Calculation on sulphur isotope fractionation between sphalerite and galena using lattice dynamics

Increasing use is being made of sulphide minerals in isotope geothermometry. Sulphur isotope fractionation factors for³⁴S exchange between sphalerite (ZnS) and galena (PbS) have been calculated for various temperatures between 0 and 1000°C. The reduced partition function ratios have been calculated using a “shell” model for the forces derived from inelastic neutron scattering studies of the lattice dynamics of sphalerite and galena. A new formalism of the current theory has been developed which enables the accuracy of the calculation to be determined.The Zn³⁴S—Pb³⁴S equilibrium constant obtained by the shell model calculations is 1.0060 to 100°C, 1.0031 at 250°C and 1.0005 at 1000°C, in agreement with experimental determinations.