Investigation of Fe Isotope Fractionation During Magmatic Differentiation at the Skaergaard Intrusion

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Magma evolution is responsible for the wide range of igneous rocks observed on Earth as well as their varied chemical compositions and the genesis of associated economic mineral deposits. Of fundamental importance for understanding magmatic evolution and planetary formation processes are detailed geochemical studies of large simple igneous systems. Layered mafic-ultramafic igneous complexes are not only useful for understanding magmatic differentiation processes but also important economically for being the main source of platinum group elements, Cr, Ni, Cu, and Fe ore deposits. Iron isotope studies can help unravel the formation processes of these igneous complexes as well as their mineral deposits because the different isotopes of Fe can potentially partition into different phases during diverse processes, including crystallization of minerals from the melt, assimilation of country rocks, and reactions between rocks and late-stage hydrothermal fluids. Within individual crystals Fe isotopes can also diffuse through various minerals at different rates and may also reflect equilibrium (magmatic) or non-equilibrium (kinetic) processes. Improvements in the precision of Fe stable isotope measurements of bulk rocks and minerals by multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS) show that significant variations exist in high-temperature mafic and ultramafic terrestrial crustal and mantle igneous rocks, lunar mafic rocks, and meteorites. These findings promise success for understanding magmatic differentiation and planetary formation processes. However, the current knowledge of Fe isotope compositions of individual minerals and fractionations among them in these rocks is still limited, and the causes, mechanisms, and implications of Fe isotope fractionations in high-temperature terrestrial and extraterrestrial igneous systems are still poorly understood. Only a very limited number of igneous intrusions have been investigated in detail with Fe isotopes. In addition, in situ Fe isotope measurements of individual minerals in extraterrestrial rocks are not available and only very few exist for terrestrial rocks. This means that Fe isotope compositions previously measured in bulk minerals may well be, in many cases, an average of the complex compositions recorded during crystal growth, diffusion, or late-stage hydrothermal alteration. This project will determine the extents, causes, and mechanisms of Fe isotope fractionations in a simple high-temperature magmatic system combining Fe and O isotope analysis of bulk-rocks and minerals with high-spatial resolution in situ Fe and O isotope analysis of minerals by femtosecond laser ablation (fs-LA) MC-ICP-MS and secondary ion mass spectrometry (SIMS), respectively, in the mafic-ultramafic layered intrusion of Skaergaard, Greenland, the most studied intrusive complex on Earth. This intrusion exemplifies a highly differentiated magma chamber originated from a single, large, magma body that underwent extensive, closed-system evolution through fractional crystallization that later underwent hydrothermal alteration. The results of this research will help understand the processes of formation of mafic-ultramafic layered intrusions and their host magmatic mineral deposits and has implications for understanding planetary differentiation processes.The emphasis of this project is on systematic, high resolution, inter-mineral Fe isotope fractionations and in situ intra-mineral Fe isotope compositions (by fs-LA-MC-ICP-MS). Because fs-LA-MC-ICP-MS is a new technique in geochemistry, this research will help develop the method that may ultimately benefit the broader geoscience community. Detailed bulk-rock and mineral Fe and O isotope compositions combined with in situ Fe isotope compositions, mineral chemistry, O isotope cooling temperatures, bulk-rock major and trace element compositions, and modeling will produce the most comprehensive and detailed study of a single large igneous intrusion. Oxygen isotope compositions will help discern high-temperature magmatic Fe isotope compositions (equilibrium) from kinetic and late-stage hydrothermal effects. All these data together will help identify the origin of the measured fractionations (fractional crystallization, chemical diffusion, thermal diffusion, late-stage hydrothermal alteration). The inter-mineral Fe isotope fractionation factors as a function of temperature calculated for Skaergaard will be useful for understanding those in other terrestrial igneous systems, lunar and Martian rocks, and other planetary bodies. Because large samples are hard to obtain from meteorites, this study will provide much needed information to determine the best approach for extraterrestrial sample studies. The results of this work will improve our understanding of large-scale evolution of Fe isotopes at the intrusion level as well as small scale, Fe isotope heterogeneities within crystals. This project combines the expertise of mineralogy, petrology, economic geology, geochemistry, and Fe and O isotope geochemistry of the PI and collaborators from the University of Wisconsin-Madison. This project will support an early career female scientist, fund two M.S. theses and undergraduate student researchers, and support the development of the physical infrastructure for research on state-of-the-art high-temperature isotope geochemistry at East Carolina University. This collaboration will also provide graduate and undergraduate students at ECU the experience of working with state-of-the-art analytical facilities at UW-Madison and interact with top leaders in geochemistry.