Speciation and fractionation of toxic metals (Pb, Se, Te) in volcanic geothermal systems: Insights from partitioning experiments and in-situ spectroscopic measurements to high P-T conditions.

Fluids in volcanic systems (supercritical fluids produced by magma degassing at depth, brines, vapors or mixed geothermal waters) are tremendous vectors of energy and volatiles (CO₂, CH₄, CO, HCl, H₂S, etc…). They may also carry consequent and sometimes toxic amounts of metals, with recent estimates of metal emissions from Masaya, Etna or Iceland even suggesting that short-term metal release associated with volcanic activity may be comparable to anthropogenic emissions from rich industrial countries [1].

While an increasing amount of experimental data is already available to describe the speciation, solubility or fluid-melt partitioning and model the release of metals of economic interest (Cu, Au) in arc settings [2,3,4], how metals as As, Hg, Pb, Cd, Se or Te are extracted from underlying magmas and transferred towards the surface remains poorly constrained. Furthermore, underground reactions between the high-temperature fluids and rocks may favour precipitation, incorporation in sulfosalts or adsorption on mineral surface and thus complicate the interpretation of ‘quenched’ signal from fluid inclusions or fumarole analyses [5]. The development and validation of precise THMC models of fluid-rock interactions and precipitation patterns in volcanic geothermal systems thus requires new experimental data.

Here, we will present two different approaches that have been developed to enable the characterization of Pb, Se and Te behaviour in silicate melts, supercritical fluids, brines and vapors, as a function of P-T conditions, melt and fluids composition, down to the molecular level. They first one relies on the recovery of quenched fluids for detailed chemical analysis by ion chromatography (major elements + Cl or S) and ICP-MS to determine fluid-melt partition coefficients to 800-1200 °C and 2-4 kbar, whether the second one takes advantage of in situ Raman or X-ray Absorption Spectroscopy to assess the speciation and brine-vapor fractionation of the metals to 200-600 °C and 0.5-1.5 kbar. Ultimately, these complementary results will enable estimating the budget of Pb, Se and Te transferred to the geothermal system and the atmosphere and the information it bears about the P-T-X-fO₂ conditions and processes at depth.

References: [1] Edmonds et al., 2018. Nat. Geosc. 11, 790-794. [2] Frank et al., 2002. GCA 66,3719-3732. [3] Zajacz et al., 2012. GCA 91, 140-159. [4] Pokrovski et al., 2013. Rev. Mineral. Geochem. 76, 165-218. [5] Henley and Berger, 2013. Earth Sci. Rev. 125, 146-170.