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

岩浆演化是地球上观察到的广泛的火成岩及其不同的化学成分和伴生的经济矿藏成因的原因。对于理解岩浆演化和行星形成过程，最重要的是对大型简单火成岩系统进行详细的地球化学研究。层状镁铁质-超镁铁质火成岩不仅有助于了解岩浆的分异过程，而且是铂族元素、铬、镍、铜、铁矿床的主要来源，具有重要的经济意义。铁同位素研究有助于揭示这些火成岩杂岩及其矿床的形成过程，因为不同的铁同位素可能在不同的过程中分成不同的相，包括熔体中矿物的结晶、围岩的同化以及岩石与晚期热液之间的反应。在单个晶体中，铁同位素也可以以不同的速度扩散到不同的矿物中，也可以反映平衡(岩浆)或非平衡(动力学)过程。多收集器电感耦合等离子体质谱(MC-ICP-MS)测量块状岩石和矿物的Fe稳定同位素精度的提高表明，高温镁铁质和超镁铁质陆壳和地幔火成岩、月球镁铁质岩和陨石存在显著差异。这些发现有望成功地理解岩浆分异和行星形成过程。然而，目前对这些岩石中单个矿物的铁同位素组成及其分馏的认识仍然有限，对高温陆地和地外火成岩系统中铁同位素分馏的原因、机制和意义仍知之甚少。只有数量非常有限的火成岩侵入岩进行了详细的铁同位素研究。此外，无法获得地外岩石中单个矿物的原位铁同位素测量，仅有极少数陆地岩石存在。这意味着，在许多情况下，以前在块状矿物中测量的铁同位素成分很可能是晶体生长、扩散或后期热液蚀变过程中记录的复杂成分的平均值。该项目将确定一个简单的高温岩浆系统中铁同位素分馏的程度、原因和机制，该系统将块状岩石和矿物的Fe和O同位素分析与高空间分辨率的原位矿物Fe和O同位素分析相结合，分别使用飞秒激光烧蚀(fs-LA)、MC-ICP-MS和二次离子质谱仪(SIMS)，在地球上研究最多的侵入杂岩斯凯尔加德的镁铁质-超镁铁质层状侵入体中。这种侵入说明了一个高度分化的岩浆室，它起源于一个单一的、大的岩浆体，它经历了广泛的闭合系统的演化，通过分离结晶，后来经历了热液蚀变。这一研究成果将有助于理解镁铁质-超镁铁质层状侵入岩及其寄主岩浆矿床的形成过程，并对了解地球分异过程具有重要意义。该项目的重点是系统的、高分辨率的矿物间Fe同位素分馏和矿物内Fe同位素组成(用fs-LA-MC-ICPMS)。由于FS-LA-MC-ICP-MS是一种新的地球化学技术，这项研究将有助于开发最终可能造福于更广泛的地球科学界的方法。详细的块状岩石和矿物Fe和O同位素组成，结合原位Fe同位素组成、矿物化学、O同位素冷却温度、块状岩石主要元素和微量元素组成以及模拟，将产生对单个大型火成岩侵入体最全面和详细的研究。氧同位素组成有助于从动力学和晚期热液效应中区分高温岩浆铁同位素组成(平衡)。所有这些数据将有助于确定所测分馏的来源(分步结晶、化学扩散、热扩散、晚期热液蚀变)。为斯卡尔加德计算的矿物间Fe同位素分馏系数随温度的变化将有助于理解其他陆地火成岩系统、月球和火星岩石以及其他行星体中的分馏系数。由于很难从陨石上获得大样本，这项研究将提供急需的信息，以确定研究地外样本的最佳方法。这项工作的结果将加深我们对侵入体水平上铁同位素的大尺度演化以及晶体内小尺度铁同位素不均质性的理解。该项目结合了来自威斯康星大学麦迪逊分校的矿物学、岩石学、经济地质学、地球化学和铁、氧同位素地球化学的专业知识。该项目将支持一名职业生涯早期的女科学家，资助两篇硕士论文和本科生研究人员，并支持东卡罗来纳大学最先进的高温同位素地球化学研究的物理基础设施的发展。这项合作还将为ECU的研究生和本科生提供使用威斯康星大学麦迪逊分校最先进的分析设施的经验，并与地球化学领域的顶级领导人互动。

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.