Volatility of mercury and related volatile metals at magmatic temperatures

Abstract

The volatilities of metals in igneous systems are important constraints in the contexts of magmatic degassing, ore deposition and the volatile budgets of the Earth and other planets. In order to develop a systematic understanding of such volatilities, we have made the first experimental measurements of the volatility of Hg from molten basalt at high temperatures and compared it directly with other volatile trace elements (i.e., Ag, As, Cd, Cr, Cs, Cu, Ga, Ge, In, Li, Pb, Rb, Sb, Sn, Tl and Zn). We placed 100 mg of crushed basalt, containing 93 ppm of Hg and ∼ 500 ppm of most of the other volatile metals in an alumina crucible which was sealed under vacuum inside a silica glass tube of volume ∼ 4.4 × 10⁻⁶ m³. After holding them at high temperature (1250, 1300 or 1400 °C) between 5 min and 24 h, samples were quenched in air. Analysis of the quenched glasses using a Direct Mercury Analyser shows that almost all Hg (>99 %) is lost within the first few minutes, establishing a total pressure of Hg species of 150 Pa (calculated from the known mass of Hg in the system and the ampoule volume). Apparent steady-state concentrations of ∼40 ppb Hg at 1400 °C and ∼ 100 ppb Hg at 1300 °C are achieved after 1 and 6 h respectively. The oxidation state of Fe in the products, measured by XANES, indicates that Fe³⁺/(Fe³⁺+Fe²⁺) remains constant at ∼0.1 during the experiments. The behavior of elements other than Hg in the glasses were determined by Electron Microprobe and Laser Ablation ICP-MS. In our experiments we find that the fraction of each element lost to the atmosphere decreases in the following order: Hg>>Tl>Cd>In>Zn*>Li>Cs>Cu*>As>Ag>Pb∼Sn∼Ga>Ge∼Sb (with the position of asterisked elements uncertain and maximum possible volatilities adopted). These results show broad similarities to, but also important differences from volatilities estimated from field measurements of gas/aerosol compared to concentrations in lava. They also show important differences from volatilities obtained from open-system experiments and those estimated from the condensation temperatures of elements in the relatively reduced environment of a protoplanetary gas disc.