In the temperature-gradient-driven mass diffusion (so-called thermal diffusion or Ludwig–Soret effect) processes, not only elements, but also isotopes will differentiate between the hot and the cold ends. Molecular-level knowledge of such processes is extremely important in understanding concentration and isotopic distributions in many geologic systems. However, the molecular-level mechanisms and theoretical treatments of these processes are still under debate.
LI Xuefang and LIU Yun from the State Key Laboratory of Ore Deposit Geochemistry (SKLODG) at the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) provide a unified theory based on local thermodynamic equilibrium approach to evaluating thermal isotope fractionations under a wide range of temperatures. For high temperature silicate melts, the theory offers a simple equation for calculating isotopic fractionations of all isotope systems.
In their study, a unified equation for calculated isotope fractionation in thermal diffusion for isotope system M (ΔXM) is deduced, and it is for all temperature conditions.
where superscript “*” means heavier isotope substituted species. T0 is called reference temperature, which denotes a starting point of temperature change.
The magnitude of isotope fractionation can be calculated by the above equation, if knowing the relationship between temperature and the partition function ratio(Q∗/Q) of the heavy and light isotopes.
Furthermore, in high temperature (>800ºC), there is a simpler equation,
where A and B are constants, T and T0 denote the end temperatures. Because the second term is only 0.0001 or smaller in high temperature, it can be neglected in general,
m* and m are the mass of heavy and light isotopes.
The equation shows that isotope fractionation only depends on temperature gradient, and it isn’t related to chemical environment, which helps to clarify the previous debate among the researchers. The results from this equation agree with the observed data for the most of network modifiers. The equation greatly simplifies the complex treatment of thermal diffusion. In the study, the estimations about Mg, Fe, Ca isotopes are in accord with experimental results, and the theory can be applied to the other isotope studies in the future.
The study has been published in Geochimica et Cosmochimica Acta.
Reference:
Xuefang Li and Yun Liu* (2015) A theoretical model of isotopic fractionation by thermal diffusion and its implementation on silicate melts. Geochimica et Cosmochimica Acta, 154, 18-27.
Contacts:
LIU Yun
Institute of Geochemistry, Chinese Academy of Sciences
E-mail: liuyun@vip.gyig.ac.cn
(By LI Xuefang)
