SYNCHROTRON STUDIES OF SULFUR SPECIATION IN SILICATE GLASSES: IMPLICATIONS FOR THE SULFUR CONTENT OF MAGMATIC SYSTEMS

Sulfur is an element of interest in magmatic-hydrothermal systems because its behavior is directly related to a variety of geological processes (e.g. formation of ore deposits, global cooling by S-rich explosive volcanism). Sulfur is sensitive to oxygen fugacity (fO₂) variations and can be present in silicate melts as sulfide (S^2-), sulfate (S⁶⁺), sulfite (S⁴⁺) or combinations of these species. Understanding how S speciation changes as a function of fO₂ is of interest because S speciation determines the amount of sulfur that magmas can dissolve and that can be transferred to a magmatic volatile phase. We used X-ray Absorption Near Edge Structure (XANES) at the S K-edge to determine S speciation in natural basaltic glasses (from several locations worldwide) and in glasses of basaltic and andesitic composition synthesized in a piston-cylinder apparatus at 1 GPa and between 1100 ^oC and 1350 ^oC. In some experiments a strong fO₂ gradient was imposed, creating reduction profiles that record the continuous transition from sulfide to sulfate species. The analyses were performed at the European Synchrotron Radiation Facility (ESRF), beamline ID21, using a combination of “broad” and “focused” beams (200 and 0.8µm diameter, respectively). The high spatial resolution of the focused beam, in combination with our experimental approach, provided unique insights into the S speciation transition in silicate melts. Our results show the coexistence of S⁴⁺ and S⁶⁺ species and S^2- and S⁶⁺ species. However, we did not observe coexistence of S^2- and S⁴⁺ species in our samples. The results from “reduction-profile” experiments (showing a continuous variation in the proportion of S^2- and S⁶⁺) were used to calculate how S solubility in silicate melts changes from sulfide dominated to sulfate dominated system. This is especially relevant for arc magmas because their fO₂ overlaps with the range in fO₂ for the transition from S^2- to S⁶⁺ in silicate melts.