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

这个项目试图通过对岩浆如何化学演化的实验室研究来了解地壳的形成过程。岩浆分异过程改变了火成岩的化学成分，从而导致地球上浮力的大陆地壳(可能还有复杂的生命)，但细节上仍然模糊不清。该项目将使用受控的实验室实验来限制当岩浆与共存的晶体相互作用时发生的镁、铁和锂同位素比率的变化。这些数据将允许比较不同的岩浆分异模式；具体地说，这些数据将提供区分1)晶体与岩浆的机械分离；2)扩散和反应过程的分异。由此得到的答案将影响我们对地球历史以及它是如何随着时间的化学演变的理解。该项目旨在确定在超临界流体和热梯度存在的情况下发生的扩散反应过程是否对火成岩中发生的化学差异有重大贡献。有数据显示，钚可以长时间保持热，超临界水极大地加速了扩散速度，因此需要评估扩散反应的作用。此外，质谱学的技术发展现在可以直接识别扩散过程。锂、镁或铁等“非传统同位素”比率的空间变化提供了识别在火成岩中发生的以扩散为基础的过程的能力，因为当较轻的同位素扩散速度快于较重的同位素时，就会形成同位素梯度。拟议的工作将侧重于高温和高压下的实验室实验，旨在平衡和动力学条件下限制部分熔融材料中的同位素分馏。此外，还将开发扩散-反应数值模型，以预测各种条件下的同位素比值的梯度和大小。将使用伊利诺伊大学新的MC-ICPMS设施进行实验和可能的侦察现场研究的同位素分析。一个使用磁化率各向异性(AMS)和电子背散射衍射(EBSD)测量的试点项目将使我们能够确定在存在热梯度的情况下扩散反应是否也会导致颗粒尺度各向异性的发展。

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.