AgCl(s) was measured in solutions of fixed HCl + NaCl concentration (0.01 to 5.0 m) and varying ΣNdΣCl molar ratio (0.0 to 0.5), following the method of Gammons (1995). The results of over 250 individual solubility experiments were regressed to obtain the following smoothed values for the first and second cumulative association constants for the Nd(III) chloride complexes: Nd3+ + Cl- = NdCl2+ (β1); Nd3+ + 2Cl- = NdCl+2 (β2):These are the first experimentally determined equilibrium constants for chloride complexes of any rare earth element (REE) at elevated temperature. At 25°C, neodymium exists mainly as Nd3+ in the absence of high concentrations of Cl- and other ligands (F-, CO2-3, SO2-4). However, complexation with chloride is greatly enhanced by increase in temperature, such that NdCl2+, NdCl+2, and possibly NdCl03 become the dominant species for NaCl-HCl-H2O brines at 300°C. The experimental data indicate a higher degree of complexation than predicted from earlier theoretical studies (Wood, 1990b; Haas et al., 1995), particularly in the case of log β2.Calculations of monazite solubility in seafloor hydrothermal systems (Wood and Williams-Jones, 1994) are re-evaluated in light of our new experimental data. Chloride complexes are shown to dominate the aqueous Nd speciation at 300°C, and lead to solubilities that are (1) much higher than previously estimated and (2) much closer to the maximum concentrations that have been reported from active black smokers. However, the large fluxes of REEs in altered rock beneath ancient massive sulfide deposits are still difficult to explain assuming that modern seafloor hydrothermal systems are direct analogs to ore-forming processes. Significant differences in fluid chemistry (e.g., lower pH, higher Cl- or F- concentrations) and/or duration and intensity of hydrothermal activity (higher water/rock ratio) are required to explain the REE systematics in ancient volcanogenic massive sulfide deposits.