Zinc (Zn) is nowadays one of the most important metal commodities in view of the energy transition being used in varied industrial applications. It is also employed as a reference element for several magmatic processes as a tracer (Zn/Fe ratios) and its isotopes can inform on mechanisms concerning planetary evolution. It is commonly found on Earth in sedimentary ore deposits and volcanism associated ones. The common factor between them being that Zn is mobilized through high-temperature fluids before deposition and a good way to study this process is to investigate its speciation among others. Zn speciation in Cl-bearing fluids has been studied before but mostly with thermodynamic and molecular simulations with very few experimental confirmations and only for a limited P-T-X range (>1000bar, <500°C) [1,2]. They highlighted that Zn is transported in mixed oxy/hydroxyl/chloride complexes in low temperatures (<200°C) while chloride complexes only dominate for higher temperatures in acidic fluids. Here, a new set of in situ experiments on Zn speciation in magmatic-hydrothermal fluids of different salinities (0.1mHCl to 4.5mLiCl) and acidities (from pH = 0.6 to pH = 6.3) will be presented. It involved the use of a HP-HT transparent autoclave while combining high-resolution synchrotron X-ray absorption and Raman spectroscopies in shallow crustal conditions (30-600°C – 600 bar) relative to magmatic degassing. Processing the data revealed an important effect of temperature with the Zn speciation changing progressively from octahedral aqueous Zn(H2O)62+ to chloride ZnCl42- complex in most of the solutions. The initial Cl content also showed an important effect as the chloride complex was already present at room temperature for the most Cl concentrated solution. Further discussion will be given to the implications of these results for Zn transport and deposition.
[1] Mei. et al., 2015. Geochim. Cosmochim. Acta, 150, 265-284
[2] Etschmann et al., 2019. Am. Mineral., 104, 158–161