High-temperature equilibrium isotope fractionation of non-traditional stable isotopes: Experiments, theory, and applications
High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using non-traditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors.
© 2014 Elsevier B.V. Received 14 August 2014, Revised 15 December 2014, Accepted 17 December 2014, Available online 26 December 2014. Funding for the UCLA research described here has come from the National Science Foundation (EAR-0711411) and NASA (NNX10A175G). A.S. acknowledges support from NSF (EAR-1321858). Reviews by three anonymous reviewers and the editor improved the presentation and are greatly appreciated by the authors.