TY - JOUR
T1 - The oxidation state of sulfur in synthetic and natural glasses determined by X-ray absorption spectroscopy
AU - Métrich, Nicole
AU - Berry, Andrew J.
AU - O'Neill, Hugh St C.
AU - Susini, Jean
PY - 2009/4/15
Y1 - 2009/4/15
N2 - Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO2) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO2 < -9) or oxidised (log fO2 > -2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S2-. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV. The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO42-. Sulfite (SO32-) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S6+). S4+ does, however, occur unambiguously with S6+ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO4 at 4-16 kbar, and when buffered with Re/ReO2 at 10 kbar. Solubility of S4+ thus requires partial pressures of SO2 considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO2s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S2- and S6+ were found to coexist in some samples that were not in equilibrium with the imposed fO2. The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S2-, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S2- + 8Fe3+ = S6+ + 8Fe2+, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S2- and S6+ are frequently found together despite the very limited fO2 range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.
AB - Sulfur K-edge X-ray absorption near edge structure (XANES) spectra were recorded for experimental glasses of various compositions prepared at different oxygen fugacities (fO2) in one-atmosphere gas-mixing experiments at 1400 °C. This sample preparation method only results in measurable S concentrations under either relatively reduced (log fO2 < -9) or oxidised (log fO2 > -2) conditions. The XANES spectra of the reduced samples are characterised by an absorption edge crest at 2476.4 eV, typical of S2-. In addition, spectra of Fe-bearing compositions exhibit a pronounced absorption edge shoulder. Spectra for all the Fe-free samples are essentially identical, as are the spectra for the Fe-bearing compositions, despite significant compositional variability within each group. The presence of a sulfide phase, such as might exsolve on cooling, can be inferred from a pre-edge feature at 2470.5 eV. The XANES spectra of the oxidised samples are characterised by an intense transition at 2482.1 eV, typical of the sulfate anion SO42-. Sulfite (SO32-) has negligible solubility in silicate melts at low pressures. The previous identification of sulfite species in natural glass samples is attributed to an artefact of the analysis (photoreduction of S6+). S4+ does, however, occur unambiguously with S6+ in Fe-free and Fe-poor compositions prepared in equilibrium with CaSO4 at 4-16 kbar, and when buffered with Re/ReO2 at 10 kbar. Solubility of S4+ thus requires partial pressures of SO2 considerably in excess of 1 bar. A number of experiments were undertaken in an attempt to access intermediate fO2s more applicable to terrestrial volcanism. Although these were largely unsuccessful, S2- and S6+ were found to coexist in some samples that were not in equilibrium with the imposed fO2. The XANES spectra of natural olivine-hosted melt inclusions and submarine glasses representative of basalts at, or close to, sulfide saturation show mainly dissolved S2-, but with minor sulfate, and additionally a peak at 2469.5 eV, which, although presumably due to immiscible sulfide, is 1 eV lower than that typical of FeS. These sulfate and sulfide-related peaks disappear with homogenisation of the inclusions by heating to 1200 °C followed by rapid quenching, suggesting that both these features are a result of cooling under natural conditions. The presence of small amounts of sulfate in otherwise reduced basaltic magmas may be explained by the electron exchange reaction: S2- + 8Fe3+ = S6+ + 8Fe2+, which is expected to proceed strongly to the right with decreasing temperature. This reaction would explain why S2- and S6+ are frequently found together despite the very limited fO2 range over which they are thermodynamically predicted to coexist. The S XANES spectra of water-rich, highly oxidised, basaltic inclusions hosted in olivine from Etna and Stromboli confirm that nearly all S is dissolved as sulfate, explaining their relatively high S contents.
UR - http://www.scopus.com/inward/record.url?scp=62349123205&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2009.01.025
DO - 10.1016/j.gca.2009.01.025
M3 - Article
SN - 0016-7037
VL - 73
SP - 2382
EP - 2399
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 8
ER -