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Compositional and temperature effects on sulfur speciation and solubility in silicate melts

DOI: 10.1016/j.epsl.2018.12.006 DOI Help

Authors: William M. Nash (University of Oxford) , Duane J. Smythe (University of Oxford) , Bernard J. Wood (University of Oxford; University of Bristol)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Earth And Planetary Science Letters , VOL 507 , PAGES 187 - 198

State: Published (Approved)
Published: February 2019

Open Access Open Access

Abstract: We have determined the chemical speciation of dissolved sulfur and the sulfur concentration at fixed oxygen and sulfur fugacities for a wide range of silicate melt compositions (from Fe-rich basalt to dacite). Each melt was equilibrated at 1300 °C and 1-atmosphere pressure at oxygen fugacities (fO2) between −1.67 and +1.6 log units relative to the Fayalite–Magnetite–Quartz (FMQ) buffer and absolute sulfur fugacities between −5.1 and −1.2 log units. The fO2 and fS2 of the experiments were controlled by using gas mixtures of CO–CO2–SO2. The speciation of sulfur in the quenched glasses was determined using both X-ray Absorption Near-Edge Spectroscopy (XANES), and from the dependence of equilibrium sulfur concentration on the fS2/fO2 ratio measured by secondary-ion mass spectrometry (SIMS) and electron microprobe. The speciation of dissolved sulfur in each melt undergoes an abrupt transformation from S2− to S6+ with increasing fO2, and this transition is shifted ∼0.5 log units higher in fO2 as melt FeO concentration increases from ∼5 wt% to ∼18 wt%. Since sulfide concentrations at constant fO2 and fS2 are consistently greater for more FeO-rich melts, the compositional effect on speciation may be explained by the well-known sensitivity of the sulfide capacity ( CS2− ) of the melt to FeO concentration. S6+/S2− ratios for the glasses exhibit a linear relationship with Fe3+/Fe2+, indicating that the redox couples for iron and sulfur can be directly related to one another. We used thermodynamic data to model the interrelationship between Fe and S oxidation states in terms of the equilibrium FeS+8FeO1.5=8FeO+FeSO4 Fitting the data to our experiments at 1300 °C we obtained the following expression for the temperature-dependence of speciation: log⁡(S6+S2−)=8log⁡(Fe3+Fe2+)+8.7436×106T2−27703T+20.273 This equation fits the data for all our compositions and is also consistent with earlier results at 1050 °C and 950 °C. We used the interdependence of S and Fe oxidation states to infer electron transfer between Fe2+ and S6+ during quenching of glasses from Mauna Kea, Hawaii. The effect is sufficient to cause significant overestimation of equilibrium Fe3+/ΣFe in natural glasses and corresponding overestimate of fO2 by about 0.8 log units. Glasses equilibrated under the most oxidizing conditions (containing S6+ only) have equilibrium S concentrations that are negatively correlated with their mole fractions of tetrahedral (Si + Ti) cations.

Keywords: sulfur; oxidation state; silicate melt; XANES; SIMS

Subject Areas: Earth Science, Chemistry


Beamlines: I18-Microfocus Spectroscopy

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