Abstract
S occurs in both oxidized (S6+) and reduced (S2−) forms in natural silicate liquids. The wavelength of SKα radiation changes as a function of the oxidation state of S and can be used to determine the proportions of sulfide and sulfate in unknown samples by comparison with reference standards of a known valence state. We have measured the wavelength of SKα X-rays in a variety of glassy submarine lavas from spreading centers (Juan de Fuca, Galápagos, Kane, and Southeast Indian Ridge), seamounts (Loihi and Lamont seamounts), and back-arc basins (Lau Basin). The relative fO2 values of these glasses are known from wet-chemical determinations of FeO and Fe2O3 and vary from slightly above the Ni + NiO buffer (NNO) to 3.2 log units below NNO. Measured values of sulfate/Stot (S6+/Stot) vary from 0.02 to 0.25 and increase with increasing relative fO2. Our results demonstrate that the dissolved S in these samples occurs predominately as S2−. Large variations in H2O content have no measurable effect on the redox state of S, suggesting that H2S or HS− are not abundant S species in solution in silicate liquids. Thermodynamic calculations show that the speciation of S in coexisting gas during submarine eruptions of basaltic magma is also dependent on relative fO2. Many lavas from seamounts and back-arc basins are predicted to have S present mainly as SO2 in coexisting magmatic gas, whereas the lower fO2 of midocean ridge basalts favor H2S as the dominant form of gaseous S.
