Radiometric Methods

Time Scales Absolute Dating Outline

Radiometric dating was first applied by Bertram Boltwood who postulated that older uranium bearing minerals should contain a higher proportion of lead. Both uranium-238 and uranium-235 decay to produce lead-206 and lead-207 respectively.

The time for 50% of the atoms in a sample to decay is called the half-life. After one half-life, the ratio of parent to product is 1:1 (one). After two half-lifes the ratio is 1:3, after three half-lives 1:7 and so on until after ten half-lives the ratio is 1:1023 and less than 0.1% of the original parent isotope remains.

All radiogenic isotopes have charcateristic decay series. The most useful series for dating rocks are those involving elements found in common silicate minerals. Potassium-Argon dating and Rubidium-Strontium dating are most useful in this respect.

The commonly used decay series and their half-lives are given in the table:

Parent Isotope Daughter Isotope Half-Life (Years)
Carbon-14 Nitrogen-14 5,730
Uranium-235 Lead-207 710,000,000
Potassium-40 Argon-40 1,300,000,000
Uranium-238 Lead-206 4,500,000,000
Thorium-232 Lead-208 15,000,000,000
Rubidium-87 Strontium-87 47,000,000,000

The age given by a mineral separated from a rock is the age of crystallization. Metamorphic recrystallization will reset the radiometric clock. Sediments are difficult to date radiometrically because few minerals crystallize in sediments at the time they are formed.

Time Scales Absolute Dating Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9