Radiometric age dating theory

· Unstable naturally occurring isotopes emit particles in a process known as radioactive decay

· Radioactive decay occurs at known rates and using this you can determine the age of certain types of rocks.

· Read the course notes (section #3) for the complete theory

Principles of radioactive dating

· Parent: unstable radioactive isotope

· Daughter product: results from the decay of a parent

· Half-life: the time required for one-half (50%) of the parent to change to daughter product

· Comparing the ratio of parent to daughter yields the age of the sample

1) Examples of Common Half-lifes

Parent Daughter Half life

Rubidium 87 Strontium87 48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 238 Lead 206 4.47 billion years

Potassium 40 Argon 40 1.25 billion yrs

Uranium 235 Lead 207 704 million years

Carbon 14 Nitrogen 14 5730 years

2) Chart for half-lifes elapsed

How it really works

You need to have:

1. The half-life of the material (above)

2. The number (or fraction) of half-lives that have elapsed. This starts with a ratio of parent to daughter, then using the chart you get the half-lives elapsed.

3. Then you multiply #1 times #2.

Examples:

a. 50% Uranium 238 and 50% Lead 206

4.47 Billion Years x 1 (half-life) = 4.47 Billion Yrs

b. 25% Uranium 238 and 75% Lead 206

4.47 Billion Years x 2 (half-lives) = 8.94 Billion Yrs

c. 70% C14 and 30% N14

5730 years x 1/2 (of a half-life) = 5730/2 or 2865 yrs

d. 86% C14 and 14% N14

5730 years x 1/4 (of a half-life) = 5730/4 or 1433 yrs

You have to estimate this one for the ½ lives-elapsed