First in situ measurement of sulfur speciation under high pressure using synchrotron X-ray absorption spectroscopy

The chemical speciation of sulfur in aqueous fluids and silicate melts is a controlling factor in magmatic-hydrothermal processes on Earth. However, this speciation is poorly understood because many sulfur chemical species cannot be preserved in fluids and melts during cooling and decompression thereby requiring in situ analysis. Here we employed synchrotron X-ray absorption spectroscopy (XAS) to investigate sulfur speciation under high pressure in blue-green varieties of sodalite minerals and their synthetic analogs ultramarine pigments. These phases host a diverse array of coexisting sulfur species of different redox states. These solids can, therefore, be regarded as proxies for sulfur speciation in fluids and melts. Our measurements were performed using a recently designed perforated diamond anvil cell at the ID12 beamline of the European Synchrotron Radiation Facility (ESRF). This innovative design enables to directly probe the sulfur K-edge (∼2470 eV) at elevated pressures. The obtained high-resolution spectra, when combined with those of sulfur reference compounds, demonstrate an excellent sensitivity of the method at pressures up at least to 6 GPa. Our spectroscopic approach offers straightforward identification of the different sulfur species, which are difficult to discriminate using more traditional laboratory methods (e.g., Raman spectroscopy, electron paramagnetic resonance or X-ray photoelectron spectroscopy). In particular, it enables the disulfur and trisulfur radical ions to be detected and quantified using their characteristic pre-edge peaks and their evolution with pressure to be unraveled. Our first results provide a ground for future in situ XAS studies of sulfur speciation in mineral, fluid, and melt phases of the Earth’s interior, overcoming the limitations of less direct analyses.