Cutting Edge > Early Earth > What can zircons tell us about the Early Earth?

What can zircons tell us about the Early Earth?

Page developed for SERC by Aaron Cavosie (University of Puerto Rico), Julie Baldwin (University of Montana), and Darrell Henry (Louisiana State University)
,

This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are

  • Scientific Accuracy
  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
  • Robustness (usability and dependability of all components)
  • Completeness of the ActivitySheet web page

For more information about the peer review process itself, please see http://serc.carleton.edu/NAGTWorkshops/review.html.


This page first made public: Jun 13, 2007

Summary

This image shows an array of Jack Hill zircons arranged on a piece of tape prior to casting in epoxy. Details
Students create U-Pb concordia plots in Excel to experience the methods used to date the worlds oldest zircons and use these results to explore what geologic processes were active on the Early Earth.

Used this activity? Share your experiences and modifications

Context

Audience


Two levels:

Skills and concepts that students must have mastered


Historical Geology level: Advanced undergraduate / graduate

How the activity is situated in the course


This activity can follow a discussion that introduces: This activity can form the basis of (1) a single class laboratory, or (2) a homework exercise.

Goals

Content/concepts goals for this activity

Higher order thinking skills goals for this activity

Other skills goals for this activity

Description of the activity/assignment

A detailed two page Word document with activity instructions that can be tailored prior to handout. (Microsoft Word 34kB Jun7 07)

Give students a synthetic data set (Excel 38kB Jun7 07) of 206Pb/238U and 207Pb/235U isotope ratios. The data set will define two age populations (A and B) that can be assigned to either of the following scenarios. The data set is given to the students with the intention that Historical Geology level students will not be required to have advanced knowledge.

PART I: Data plotting

Students are to make concordia plots for use with the provided data sets (Excel 38kB Jun7 07) using the plotting program Excel.

PART II: Data analysis

Data analysis. Experience the discovery of finding two age populations on a concordia plot. Discuss U-Pb concordance. Distinguish different populations using concordia diagram, discuss uncertainty in data.

PART III: Contextual basis

Introduce the two different scenarios (see below) for encountering the age populations A and B. Explore the implications of finding two different age populations within single grains from zircons in an igneous rock (i.e. zircon inheritance).
Explore the ability to discriminate different sedimentary components within a detrital population.
Explore what other aspects of zircon could be used to distinguish different age populations.


Question: What are you really dating when you analyze a zircon?

Two examples of the concept of multiple age populations:

Example 1: Discuss concept that zircons from an igneous rock can record multiple age populations (A and B) that result in grains with different age cores and rims. As a 'hook', illustrate this concept with images of well-known Early Earth examples, demonstrating that this exercise is a real-world problem.

  1. Acasta gneiss zircons (images from S. Bowring)
  2. Investigating the Jack Hills zircons (PowerPoint 17.1MB Apr23 07) (images from A. Cavosie)
  3. Wyoming province zircons (PowerPoint 965kB Jun8 07) (images from D. Henry)

Example 2: Discuss concept of using zircon geochronology for sedimentary provenance. Use, as an example, two age populations (A and B, same data set as in ex. 1) of rounded detrital igneous zircons that end up in the same sedimentary rock. As a 'hook', illustrate this concept with images of well-known Early Earth examples

  1. Field shots of Jack Hills siliciclastic sediments (images from A. Cavosie)
  2. Field shots of Wyoming siliciclastic sediments (See #2 just above.)
  3. Petrographic images (CL, BSE, TL, etc.) of the above.
  4. Demonstrate that both Jack Hills and Wyoming zircons occur in siliciclastic rocks but are very different.

Determining whether students have met the goals

More information about assessment tools and techniques.

Download teaching materials and tips

Other Materials

Supporting references/URLs

Elements , Early Earth, 2(4), Aug. 2006

Elements , Zircon, 3(1), Feb. 2007

Zircon, Reviews in Mineralogy and Geochemistry, v. 53 (2003) (Hanchar and Hoskin, Eds.)

Geochemical Instrumentation and Analysis: This site from Integrating Research and Education supplies an on-line tutorial of the analytical techniques commonly used to characterize geological materials.

ISOPLOT/EX: For more advanced exercises that require the plotting of isotopic data, instructors can request a free copy of this Excel plug-in plotting program from Dr. Ken Ludwig (Berkeley Geochronology Center).

See more Early Earth »