Radioactive Decay and Geochronology

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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.

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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.

Description and Teaching Materials

radioactive_decay_lab-student_version.pdf (Acrobat (PDF) PRIVATE FILE 38kB Jun13 05)
Documented_Isotope_Model.stm ( PRIVATE FILE 25kB Jun13 05)
Documented_UPb_Model.v.1.STM ( PRIVATE FILE 589kB Jun13 05)
Documented_UPb_Model.v.2.stm ( PRIVATE FILE 705kB Jun13 05)
PBLOSS.F90 ( PRIVATE FILE 4kB Jun13 05)
U_series.f90 ( PRIVATE FILE 4kB Jun13 05)
fortran_radioactive_decay.pdf (Acrobat (PDF) PRIVATE FILE 8kB Jun13 05)

Teaching Notes and Tips

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

Assessment

Radioactive_Decay_Lab-Answer_Key.pdf (Acrobat (PDF) PRIVATE FILE 213kB Jun13 05)
assignment_assessment_2.pdf (Acrobat (PDF) PRIVATE FILE 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.