University of Michigan - Department of Astronomy

Version: Rad

Geologic Age

Radiometric Dating

Information you need to complete this is in the Introduction

Please check with your GSI about what sections to complete.

Part 1: Half-life and the K-Ar method

The graph below shows the relationship between the daughter-parent ratio at the time of the test and the number of half lives elapsed for a limited number of half-lives.

Half life graph

Using this graph, you can determine the age of a few samples.

  1. Calculate the daughter to parent ratio for the samples in table 2.1. Enter the values in table 2.1.
  2. Use the graph to determine the number of half-lives that passed for each sample. Enter your answers in the table.
  3. Calculate the age of the rocks if the parent is potassium-40, which has a half life of is 1.26x109 years (1.26 Gyr). Record your answers in the table. (Note this assumes that Dt is the total amount of daughter isotopes)




Half-lives elapsed

Age (Gyr)







  1. What is/are the daughter isotope(s) of K-40?

  2. What is assumed about the initial value of the daughter isotope D0?

  3. Why is it safe to make this assumption about D0?

Part 2: Simple Isocrons

Below are three isocrons for 3 different rocks using the Rb-Sr method described in the introduction. For each of the three graphs below: State whether or not this is a valid or invalid method. If it is not a valid method, explain why. If it is a valid method, draw a best-fit line and write the equation of the line in the space beside the graph.  Calculate the age of the rock from the slope of the line.  The half-life of rubidium-87 is 48.8 billion years.

Sample 1 graph




Part 3: Age spectrum

The graphs below show the graph of age or argon-40:argon-39 vs argon-39 for three samples. 



  1. Which sample was disturbed the least during it's lifetime? How can you tell?

  2. Which sample underwent some sort of metamorphic event? How can you tell from the graph?

  3. Which sample is 120 million years old? How can you tell?

  4. How old is the meteorite?

Last modified: 2/19/08 by SAM with material/suggestions from EP

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