Sulfur K-edge XANES spectroscopy: Chemical state and content of sulfur in silicate glasses

The chemical state and local environment of sulfur in basaltic glasses from the eastern Galapagos rift and quenched laboratory experiments have been investigated by synchrotron-radiation S K-edge X-ray absorption near-edge structure (XANES) spectroscopy, using an energy-selective fluorescence detector. We have also investigated sulfur in melts in the system 6NaAlSiO ₄ -CaS-CaSO ₄ quenched from 1500°C and various reference compounds, using conventional fluorescence yield and total-electron yield (TEY) recording modes. The S K-edge XANES spectrum of CaS dissolved in Na-aluminosilicate glass is dominated by a broad singlet edge-feature near 2475 eV, consistent with isolated SCa _a coordination polyhedra in the melt, whereas that of quenched end-member CaSO ₄ -bearing melt is similar to XANES spectra of crystalline sulfates. The linear correlation between relative XANES area and wt% content of sulfur has been used to estimate the content of sulfur in samples of lazurite (5.6-6.3 wt% by EPMA), scapolite-group minerals (0.4 wt%) and the basaltic glasses (0.04-0.15 wt%). The S K-edge XANES spectra of basaltic glasses synthesized under reducing (WM and IQF solid oxygen-buffer) conditions are dominated by a single broad absorption peak at 2470-2482 eV in combination with a weak peak or shoulder at 2470-2471 eV, and are reproduced by mixing XANES spectra of FeS and alkaline-earth monosulfides. The S K-edge XANES spectra for glasses from the eastern Galapagos rift are also dominated by S ^2- , but one TEY spectrum shows possible thiosulfate and another, trace sulfate.