Sulfur radical species and their applications for metal resources, magma evolution, and isotope geochemistry – RADICALS

Sulfur in aqueous fluids and silicate melts is a key element in many natural processes controlling the formation and exploitation of mineral resources, magma evolution and volatile degassing and isotope fractionation in mineral-fluid systems. For technological applications, sulfur is an important player in chemical catalysis and nanotechnology. Understanding the chemical speciation of sulfur is thus critical across a wide range of temperatures (T), pressures (P) and physical-chemical environments. For decades, the major chemical forms of sulfur have been believed to be sulfate, sulfide, and sulfur dioxide; however, our recent discoveries of the sulfur radical ions S3- and S2- in geological and technological fluids may dramatically change our vision of a number of processes in which sulfur is involved. This is because the S radicals have unique properties, such as enhanced chemical reactivity and selective affinity for some metals, recognized in our previous program ANR SOUMET. However, very little is still known about their respective roles in natural processes. Yet, these radicals may significantly affect magma evolution, metal transport and ore deposit formation, sulfate-sulfide reaction kinetics and associated sulfur isotope fractionations. The goal of this project is therefore to bring quantitative constraints on the amount, stability and reactivity of these potentially important new sulfur forms in fluid and melt phases across the wide range of temperatures and pressures of the lithosphere.

To achieve this goal, we will combine original experimental tools (hydrothermal reactors, piston-cylinder apparatuses), in situ spectroscopic techniques (Raman and XAS in optical cells), cutting-edge analytical facilities for metal, sulfur and isotope measurements (LA-ICPMS, GS-MS, SIMS), and molecular dynamics and thermodynamic modeling. Most of these techniques are already established and several of them have been developed with the support of the past ANR SOUMET research program. Specifically, we will achieve the following major objectives: 1) to quantify the stability and abundance of radical ions in silicate melts and aqueous and mixed (H2O-CO2-salt) fluids across the T-P range of the lithosphere (to ~1200°C and ~15 kbar), 2) to elucidate the transport capacities of these radicals for selected economically critical metals (Cu, Mo, Re, Platinoids) in melts and fluids and the resulting ore deposit formation, 3) to investigate, in hydrothermal fluid-mineral systems, the fractionation of stable sulfur isotopes, including so-called mass-independent fractionation for the S3- ion, which has an ozone-like molecular and electronic structure, and 4) to create robust physical-chemical models enabling predictions of radical ions abundances, stability and solubility of their complexes with metals and S isotope fractionation factors allowing, for the first time, quantitative assessment of the role of these fascinating sulfur species in magmatic-hydrothermal processes.

We believe that this program will profoundly change paradigms in geochemistry, petrology and mineral resources research, with far-reaching future applications for ore processing, nanotechnology and chemical catalysis. The program involves five French teams - leaders in experimental geochemistry and petrology, synchrotron-based methods, physical-chemical modeling of fluids and melts, in situ microanalysis, isotope geochemistry, volcanology, and economic geology. This interdisciplinary consortium, built at the interface “geochemistry-petrology-ore deposit geology-chemistry-physics”, is expected both to reinforce the position of France in these scientific domains and to allow, in the future, the results of this fundamental research to be applied in various natural and industrial environments.