The Selective Flux of Sulfur and Implications for Magmatic Sulfur Isotope Fractionation

The selective flux of sulfur during magma emplacement is proposed to explain some abnormal δ³⁴S data from granitic and basaltic rocks. It is assumed that on the one hand a quasi-equilibrium isotope fractionation exists between sulfate and sulfide during magma cooling, and on the other hand a non-equilibrium fractionation occurs between the fluxed sulfur and the magmatic sulfur. The results show that at high fO₂, ³⁴S is preferably enriched in sulfate with decreasing temperature, without a corresponding depletion of sulfide in ³⁴S. The δ³⁴S value of solidified rocks is then significantly shifted in the positive direction due to the selective degassing or assimilation. Conversely, at low fO₂, ³⁴S is preferably depleted in sulfide as temperature declines, while a corresponding ³⁴S-enrichment in sulfate does not occur. As a consequence, δ³⁴S value of the rocks is driver in the negative direction.