ABSTRACT

Sulfur isotope fractionation factors between sulfate and sulfide at 600 to 1, 000°C were experimentally determined by decomposing anhydrous Na₂SO₃ in melts of NaCl and LiCl-KCl mixture (6 : 4 molar ratio). Sulfur isotope exchange equilibrium was attained between sulfate and sulfide formed by decomposition of sodium sulfite within 120 minutes at 600°C in the LiCl-KCl melt and within 30 minutes at 810°C in the NaCl melt. The observed fractionation factors (α) obey the relationship in the experimental temperature range: 1000 lnα_{sulfate-sulfide} = 7.4 × 10⁶/T² − 0.19. In alkali chloride melts, sulfide would exist as HS^-, S^2-, NaHS and NaS^-, and sulfate as SO₄^2- and NaSO₄^-, respectively, in an analogy with aqueous sodium chloride solution. The observed fractionation factors are considered to represent those between bulk sulfate and sulfide ions. The fractionation factor between gaseous H₂S and dissolved sulfate in melt at 600 and 700°C is about 1 and 0.7‰ smaller than those between dissolved sulfide and sulfate in melt, respectively, and may be approximated by the following equation in the experimental temperature range: 1000 lnα_sulfate-H2S = (6.5 ± 0.3) × 10⁶/T².