Radiometric Dating

Pamela J. W. Gore
Georgia Perimeter College

How do we determine the age of a rock?

  1. Relative dating - Steno's Laws, etc.
    "A is older than B"
  2. Absolute dating
    Quantify the date in years. Radiometric Dating


Principles of Radiometric Dating

Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

Radioactive parent elements decay to stable daughter elements.

Radioactivity was discovered in 1896 by Henri Becquerel. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium. In 1907, Boltwood dated a sample of urananite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately.

The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes.

Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

Examples:

Radioactive parent isotopes and their stable daughter products

Radioactive Parent

Stable Daughter

Potassium 40

Argon 40

Rubidium 87

Strontium 87

Thorium 232

Lead 208

Uranium 235

Lead 207

Uranium 238

Lead 206

Carbon 14

Nitrogen 14

In the above table, note that the number is the mass number (the total number of protons plus neutrons).
Note that the mass number may vary for an element, because of a differing number of neutrons.
Elements with various numbers of neutrons are called isotopes of that element.

Each radioactive isotope has its own unique half-life.
A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

Examples:

Half Lives for Radioactive Elements

Radioactive Parent

Stable Daughter

Half life

Potassium 40

Argon 40

1.25 billion yrs

Rubidium 87

Strontium 87

48.8 billion yrs

Thorium 232

Lead 208

14 billion years

Uranium 235

Lead 207

704 million years

Uranium 238

Lead 206

4.47 billion years

Carbon 14

Nitrogen 14

5730 years


Radioactive decay occurrs at a constant exponential or geometric rate.
The rate of decay is proportional to the number of parent atoms present.

The proportion of parent to daughter tells us the number of half-lives, which we can use to find the age in years.
For example, if there are equal amounts of parent and daughter, then one half-life has passed.
If there is three times as much daughter as parent, then two half-lives have passed. (see graph, above)

Radioactive decay occurs by releasing particles and energy.

Uranium decays producing subatomic particles, energy, and lead.

As uranium-238 decays to lead, there are 13 intermediate radioactive daughter products formed (including radon, polonium, and other isotopes of uranium), and 8 alpha particles and 6 beta particles released. There are three types of subatomic particles involved:

  1. Alpha particles
    large, easily stopped by paper
    charge = +2
    mass = 4
  2. Beta particles
    penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.
    charge = -1
    mass = negligible
  3. neutrons
    highly penetrating
    no charge
    mass = 1
Gamma rays (high energy X-rays) are also produced.

Minerals you can date

Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time elapsed since the magma cooled.