Seminar: Temperature, melt composition and oxygen fugacity controls on sulfur behavior in silicate melts: an experimental study
E001
Abstract
Sulfur plays a key role in igneous processes such as volcanic degassing, magmatic ore formation, and planetary differentiation. Its multiple oxidation states (–2, 0, +4, +6) govern its distribution among terrestrial reservoirs. In silicate melts, S mainly occurs as sulfide (S²⁻) and sulfate (S⁶⁺ or SO₄²⁻). Despite this understanding, experimental data on S²⁻ and S⁶⁺ concentrations in natural melts remain limited across compositions and temperatures.
We experimentally examined the effects of temperature (1200–1500 °C) and melt composition (14 types) on sulfide and sulfate solubility at atmospheric pressure, under controlled oxygen and sulfur fugacities. Sulfur and major elements in quenched glasses were measured by electron microprobe and SIMS (< 60 ppm S). Sulfide and sulfate solubilities were expressed as capacities (CS²⁻ and CS⁶⁺) relative to fO₂ and fS₂.
Our data show that sulfide capacity increases with FeO content and temperature, while sulfate capacity increases with CaO content and decreases with temperature. These data allow estimation of the melt S redox ratio (S⁶⁺/S²⁻) as a function of composition, temperature, and oxygen fugacity. The findings agree with previous work and indicate that the oxygen fugacity of the S²⁻–S⁶⁺ transition rises significantly relative to FMQ as temperature drops from 1500 to 1200 °C. Simulations of sulfur degassing show that an H₂O-rich fluid generally raises fO₂ — via degassing of H₂ and H₂S at low fO₂, and H₂O and SO₂ at high fO₂.