As part of a project to investigate the volatilities of so-called "moderately volatile elements" such as Zn, In, Tl, Ga, Ag, Sb, Pb, and Cl during planetary formation, we began by recalculating the condensation temperatures of these elements from a solar gas at 10-4 bar. Our calculations highlighted three areas where currently available estimates of condensation temperature could be improved. One of these is the nature of mixing behavior of many important trace elements when dissolved in major condensates such as silicates, Fe-rich metals, and sulfides. Nonideal solution of the trace elements can alter (generally lower) condensation temperatures by up to 500 K. Second, recent measurements of the halogen contents of CI chondrites (Clay et al. 2017) indicate that the solar system abundance of chlorine is significantly overestimated, and this affects the stabilities of gaseous complexes of many elements of interest. Finally, we have attempted to improve on previous estimates of the free energies of chlorine-bearing solids since the temperature of chlorine condensation has an important control on the condensation temperatures of many trace elements. Our result for the 50% condensation temperature of chlorine, 472 K is nearly 500 K lower than the result of Lodders (2003), and this means that the HCl content of the solar gas at temperatures <900 K is higher than previously estimated. We based our calculations on the program PHEQ (Wood and Hashimoto 1993), which we modified to perform condensation calculations for the elements H, O, C, S, Na, Ca, Mg, Al, Si, Fe, F, Cl, P, N, Ni, and K by free energy minimization. Condensation calculations for minor elements were then performed using the output from PHEQ in conjunction with relevant thermodynamic data. We made explicit provision for nonidealities using information from phase diagrams, heat of solution measurements, partitioning data and by using the lattice strain model for FeS and ionic solids and the Miedema model for solutions in solid Fe. We computed the relative stabilities of gaseous chloride, sulfide, oxide, and hydroxide species of the trace elements of interest and used these, as appropriate in our condensation calculations. In general, our new 50% condensation temperatures are similar to or, because of the modifications noted above, lower than those of Lodders (2003).

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The extent to which sulfur dissolves in silicate melts saturated in an immiscible sulfide phase is a fundamental question in igneous petrology and plays a primary role in the generation of magmatic ore deposits, volcanic degassing, and planetary differentiation. In igneous systems, sulfide melts can be described as FeS-NiS-CuS 0.5 solutions with Fe/(Fe+Ni+Cu) significantly less than 1. Despite the presence of Ni and Cu in the sulfide, however, most experimental studies to date have concentrated on the effects of silicate melt composition on sulfur solubility and have used essentially pure FeS as the sulfide liquid. We have carried out 49 new experiments at pressures of 1.5-24 GPa and temperatures of 1400 to 2160 °C to investigate the effects of sulfide composition on sulfur solubility as well as extending the pressure and temperature ranges of the available data on sulfide saturation. We find that in the compositional range of most igneous sulfide melts [Fe/(Fe+Ni+Cu) > 0.6] sulfur solubility decreases linearly with Fe content such that at Fe/(Fe+Ni+Cu) of 0.6 the sulfur content at saturation is 0.6 times the value at pure FeS saturation. At lower values of Fe/(Fe+Ni+Cu), however, deviations from this ideal solution relationship need to be taken into consideration. We have treated these non-idealities by assuming that FeS-NiS-CuS 0.5 liquids approximate ternary regular solutions. We have fitted our data, together with data from the literature (392 in total), to equations incorporating the effects of silicate melt composition, sulfide liquid composition, and pressure on the solubility of sulfur at sulfide saturation ([S] SCSS). The temperature dependence of [S] SCSS was assumed either to be an unknown or was taken from 1 bar thermodynamic data. The most important best-fit silicate melt compositional term reflects the strongly positive dependence of [S] SCSS on the FeO content of the silicate melt. The best-fit value of this parameter is essentially independent of our assumptions about temperature dependence of [S] SCSS or the solution properties of the sulfide. All natural compositions considered here exhibit positive dependences of [S] SCSS on temperature and negative dependences on pressure, in accord with previous studies using smaller data sets.

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