"ZirChron" Virtual Zircon Analysis App

Mark Schmitz and Karen Viskupic
Boise State University
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This web-based app is designed to help students evaluate a radioisotopic age and its uncertainty based upon the collection of data, the application of statistics, and the interpretation of geological and analytical sources of complexity and bias.

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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.

Skills and concepts that students must have mastered

A basic understanding of the use of relative dating principles; an introduction to numerical dating using radioactive decay; a familiarity with concepts of the mean, the standard deviation, and the normal "bell-shaped curve" distribution.

How the activity is situated in the course

This activity can be used as a stand-alone in-class activity, a completely on-line activity, or a hybrid of both.


Content/concepts goals for this activity

By the end of this set of activities, 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
- Compare and contrast the LA-ICPMS and CA-IDTIMS analysis techniques with respect to precision and accuracy

Higher order thinking skills goals for this activity

By the end of this set of activities, students will be able to:
- Form hypotheses that relate the statistical distribution of crystal ages to the physical characteristics of those crystals
- Form and test hypotheses about the geologic and/or analytic processes that could account for the distribution of crystal ages in a population

Other skills goals for this activity

By the end of this set of activities, students will be able to:
- Describe the decisions that are made to determine the geologic age of a rock sample from a set of crystals

Description of the activity/assignment

This application was developed to promote a 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—both in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and high-precision chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-IDTIMS)—to the same crystals, and link the resulting radioisotopic ages to the textures within crystals revealed by cathodoluminescence (CL) imaging.

The application presents the student with a set of images of zircon crystals from a single sample, illustrating their internal zoning and complexities. Two modules — LA-ICPMS and CA-IDTIMS — are available, with the same set of crystals available for analysis in each module. The exercises are designed around the ability of the student to conduct a virtual experiment by clicking on either labeled laser ablation "spots" (in the LA-ICPMS module) or individual crystals (in the CA-IDTIMS module) to obtain a set of radioisotopic ages. These ages are illustrated in both tabular and graphical formats to allow the student to visualize the distribution of data, and assess the relative similarities and differences between ages using common statistical tools.

Determining whether students have met the goals

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.

More information about assessment tools and techniques.

Teaching materials and tips

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