THERMODYNAMIC CALCULATIONS OF THE VOLATILITY OF THE PLATINUM GROUP ELEMENTS (PGE): THE PGE CONTENT OF FLUIDS AT MAGMATIC TEMPERATURES

Abstract

The volatilities of the platinum-group elements as metals, oxides and chlorides were calculated at temperatures of 800–1600 K. Only Pd is significantly volatile as the metal. At logfH2O = 1 Kbar and 1200 K., the concentration (weight) of Pd in the vapor reaches 1 ppt and at 1600 K attains several ppb. The PGE oxides are extremely volatile at atmospheric oxygen fugacities. However, only Os and Ru have significant volatilities (≥ ppt) as oxides (OsO4, RuO3) at oxygen fugacities typical of magmatic PGE deposits (near QFM) and only at temperatures greater than 1400 K.Data on the volatility of PGE chlorides exist only for Pd and Ru, both of which are somewhat more volatile as chlorides than as oxides. At 1400 logfH2O = 1 bars, fHCl = 100 bars and at QFM, the calculated vapor concentrations of PdCl2 and RuCl3 are 500 ppt and 20 ppt, respectively (and less in the presence of sulfur). However, higher concentrations of PGE may be attained at higher temperatures, higher fO2, higher fHCl or lower fH2. Also, any interactions between water vapor and PGE vapor species (e.g. ionization, solvation) would tend to increase the vapor concentration of PGE. Volatility of Ir as IrF6 is insignificant at all conditions.Vapor transport of the more volatile PGE as chlorides may play some role in the transport of these metals in mafic igneous complexes such as the Stillwater or the Bushveld. However, under the conditions where the PGE are most volatile, the metals Fe, Ni and Cu are several factors often more volatile, so that enrichment of the PGE and Cu over Ni and Fe cannot be explained by chloride transport alone.