This postdoctoral fellowship is awarded to Dr. Tyler Blum to work at the University of Wisconsin Madison and Max-Plank-Institut fur Eisenforschung GmbH (MPIE) in Dusseldorf, Germany. The proposed work will analyze several 3.3-4.0 billion-year-old zircons from the Beartooth Mountains, USA, in order to study the geological controls (thermal, temporal, and crystallographic) on the formation of small, trace element-rich clusters, and how they can be used to deduce unique information about the thermal histories for old, out of context zircons. Recent technological advances have enabled the application of atom probe tomography (APT) to geological materials, where it offers a unique means to study mineral structure and chemistry (with isotope sensitivity) in three dimensions and at the near atomic scale. This work expands upon recent atom probe analysis of zircon, the mineral most widely relied upon for dating geologic events and looks to improve our understanding of how clustering of trace elements at the nanometer scale relates to changes in mineral structure and temperature over geologic time. Importantly, it will help to delineate the applications and limitations of APT in geochronology, the time periods in Earth's history where it can be utilized, and the geological variables important to modeling cluster formation ages. This work will also contribute to a greater understanding of the nanometer-scale defect structures produced during radioactive decay and their characteristics over geologic time; the stability and behavior of these crystallographic domains is highly relevant to more effective engineering of nuclear waste forms. In collaboration with the University of Wisconsin -Madison Geology Museum, this work will also include outreach and education on the geological time scale, our understanding of the early earth, and the mineral zircon.
The formation of nanoscale trace element-rich clusters is thought to represent a complex interaction between the atomic-scale processes of radiation damage accumulation, structural annealing, and trace element migration. As such, there is potential for the chemistry of these clusters, as well as their phase relations and formation ages, to record previously unrecognized high-temperature episodes early in Earth's history. The ability to extract information from clusters hinges on a thorough understanding of their formation and stability, as well as the instrumental and geological limitations in age determination. This project combines secondary ion mass spectrometry (SIMS), electron backscatter diffraction (EBSD), hyperspectral Raman spectroscopy, transmission electron microscopy (TEM) and APT to characterize zircon structure and chemistry from the grain scale to the atomic scale. By applying this set of characterization tools to both Archean zircon grains from the Beartooth Mountains, USA, and laboratory annealed zircon, this proposal seeks to resolve: (1) the structural and thermal controls on formation of trace element-rich clusters, (2) the geological and instrumental limitations for determination of 207Pb/206Pb ratios (and modeled formation ages for clusters), and (3) where APT analysis of zircon can be applied within the geological record to extract novel information about zircon thermal histories.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
