Estimate of mole sulfate ratios in magmatic rocks

Abstract

This study presents a numerical method to estimate the mole fractions of sulfate species in magmatic rocks using sulfur isotopes, which can be illustrated in a δ-δ diagram. The mole sulfate ratio in an igneous rock sample can be calculated from the δ 34 S rock value of the whole-rock sample and the δ 34 S sulfide (or δ 34 S sulfate ) value of a sulfide and (or) a sulfate mineral (species) in the sample on the basis of S-isotope fractionation and mass balance under the conditions of magmatic equilibrium. This method can be used to test if magmatic equilibrium between sulfate and sulfide species in magmatic rocks is retained, and to predict the δ 34 S value of one of them at a certain temperature if sulfur isotopic equilibrium is maintained. The initial sulfur isotope composition of the system in question may be estimated based on a set of samples from an igneous suite. Mole sulfate ratio in the magmatic rocks is then used to estimate redox conditions under which they are formed. This practice is important in understanding the genesis of mineral deposits associated with magmatic rocks.