Trace Element Partitioning in Magmatic Zircon: Empirical and Experimental Studies at Sub-micron Scales

“该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。“知识价值：锆石是硅质火成岩的一种普遍成分，含有多种微量元素（例如，REE）和同位素物种（例如，O、Hf、Pb、U等元素，对地壳的年代学、岩石成因和地球化学研究具有重要意义。锆石可以提供独特的约束早期地球历史，因为它的耐火性质允许它被保存在沉积岩和变质沉积岩，当其母岩的其他遗迹已被破坏。然而，锆石通常很小（几十到几百微米），并且通常保留了复杂的微米级成分环带，这些成分环带不能通过常规的微量分析技术（例如，激光烧蚀或大多数二次离子质谱仪）。因此，它是至关重要的发展分析方法来研究锆石地球化学在小的空间尺度上的组成分带的特点，并了解在这样的尺度上控制锆石组成的过程。在这项研究中，微米级的微量元素分布将确定在自然锆石和人工锆石在受控条件下生长。具体目标包括：o使用纳米SIMS高分辨率离子微探针，开发微米和亚微米级锆石微量元素定量分析方法，并使之标准化。o检查天然锆石中的微量元素分布，努力确定元素/元素比率和/或平衡与非平衡生长过程典型的分带空间模式。特别是在与天然花岗岩熔体相当的温度下，实验研究在受控冷却过程中生长的岩浆锆石中微量元素掺入的动力学控制。Cameca nanoSIMS离子微探针，这是唯一能够在小于1微米的尺度上进行灵敏（ppm级）和精确（%级相对精度）测量的现有仪器。这种仪器已经存在了十多年，但直到最近才提供给地球科学界。该项目代表了第一个实质性的研究，使用nanoSIMS来检查微量元素分布在陆地火成岩mineres.Broader影响：这项研究将支持一个博士生在加州理工学院和间接支持高中和本科生暑期实习，这取决于一个骨干的资助研究生在活跃的实验室工作的存在。这项研究还将展示一种在亚微米尺度上测量锆石中微量元素丰度的方法，其灵敏度和精度适合于天然物质的岩石成因研究;因此，它将有助于开发地球化学仪器和方法。最后，这项研究将提供深入了解锆石微量元素变化的μ m尺度变化的物理控制，促进其在广泛的地质，岩石学和地质年代学应用中的使用。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."Intellectual merit: Zircon is a widespread component of siliceous igneous rocks and hosts a wide range of trace elements (e.g., REE) and isotopic species (e.g., O, Hf, Pb and U) that are useful for geochronologic, petrogenetic and geochemical studies of the crust. Zircon can provide unique constraints on early earth history because its refractory nature allows it to be preserved in sedimentary and metasedimentary rocks when other vestiges of its parent rock have been destroyed. However, zircons are typically small (tens to hundreds of microns) and often preserve complex, micron-scale compositional zonation that cannot be resolved by conventional microanalytical techniques (e.g., laser ablation or most secondary ion mass spectrometers). Therefore it is critical to develop analytical approaches to studying zircon geochemistry at the small spatial scales characteristic of its compositional zonation, and to understand the processes that control zircon compositions on such scales. In this study, micron-scale trace-element distributions will be determined in natural zircons and in synthetic zircons grown under controlled conditions. Particular goals include:o Develop and standardize methods for quantitative analysis of trace elements in zircons at µm and sub-µm scales using the nanoSIMS high-resolution ion microprobeo Examine trace element distributions within natural zircons, in an effort to identify element/element ratios and/or spatial patterns of zonation that are typical of equilibrium vs. non-equilibrium growth processeso Experimentally extend calibrations of equilibrium partitioning of trace elements between zircon and melt, particularly at temperatures comparable to natural granitic meltso Experimentally examine kinetic controls of trace element incorporation in magmatic zircons grown during controlled cooling.Analytical will be obtained using the Cameca nanoSIMS ion microprobe, which is the only existing instrument capable of sensitive (ppm-level) and precise (% level relative precision) measurements at scales less than 1 micron. Such instruments have existed for more than a decade, but have only recently become available to the earthscience community. This project represents the first substantive study to use the nanoSIMS to examine trace element distributions in terrestrial igneous minerals.Broader impacts: This study will support one Ph.D student at Caltech and indirectly support high school and undergraduate student summer internships, which depend on the existence of a cadre of funded graduate students working in active labs. This study will also demonstrate a method for measuring trace element abundances in zircons at sub-micron scales, with sensitivity and precision suitable for petrogenetic studies of natural materials; thus it will contribute to the development of instruments and methods for geochemistry. Finally, this study will provide insight into the physical controls of µm-scale variations in trace element variations of zircons, facilitating their use in a wide range of geologic, petrologic and geochronologic applications.