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 lnαsulfate-sulfide = 7.4 × 106/T2 − 0.19. In alkali chloride melts, sulfide would exist as HS-, S2-, NaHS and NaS-, and sulfate as SO42- 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 lnαsulfate-H2S = (6.5 ± 0.3) × 106/T2.
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