Assessing Diffusive Differentiation During Igneous Intrusion Using Integrated Theoretical, Experimental and Field Studies

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This project seeks to understand the processes forming the Earth's crust through a laboratory study of how magmas chemically evolve. The magma differentiation process, which changes the chemical composition of igneous rocks and is thus responsible for buoyant continental crust (and probably complex life) on Earth, remains nebulous in detail. This project will use controlled laboratory experiments to constrain the changes in isotopic ratios of Mg, Fe and Li occurring as magmas interact with coexisting crystals. These data will allow comparison of different models of magma differentiation; specifically, the data will provide the ability to discriminate 1) mechanical separation of crystals from magma; from 2) differentiation by diffusion and reaction processes. The resulting answers will impact our understanding of the history of Earth and how it has chemically evolved through time.This project is designed to determine if diffusion-reaction processes, taking place in the presence of supercritical fluids and a thermal gradient, could contribute significantly to chemical differentiation occurring within igneous plutons. The need to evaluate the role of diffusion-reaction has been stimulated by the emergence of data showing that plutons stay hot for a long time and that supercritical H2O greatly accelerates diffusion rates. Additionally, technological developments in mass spectrometry now allow direct identification of diffusive processes. Spatial variations in the ratios of "non-traditional isotopes" such as Li, Mg or Fe provide the ability to identify diffusion-based processes occurring within igneous plutons because isotopic gradients develop when lighter isotopes diffuse faster than heavier isotopes. The proposed work will focus on laboratory experiments at high temperature and pressure designed to constrain the isotope fractionation within partially molten materials under both equilibrium and kinetic conditions. In addition, numerical models of diffusion-reaction will be developed to predict gradients and magnitudes of isotope ratios under a variety of conditions. Isotopic analyses for experiments and possible reconnaissance field studies will be conducted using the new MC-ICPMS facility at the University of Illinois. A pilot project using measurement of anisotropy of magnetic susceptibility (AMS) and Electron BackScatter Diffraction (EBSD) will allow us to determine if diffusion-reaction in the presence of a thermal gradient can also lead to the development of grain-scale anisotropy.