lab 3

Lab 3: Absolute Geologic Dating Dr. Khaled Chowdhury Absolute geologic time studies involve finding a numeric age for a sample or a geologic event. The common approach to obtaining a numeric age requires careful analysis of unstable, radioactive isotopes. All radioactive ‘parent’ isotopes decay into stable ‘daughter’ isotopes at a specific rate of time known as the half-life. The half-life is the amount of time it takes half of the parent atoms to decay to the daughter atoms. As an example, if you start with 100 atoms of radioactive element ‘X’, how many atoms of ‘X’ you have after 5 half-lives had passed? 0 half life 1 half life 2 half lives -----> -----> -----> 100 atoms of X 50 atoms of X 25 atoms of X 3 half lives -----> 12.5 atoms of X 4 half lives -----> 6.25 atoms of X 5 half lives -----> 3.125 atoms of X Plotting number of half-lives versus number of parent atoms remaining produces a graph known as a Generic Radioactive Decay Curve (see below). It is generic in the sense that it works for all radioactive parent - stable daughter pairs. 100 J 90 80 70 60 50 J 40 30 J 20 10 J J J 0 0 1 2 3 4 5 Number of Half Lives Once the number of half-lives is known an age can be determined by multiplying the number of half-lives by the amount of time that it takes for each half-life to take place. The half-lives for several frequently used radioactive isotopes are listed below: Radioactive Parent Stable Daughter Half Life Values Uranium-238 Lead-206 4.5 billion years N u m be r of P ar en t At o m s R

Uranium-235 Lead-207 713 million years Thorium-232 Lead-208 14.1 billion years Rubidium-87 Strontium-87 47.0 billion years Potassium-40 Argon-40 1.3 billion years Carbon-14 Nitrogen-14 5730 years