Sulfur dissolves in silicate melts in at least two and possibly three oxidation states: sulfide (S2-), sulfate (S6+), and possibly sulfite (S4+). The solubility of sulfur in magmas is directly related to its oxidation state, and is important for understanding geological processes such as the origin of magmatic sulfide ores, sulfur degassing from volcanic eruptions and hence global climate change, and the geochemical behavior of the chalcophile trace elements (Re-Os, PGE).
We have already done different series of XANES experiments on natural glass/melt inclusions and sulfide compounds [1,2]. The XANES spectra of glass inclusions, representative of basaltic magmas saturated with respect to the immiscible FeS sulfide liquid, showed the coexistence of different sulfur species (sulfate, sulfide, even sulfite, ..), difficult to interpret in terms of redox conditions (fO2, fS2), only.
In order to determine the oxidation state and speciation of sulfur in silicate glasses, µXANES spectra at the sulfur K-edge, were recorded for a wide range of experimental samples. XANES experiments were performed at the European Synchrotron Radiation Facility (ESRF, France), using the X-ray microscopy beamline, ID21.
Glasses ranging from typical MORB-type basalts to Ca-(Na, Mg) rich compositions were synthesized under reducing conditions (log fO2 –8.79 to – 10.92; log fS2 –1.91) at 1200-1400°C and 1 bar. The XANES spectra of all glasses show a main peak at 2476.3 eV (S2-), while those of the Fe-bearing glasses also contain a shoulder on the absorption edge (2471.7 eV) and differ in their structure after the edge. All these XANES spectra are clearly different in both the energy position of the first peak and the structure after the main edge from those recorded on sulfide-bearing minerals.
High-pressure experiments saturated with FeS display a low energy peak at 2470.5 eV, indicating a contribution from sulfide microglobules, that can be easily assessed by combining the XANES spectrum of the glass anf that of the sulfide globule.
For glasses synthesized under oxidizing conditions (log fO2 –0.49 to – 0.30; log fS2 –0.18 to –0.31; log fSO2 –1.47 to –2.06) at 1200-1400°C and 1 bar, all spectra are essentially identical, with a main peak at 2482 eV (sulfate), despite large variations in composition.
We show that the sulfite (S4+) species may co-exist with sulfate (S6+), in Fe-free glasses under highly oxidizing conditions. Conversely, S2- and S6+ species were not found to coexist in the experimental glasses. The spectra of these samples differ significantly from those of melt inclusions in addition to the sulfate peak at 2482.2 eV, and an unassigned peak at 2469.4 eV. The presence of both S2- and S6+ may reflect intermediate redox conditions, although this is unlikely given the very narrow fO2 range over which the two oxidation states are expected to coexist. An alternative explanation is post-entrapment oxidation, which is supported by experiments on melt inclusions heated as a funtion of time.
 M. Bonnin-Mosbah N. Métrich N., J. Susini., M. Salomé, D. Massare, B. Menez (2001). Spectr. Chim. Acta B57, 711-725.
 Metrich N., Bonnin-Mosbah M., Susini J., Menez B. and Galoisy L. (2002), Geophys. Res. Lett. 29, 11.
1. Laboratoire Pierre Sue, CEA-CNRS, Saclay, France
2. Research School of Earth Sciences, ANU, Canberra, Australia
3. European Synchrotron Radiation Facility, Grenoble, France
Goldschmidt Conference 20-25 May 2005, Session S05 – Advances in in-situ microanalysis of trace elements