Equilibrium Inter-Mineral Titanium Isotope Fractionation: Implication for High-Temperature Titanium Isotope Geochemistry

Wenzhong Wang, University of Science and Technology of China
Shichun Huang, University of Nevada, Las Vegas
Fang Huang, University of Science and Technology of China
Xinmiao Zhao, Chinese Academy of Sciences, Beijing
Zhongqing Wu, University of Science and Technology of China


Equilibrium Ti isotope fractionation factors among major Ti-bearing minerals are critical for understanding Ti isotope fractionation during magmatic processes. We use the first-principles calculations based on the density functional theory (DFT) to obtain Ti isotope reduced partition function ratios (103lnβ of 49Ti/47Ti) in a series of important Ti-bearing minerals, including Ti-doped clinopyroxene, orthopyroxene, olivine, and pyrope, geikielite-ilmenite solid solutions, and rutile. There is a large variation in our calculated 103lnβ, which are linearly correlated to their Ti force constants, a parameter related to the average TiO bond length and the Ti valence state. Among all studied minerals, silicates with Ti4+ occupying the tetrahedral Si site have the highest 103lnβ, and rutile has the lowest 103lnβ. The valence state also significantly controls the 103lnβ. Typically, Ti3+-doped silicates have lower 103lnβ than those of Ti4+-doped silicates. At the natural abundance levels, the 103lnβ of Ti4+Si-doped and Ti3+Mg-doped (Ti3+ occupying the Mg site) silicate minerals show no concentration effect. That is, their 103lnβ do not vary with their Ti4+ and Ti3+ contents, respectively. In contrast, the 103lnβ of geikielite-ilmenite solutions significantly decrease with increasing Fe/(Fe + Mg) ratio. Our calculations predict no significant Ti isotope fractionation among Ti4+Si-doped clinopyroxene, orthopyroxene, olivine, and pyrope (<0.08 % at 1200 K), whereas the 103lnα between geikielite-ilmenite solutions and Ti4+Si-doped clinopyroxene ranges from ∼−0.67‰ to −0.49‰ at 1200 K, supporting the hypothesis that FeTi oxides are important fractionating Ti isotopes during magma differentiation. Finally, the large equilibrium Ti isotope fractionation between geikielite-ilmenite solutions and clinopyroxene suggests that Ti isotopes can be used as a thermometer with precision comparable to that of elemental geothermometer.