Qualitative assessment of high–temperature disequilibrium sulfur isotope fractionation in the gas–silicate melt–sulfide liquid system from experiments

Experiments using a dynamic 1–atm gas–mixing furnace simulated oxidation and reduction processes, as observed during volcanic degassing, by manipulating the oxygen (log f O 2 ) and sulfur fugacities (log f S 2 ). The experiments were designed to identify the rate–limiting processes in sulfur isotope fractionation and determine the mechanisms of exchange between silicate melt and a sulfide liquid under dynamic conditions. Six series of experiments were conducted using high–Mg olivine crystals as capsules and basalt and Fe powder as the charges. The experimental approach involved equilibrating samples at initial conditions for eight hours at 1300 °C, followed by continuous atmospheric changes within the furnace at a constant rate until the final conditions were attained, after which samples were quenched. The imposed dynamicity varied from 15 min to 120 min, with rate of fugacity changes between –8.4 and 10.4 log unit hr –1 for f O 2 and –4 to 4.4 log unit hr –1 for f S 2 . S isotope fractionation between the silicate melt and sulfide liquid was verified to be at disequilibrium using the existing equilibrium model, with the silicate melt being isotopically lighter than would be expected at equilibrium conditions. Despite sulfur’s transfer within the basaltic melt through diffusion, diffusive isotopic fractionation was not observed in any samples. This provides us a β value of 0 for sulfur isotopic diffusion at high temperatures, where β quantifies the mass (m) dependence of the diffusion coefficient (D) of the isotopes 1,2 as ( D 1 / D 2 ) = ( m 2 / m 1 ) β , while also showing a lack of an effect of pressure on sulfur isotope diffusion. The observed disequilibrium fractionation is hypothesized to be due to isotope fractionation at the interface between the phase boundaries.