“ZIR-CHRON”: AN ONLINE APP FOR EXPLORING THE SCIENCE BEHIND U-PB ZIRCON GEOCHRONOLOGY

U-Pb zircon geochronology has proliferated over the past two decades as a tool for understanding a plethora of Earth system dynamics, spurred by developments including rapid, inexpensive analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and a renaissance in high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS). A result is that many users of U-Pb zircon geochronology are no longer deeply invested in the analytical measurements themselves, nor the assumptions that are made to transform measured isotope ratios into geologic age interpretations.

As part of a series of online activities to teach the science of geochronology, we built an on-line Javascript and HTML-based application that allows students to explore real LA-ICPMS and ID-TIMS data sets. The app is designed around a clickable montage of high-resolution cathodoluminescence images showing the growth zoning of zircon crystals. Each crystal shows the locations of LA-ICPMS spot analyses, and was also dissolved and analyzed by ID-TIMS. Students can select what analysis mode to explore (LA-ICPMS versus ID-TIMS) and can incrementally choose a set of spots or dissolved crystals to analyze by clicking appropriately; with each click the associated isotope ratio translated into a U-Pb date appears in a tabulated list and is displayed in both histogram and summed probability density function graphical formats. The inverse-variance weighted mean, standard error, and probability of fit parameters are automatically calculated and displayed.

We provide an example undergraduate curriculum for use of this app in exploring the criteria by which crystals from a population are interrogated, the use of basic parametric statistics to establish the precision and accuracy of U-Pb zircon ages, and the strengths and weaknesses of different analytical techniques. The generic nature and portability of the application readily allows for customizing its use in varied classroom and on-line settings. The data sets to be demonstrated are produced from Oligocene volcanic tuffs of the John Day Formation in Oregon, however the open-source nature of the app allows customization with different images and data sets for users familiar with HTML.