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

“该奖项是根据2009年《美国复苏与再投资法案》(公法111-5)资助的。”智力价值：锆石是硅质火成岩的广泛成分，含有广泛的微量元素(如稀土元素)和同位素物种(如O、Hf、Pb和U)，对地壳的地质年代学、岩石成因和地球化学研究有用。锆石可以对早期地球历史提供独特的限制，因为它的耐火性质允许它在母岩的其他残迹被破坏时保存在沉积和变质沉积岩中。然而，锆石通常很小(几十到几百微米)，而且往往保存着复杂的微米级的成分分带，这是传统的微分析技术(例如激光烧蚀或大多数二次离子质谱仪)无法解决的。因此，发展分析方法以研究具有成分分带特征的小空间尺度上的锆石地球化学，并了解在这种尺度上控制锆石成分的过程是至关重要的。在这项研究中，将测定微米级微量元素在天然锆石和在受控条件下生长的合成锆石中的分布。具体目标包括：开发和标准化使用纳米SIMS高分辨率离子微探针在微米和亚微米尺度上定量分析锆石中痕量元素的方法。检查天然锆石中的微量元素分布，努力识别典型的平衡与非平衡生长过程中的元素/元素比率和/或分带的空间模式。实验扩展微量元素在锆石和熔体之间平衡分配的校准，特别是在与天然花岗岩熔体相当的温度下。因此，通过实验检查在受控冷却过程中生长的岩浆锆石中痕量元素掺入的动力学控制。将使用Cameca NanSIMS离子微探针获得分析结果，这是目前唯一能够在小于1微米的范围内进行灵敏(ppm级)和精确(%级相对精度)测量的仪器。这种仪器已经存在了十多年，但直到最近才向地球科学界开放。这个项目是第一个使用纳米SIMS来检查陆地火成岩矿物中微量元素分布的实质性研究。广泛的影响：这项研究将支持加州理工大学的一名博士生，并间接支持高中和本科生的暑期实习，这取决于是否存在一批在活跃实验室工作的受资助的研究生。这项研究还将展示一种在亚微米级测量锆石中微量元素丰度的方法，其灵敏度和精密度适合于天然物质的岩石成因研究，从而有助于地球化学仪器和方法的发展。最后，这项研究将深入了解锆石微量元素变化中微米级变化的物理控制，促进它们在地质、岩石学和地质年代学的广泛应用中的应用。

"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.