# Help a geochronologist

#### Summary

This is an exercise in calculating absolute ages of zircon crystals based on raw data collected by a SHRIMP (Sensitive High-resolution Ion Micro-probe). The assignment has been used as a hands-on extension of studying methods of absolute dating in lecture and laboratory sections of Historical Geology. Students are presented with a corrupt data sheet that has lost all age calculations but retains the observed ratios of several parent-daughter isotopes. The students use the raw isotopic ratios in Excel to re-calculate ages for several zircon analyses from one of four rock samples. Accompanying questions to this activity require students to manipulate data into more accurate and meaningful formats; create and interpret graphs of calculated ages for each sample; and speculate on sources of error, protolith, and overall meaningfulness of data.

## Learning Goals

Additionally, students learn how to perform basic to intermediate functions in Microsoft Excel by organizing data, writing calculations, and creating a variety of graphs.

## Context for Use

The activity provides hands-on application of calculating radiometric ages based on real, analytically observed isotopic ratios. To be most effective, this assignment best follows after students have been exposed to the fundamental aspects of radiometric data (e.x. half-life, parent & daughter isotopes, decay constant) and rock forming processes. Any experience with Excel (or a comparable spreadsheet program) is a benefit; however, this exercise can also be used to introduce basic to intermediate spreadsheet skills.

## Description and Teaching Materials

Students are presented with isotopic data for zircon crystals analyzed on a Sensitive High-resolution Ion Micro-probe (SHRIMP). The value of a SHRIMP, compared to conventional radiometric dating techniques, is the ability to obtain many ages from several zircon crystals within an individual rock. While conventional dating techniques result in very high precision data, these data are collected from a single crystal. This technique is less reliable for metamorphosed rocks, in which individual zircons may yield a variety of ages reflecting metamorphic events and/or the sample's protolith.

The data set that students are presented with is corrupt; yet while the calculated ages are missing, the observed isotopic ratios were retained. Sadly, this corrupted file was a critical part of my Master's research, so this provides students with a real-world application of analytical data.

Students may work individually or in small groups to re-calculate the ages of zircon crystals from one sample.

Each group starts by opening up the corrupt data sheet and selecting one of four samples. Each sample includes several analyses from multiple zircons extracted from one rock sample. All rock samples are moderate to high-grade metamorphic gneiss. The students should briefly look through the data and explore what information was recorded and especially what information is missing and what was still retained.

Following along with the instructions provided, students recalculate ages for all zircons within their sample. Depending on the level of difficulty, the instructor may use more or less complicated age-calculating equations. The instructions provided include a simple equation that only requires knowing the parent-daughter isotopes and the appropriate decay constant.

After calculating ages for each analysis, students convert the ages into another units of time and calculate 5% error for each analysis. Additionally, students graph all of their calculated ages and may perform statistical calculations to interpret their calculations.

Additional questions require students to speculate on the protolith for their sample (based on age distribution alone) and consider sources of error in their calculations.

Instructions (Microsoft Word 40kB Jun15 10)

Original Corrupt Data (Excel 1.2MB Jun15 10)

Re-calculation worksheet (Excel 25kB Jun15 10)

## Teaching Notes and Tips

Additionally, I find that use of any images of zircon crystals, especially of grains that show zoning, are useful to reinforce where exactly each analysis was collected from a single crystal.

I have used this assignment in class (without use of computers) by printing the datasheets for each sample and having students calculate ages with a calculator. This adaptation seems to work well but does require printing out instructions and data.