What can zircons tell us about the Early Earth?
- Historical Geology level
- Advanced undergraduate / graduate geochemistry or petrology
Skills and concepts that students must have mastered
Historical Geology level:
- Basic physical geology concepts (knowledge of 3 rock types)
- Basic plotting skills in Excel
- Basic plotting skills in Excel
- Optical mineralogy concepts
- Basics of isotopic applications in geology
- Basics of U-Pb analytical methods (e.g., SIMS, TIMS LA-ICP, etc.)
How the activity is situated in the course
This activity can follow a discussion that introduces:
- Early Earth concepts
- sedimentary provenance
- petrogenesis (igneous or metamorphic)
- Instrumental analysis
Content/concepts goals for this activity
- Use a data set to master Excel plotting skills. This includes using Excel to solve equations, make data plots, assign error bars to data points, make regressions.
- Use a U-Pb isotopic data set to convey concepts of (1) U-Pb concordance and discordance, (2) sedimentary provenance, (3) igneous/metamorphic petrogenesis, (4) uncertainties in analytical data
- Use this exercise to introduce some of the most well-known and famous localities where samples from the early Earth are preserved: oldest rocks (Acasta, NWT, Canada), oldest minerals (Jack Hills, western Australia; Beartooth Mnts., Wyoming, USA)
- Explain the differences between a mineral age and a rock age.
Higher order thinking skills goals for this activity
- Develop an appreciation for the wide range of geologic insights, understanding, and knowledge about a wide range of geologic processes (e.g. tectonics, magmatism, sedimentary provenance) that is accessable through zircon U-Pb geochronology
- Explore the complexities of zircon geochronology, the complexity encountered in natural datasets, and the meaning of analytical uncertainty
- Better understanding of the Early Earth Geologic Timescale, absolute time, the Hadean and Archean Eons.
Other skills goals for this activity
- Appreciation for minerals with a petrographic context
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.
- Acasta gneiss zircons (images from S. Bowring)
- Investigating the Jack Hills zircons (PowerPoint 17.1MB Apr23 07) (images from A. Cavosie)
- 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
- Field shots of Jack Hills siliciclastic sediments (images from A. Cavosie)
- Field shots of Wyoming siliciclastic sediments (See #2 just above.)
- Petrographic images (CL, BSE, TL, etc.) of the above.
- Demonstrate that both Jack Hills and Wyoming zircons occur in siliciclastic rocks but are very different.
Determining whether students have met the goals
- Was the student able to make the data plots? This exercise requires using Excel functions that may not be known to all students
- Students will be graded on short-answer questions associated with the exercise. More detailed questions can be tailored to explore each of the scenarios in depth.
Teaching materials and tips
- A PowerPoint file of images from the Jack Hills zircons (PowerPoint 17.1MB Apr23 07)
A low resolution version of the same file (PowerPoint 1.1MB Apr23 07)
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).