"ZirChron" Virtual Zircon Analysis App

Tuesday 2:30pm-2:50pm Weeks Geo: 140

Teaching Demo

Leaders

Mark Schmitz, Boise State University

Karen Viskupic, Boise State University

Demonstration

We will demonstrate the first element of the student exercise available here. Participants in the teaching demonstration will use the Zir-chron app on their laptop or tablet to explore the functionality of the app and engage in the geochronological experimentation it allows, in order to understand how the curriculum scaffolds student learning.

Abstract

This application was developed to promote deeper understanding of the science of geochronology, including the integration of crystal-scale relative dating principles with numerical dating via radioisotope measurements. An integral aspect of developing this understanding is practical experience with the decision-making that goes into the selection of samples for analysis, and the subsequent interpretation of the resulting isotopic ages from those samples. The U-Pb decay system in zircon is particularly amenable to this practice, as we can apply different methods—in situ laser ablation inductively coupled plasma mass spectrometry and high-precision chemical abrasion isotope dilution thermal ionization mass spectrometry—to the same crystals, and link the resulting radioisotopic ages to the textures within crystals revealed by cathodoluminescence imaging.

By the end of this activity, students will be able to:

Apply relative dating principles at the crystal scale
Describe the statistical distribution of the crystal ages including means, modes and outliers
Form hypotheses relating the statistical distribution of crystal ages to the physical characteristics of those crystals
Form and test hypotheses about the geologic processes that could account for the distribution of crystal ages in a population
Describe the decisions made to determine the geologic age of a rock from a set of crystals

Context

This activity is designed to be flexible in its deployment, with content and activities that engage students from high school to graduate levels. I have used this activity in three courses: a lower-division undergraduate required course in field methods; an upper-division undergraduate required course in geochemistry; a graduate course in isotope geochemistry.

Why It Works

The activity is built around 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 activity is centered around hypothesis formation and experimentation. Students interact with the learning environment by using visual criteria to select zircon crystals or spots on crystals for analysis, evaluating the numerical and graphical results, and then using those results to revise their analyses and interpretations.
Students are assessed on the basis of their responses to a set of questions formulated as a student exercise, with particular emphasis on a set of wrap-up questions that prompt students to synthesize and reflect on their learning.