Sulfur Isotope Fractionation in Magmatic Systems：Models of Rayleigh Distillation and Selective Flux

The effect of Rayleigh distillation by outgassing of SO2 and H2S on the isotopic composition of sulfur remaining in silicate melts is quantitatively modelled.A threshold mole fraction of sulfur in sulfide component of the melts is reckoned to be of critical importance in shifting the δ^34S of the melts mith respect to the original magmas.The partial equilibrium fractionation in a magmatic system is evaluated by assuming that a non-equilibrium flux of sulfur occurs between magmatic volatiles and the melts,while an equilibrium fractionation is approached between sulfate and sulfide within the melts.The results show that under high fo2 conditions,the sulfate/sulfide ratio in a melt entds to increase,and the δ^34S value of sulfur in a solidified rock might then be shifted in the positive direction.This may either be due to Rayleigh outgassing in case the mole fraction of sulfide is less than the threshold,or due to a unidirectional increase in δ^34S value of the sulfate with decreaing temperature,Conversely,at low fo2,the sulfate/sulfide ratio tends to decrease and the δ^34S value of total sulfur could be driven in the negative direction,either because of the Rayleigh outgassing in case the mole fraction of sulfide is greater than the threshold,or because of a unidirectional decrease inδ^34S value of the sulfide.To establish isotopic equilibrium between sulfate and sulfide,the HM,QFM or WM buffers in the magmatic system are suggested to provide the redox couple that could simultaneously reduce the sulfate and oxidize the sulfide.CaO present in the silicatte Melts is also called upon to participate in the chemical equilibrium between sulfate and sulfide,Consequently,the δ^34S value of an igneous rock could considerably deviate from that of its original magma due to the influence of oxygen fugacity and temperature at the time of magma solidification.

Sulfur isotope fractionation in magmatic systems: Models of Rayleigh distillation and selective flux

The effect of Rayleigh distillation by outgassing of SO₂ and H₂S on the isotopic composition of sulfur remaining in silicate melts is quantitatively modelled. A threshold mole fraction of sulfur in sulfide component of the melts is reckoned to be of critical importance in shifting the δ³⁴S of the melts mith respect to the original magmas. The partial equilibrium fractionation in a magmatic system is evaluated by assuming that a non-equilibrium flux of sulfur occurs between magmatic volatiles and the melts, while an equilibrium fractionation is approached between sulfate and sulfide within the melts. The results show that under high conditions, the sulfate/sulfide ratio in a melt tends to increase, and the δ³⁴ S value of sulfur in a solidified rock might then be shifted in the positive direction. This may either be due to Rayleigh outgassing in case the mole fraction of sulfide is less than the threshold, or due to a unidirectional increase in δ³⁴S value of the sulfate with decreasing temperature. Conversely, at low , the sulfate/sulfide ratio tends to decrease and the δ³⁴S value of total sulfur could be driven in the negative direction, either because of the Rayleigh outgassing in case the mole fraction of sulfide is greater than the threshold, or because of a unidirectional decrease in δ³⁴S value of the sulfide. To establish isotopic equilibrium between sulfate and sulfide, the HM, QFM or WM buffers in the magmatic system are suggested to provide the redox couple that could simultaneously reduce the sulfate and oxidize the sulfide. CaO present in the silicate melts is also called upon to participate in the chemical equilibrium between sulfate and sulfide. Consequently, the δ³⁴S value of an igneous rock could considerably deviate from that of its original magma due to the influence of oxygen fugacity and temperature at the time of magma solidification.