Publication Type:Book Chapter
Source:Geological Society of America, 2006 annual meeting, Geological Society of America (GSA), Boulder, CO, United States, Volume 38, p.245 (2006)
Keywords:behavior, chemical fractionation, glasses, igneous rocks, magmas, P-T conditions, solubility, solution, spectra, sulfates, sulfides, sulfur, synchrotrons, volatiles, volcanic rocks, VOLCANISM, X-ray spectra, XANES spectra
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 (fO (sub 2) ) variations and can be present in silicate melts as sulfide (S (super 2-) ), sulfate (S (super 6+) ), sulfite (S (super 4+) ) or combinations of these species. Understanding how S speciation changes as a function of fO (sub 2) 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 degrees C and 1350 degrees C. In some experiments a strong fO (sub 2) 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 0.8mu m 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 S (super 4+) and S (super 6+) species and S (super 2-) and S (super 6+) species. However, we did not observe coexistence of S (super 2-) and S (super 4+) species in our samples. The results from "reduction-profile" experiments (showing a continuous variation in the proportion of S (super 2-) and S (super 6+) ) 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 fO (sub 2) overlaps with the range in fO (sub 2) for the transition from S (super 2-) to S (super 6+) in silicate melts.
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