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

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"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.