Isotopes of common rock-forming elements have great potential for understanding the redistribution of mass during geological processes. In order to make use of these isotopes one must know how they partition between different minerals in the rocks. Two of the most important elements of interest to the geological community are magnesium (Mg) and iron (Fe). This group proposes to build on their previous experimental program to test and augment theoretical predictions for iron and magnesium isotope partitioning between mineral phases at high temperatures. The project represents an uncommon integration of experimental petrology and isotope geochemistry. The investigators have trained three graduate students in recent years, two of whom are women, an underrepresented demographic in the geological sciences.
Iron and magnesium are targeted for this interdisciplinary study because of their importance as rock-forming elements and recent indications that these elements may be more mobile in Earth's mantle than previously thought. Indeed, there has been an explosion of studies on the distributions of these isotopes in natural samples. The focus of this study is on Fe and Mg isotope fractionation between spinels and other phases. Spinels are of interest because there is more than one coordination site for these cations in the spinel structure, affording tests of the principles of stable isotope partitioning. Experiments will be performed in a piston cylinder apparatus. The group will use the three-isotope method to establish equilibrium fractionation factors in these two isotope systems. This method involves spiking one of the phases with an excess of the abundant isotope in order to disturb the usual state of mass-dependent fractionation among three isotopes in the experiments. This mass fractionation relationship among the three isotopes is a necessary condition for equilibrium. The drive towards reestablishment of that condition during the experimental run is used as a monitor for isotope exchange in the experiments. The team will investigate equilibrium Mg isotope fractionation between Mg(Al,Cr)2O4 spinels and forsterite olivine and equilibrium Fe isotope fractionation between almandine garnet and magnetite (an inverse spinel). Results of the latter experiments can be combined with previous experiments to arrive at temperature-dependent garnet-olivine iron isotope fractionation factors. Both systems are pivotal in the interpretation of natural Mg and Fe isotope data.
