Equilibrium Sn Isotope Fractionation among Sn-Bearing Minerals

Abstract

Quantifying equilibrium Sn isotope fractionation among Sn-bearing minerals is fundamental for understanding the partitioning of Sn isotopes during various geological and cosmochemical processes. In this study, we used first-principles calculations to evaluate equilibrium Sn isotope fractionations among 13 Sn-bearing minerals. The Sn isotope reduced partition function ratios (RPFRs or β factors) calculated with the state-of-the-art r2SCAN functional decrease in the order of brannockite > eakerite > megawite > cassiterite > pirquitasite > kesterite > stannite > berndtite-2T ≈ hydroromarchite > romarchite > ottemannite > herzenbergite > native tin. At high temperatures, the variation range can still be large. The 1000 ln122/116 β values show a linear relationship with the average Sn force constants in minerals. The results indicate that under equilibrium, the preference of heavy Sn isotope partitioning decreases in the order of Sn­(IV) minerals > Sn­(II) minerals > native tin. Stannic silicates are enriched in heavy Sn isotopes among all of the minerals, and oxides are enriched in heavy Sn isotopes relative to sulfides in addition to the effect of the oxidation state of Sn. The RPFRs for hydrothermal Sn species are between those of oxide and sulfide minerals at the corresponding oxidation states. These results provide a data set of equilibrium Sn isotope fractionation factors, which indicates that the changes in the coordination environment and in the oxidation state can have similar effects on equilibrium Sn isotope fractionation. This could be important in interpreting natural Sn isotope variabilities.