# Radioactive Decay and Geochronology

#### Summary

Students create a STELLA model of the radioactive decay process. They then learn how the special radioactive series 238U - 206Pb and 237U - 205Pb can be used to determine both the time when a rock initially crystallized and when it underwent a recrystallization, even when the sample being dated was not a closed system.

## Learning Goals

The Radioactive Decay exercise has several purposes:

- To convey concepts necessary in modeling a system involving exponential growth and decay
- To introduce elementary differential equations
- To convey the difference between closed and open systems
- To introduce if-then-else logical statements
- To convey the importance of secular equilibrium, which allows complex isotopic decay systems to be modeled without taking into consideration radioactive daughters.
- To explore concordia-discordia diagrams and how a rock containing two isotopic systems can yield both the age of original crystallization and the time of a later metamorphic event.

## Context for Use

This activity was developed for a class introducing dynamical systems modeling to upper level undergraduate students.

## Teaching Notes and Tips

Instructor Notes (Acrobat (PDF) 44kB Apr10 06)

## Teaching Materials

radioactive_decay_lab-student_version.pdf (Acrobat (PDF) 38kB Jun13 05)

Documented_Isotope_Model.stm ( 25kB Jun13 05)

Documented_UPb_Model.v.1.STM ( 589kB Jun13 05)

Documented_UPb_Model.v.2.stm ( 705kB Jun13 05)

PBLOSS.F90 ( 4kB Jun13 05)

U_series.f90 ( 4kB Jun13 05)

fortran_radioactive_decay.pdf (Acrobat (PDF) 8kB Jun13 05)

Documented_Isotope_Model.stm ( 25kB Jun13 05)

Documented_UPb_Model.v.1.STM ( 589kB Jun13 05)

Documented_UPb_Model.v.2.stm ( 705kB Jun13 05)

PBLOSS.F90 ( 4kB Jun13 05)

U_series.f90 ( 4kB Jun13 05)

fortran_radioactive_decay.pdf (Acrobat (PDF) 8kB Jun13 05)

## Assessment

Radioactive_Decay_Lab-Answer_Key.pdf (Acrobat (PDF) 213kB Jun13 05)

assignment_assessment_2.pdf (Acrobat (PDF) 5kB Jun13 05)

assignment_assessment_2.pdf (Acrobat (PDF) 5kB Jun13 05)

## References and Resources

The following are readings for this activity:

- Dalrymple, G.B., 1991, The Age of the Earth, Stanford, CA: Stanford University Press, p. 79-90, 99-102, 115-119, and 122-124.
- Faure, G., 1986, Principles of Isotope Geology, 2nd Edition, New York: John Wiley and Sons, p. 38-45 and p. 283-296.

## Contact the Author

Please contact the author with questions or suggestions.

## Controlled Vocabulary Terms

**Subject**: Geoscience:Geology:Geochemistry:Radioisotopes

**Resource Type**: Computer Applications, Activities:Lab Activity

**Special Interest**: Computer-Based, Quantitative

**Grade Level**: College Upper (15-16)

**Quantitative Skills**: Models and Modeling

**Ready for Use**: Ready to Use

**Topics**: Time/Earth History, Solid Earth:Geochemistry:Radioisotopes