Stability and abundance of the trisulfur radical ion S-3(-) in hydrothermal fluids
Résumé
The interpretation of sulfur behavior in geological fluids and melts is based on a long-standing paradigm that sulfate, sulfide, and sulfur dioxide are the major sulfur compounds. This paradigm was recently challenged by the discovery of the trisulfur ion S-3(-) in aqueous S-bearing fluids from laboratory experiments at elevated temperatures. However, the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions. Here we used in situ Raman spectroscopy on model thiosulfate, sulfide, and sulfate aqueous solutions across a wide range of sulfur concentration (0.5-10.0 wt%), acidity (pH 3-8), temperature (200-500 degrees C), and pressure (15-1500 bar) to identify the different sulfur species and determine their concentrations. Results show that in the low-density (<0.2 g/cm(3)) vapor phase, H2S is the only detectable sulfur form. By contrast, in the denser liquid and supercritical fluid phase, together with sulfide and sulfate, the trisulfur radical ion S-3(-) is a ubiquitous and thermodynamically stable species from 200 degrees C to at least 500 degrees C. In addition, the disulfur radical ion S is detected at 450-500 degrees C in most solutions, and polymeric molecular sulfur with a maximum abundance around 300 degrees C in S-rich solutions. These results, combined with revised literature data, allow the thermodynamic properties of S-3(-) to be constrained, enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids. These predictions suggest that S-3(-) may account for up to 10% of total dissolved sulfur (S-tot) at 300-500 degrees C in fluids from arc-related magmatic-hydrothermal systems, and more than 50% S-tot at 600-700 degrees C in S-rich fluids produced via prograde metamorphism of pyrite-bearing rocks. The trisulfur ion may favor the mobility of sulfur itself and associated metals (Au, Cu, Pt, Mo) in geological fluids over a large range of depth and provide the source of these elements for orogenic Au and porphyry-epithermal Cu-Au-Mo deposits. Furthermore, the ubiquity of S-3(-) in aqueous sulfate-sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass-dependent and mass-independent fractionation of sulfur isotopes.