Experimental investigation of metal transport and deposition from fumarolic gases

Abstract:

Volcanic and fumarolic gases transport and deposit metals from the sub-volcanic environment to the surface [1,2]. Volcanic gas metal emissions range up to >1000 kg/day [3] and are an important component of the global metal cycles. Furthermore, gas-phase metal transport and deposition contribute to economically important deposits [4]. The investigation of these processes is challenging due to the inherent dangers of sampling gases on active volcanoes. Alternatively, metal speciation in gases can be investigated by thermodynamic modeling [5]. Here, we present novel gas-phase metal-transport experiments and compare the results with thermodynamic calculations, and observations from natural fumaroles reported in the literature (Kudriavy [6], La Fossa [7] & Momotombo [8]).

We conducted experiments in evacuated silica glass tubes, suspended in a vertical tube furnace, with a temperature gradient from 1250 °C to 300 °C. The metals were added as oxide powders, variably mixed with S and NaCl, and pressed into pellets. These powder pellets were placed at the bottom of the silica glass tubes before evacuating the tubes to 10^-5 bar and suspending them in the furnace for 24 h. Volatile and moderately volatile elements in the mixture (S, Cl, Cu, Zn, Ga, Ge, Mo, Ag, Cd, In, Te, Re, Tl, Pb) form a gas phase at 1250 °C, from which sulfide, chloride, elemental and oxide phases deposit along the temperature gradient in the silica glass tube. In the S- and Cl-bearing experiment observed phases include (Ag,Cu)₂S (760-1200 °C), ReS₂ (760-1040 °C), NaCl (500-1040 °C), CdIn₂S₄ (650-1040 °C), GeO₂ (580-760 °C), (Zn,Cd)S (435-760 °C), PbS (435-580 °C), PbCl₂ (420-500 °C), TlPb₂Cl₅ (420-440 °C), TlCl (340-420 °C), S and Te (<380 °C).

Thermodynamic calculations in open and closed systems provide insights into the gas-phase speciation, but they do not reproduce the deposition temperatures observed in our experiments. In contrast, our S- and Cl-bearing experiment reproduces the deposition temperatures of (Zn,Cd)S, PbS, PbCl₂, TlPb₂Cl₅, TlCl and S observed at fumaroles on the volcanoes Kudriavy [6], La Fossa [7] and Momotombo [8].

[1] Mather et al. (2012). [2] Zelenski et al. (2013). [3] Edmonds et al. (2018). [4] Henley & Seward (2018). [5] Symonds & Reed (1993). [6] Africano et al. (2003). [7] Garavelli et al. (1997). [8] Quisefit et al. (1989).