Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 µm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The (207)Pb/(206)Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages

Detecting micro- and nanoscale variations in element mobility in high-grade metamorphic rocks: Implication for precise U-Pb dating of zircon

© 2018 American Geophysical Union. Reliable geochronology is fundamental to our understanding of planetary evolution and the geological events that have shaped the Earth. Zircon has a robust crystal structure and is a key mineral in geochronological studies because it incorporates small amounts of actinides during growth. Currently, the best physical evidence for the presence of differentiated crust in the first 500 million years of the Earth’s history comes from intra-grain isotopic microanalysis of zircon crystals. However, it has been long recognized that isotope systematics in zircon can show great complexity, not only from the presence of multiple stages of growth in single grains, but also from disturbance of accumulated radiogenic Pb during subsequent geological events. Although geochronologists are aware of the problem of Pb mobilization during metamorphism, the nature of mobilization on a submicron scale is not entirely understood, especially where evidence has emerged of inhomogeneity and unsupported radiogenic Pb migration. The recent discovery of nanospheres of metallic radiogenic Pb in zircon from granulites explains, in part, instances of reversely discordant age measurements and irregular counts on Pb isotopes during analysis by secondary ion mass spectrometry (SIMS). Studies are continuing to reveal the nature of Pb nanospheres, so that possible mechanisms and conditions requisite for their formation can be proposed for testing by future investigations

GGR Biennial Critical Review: Analytical Developments Since 2014

© 2017 The Authors. Geostandards and Geoanalytical Research © 2017 International Association of Geoanalysts This GGR biennial critical review covers developments and innovations in key analytical methods published since January 2014, relevant to the chemical, isotopic and crystallographic characterisation of geological and environmental materials. In nine selected analytical fields, publications considered to be of wide significance are summarised, background information is provided and their importance evaluated. In addition to instrumental technologies, this review also presents a summary of new developments in the preparation and characterisation of rock, microanalytical and isotopic reference materials, including a précis of recent changes and revisions to ISO guidelines for reference material characterisation and reporting. Selected reports are provided of isotope ratio determinations by both solution nebulisation MC-ICP-MS and laser ablation-ICP-MS, as well as of radioactive isotope geochronology by LA-ICP-MS. Most of the analytical techniques elaborated continue to provide new applications for geochemical analysis; however, it is noted that instrumental neutron activation analysis has become less popular in recent years, mostly due to the reduced availability of nuclear reactors to act as a neutron source. Many of the newer applications reported here provide analysis at increasingly finer resolution. Examples include atom probe tomography, a very sensitive method providing atomic scale information, nanoscale SIMS, for isotopic imaging of geological and biological samples, and micro-XRF, which has a spatial resolution many orders of magnitude smaller than conventional XRF