Radiometric Dating “clocks in rocks”

Absolute Dating Gives a numerical age Gives a numerical age Works best with igneous rocks & fossils Works best with igneous rocks & fossils Uses isotopes Uses isotopes

Isotopes- different number of neutrons Carbon 14 ( 14 C) Carbon 14 ( 14 C) –2 “extra” neutrons –radioactive

Common dating isotopes ParentDaughter Half life Potassium 40 Argon by Rubidium 87 Strontium by Uranium 235 Lead by Carbon 14 Nitrogen 14 6,000 years Sodium 22 Aluminum hours

radioactivity Nuclei break apart Nuclei break apart Emit particles or waves (radiation) Emit particles or waves (radiation)

Ionizing Radiation Radiation Alpha Particle Neutron Particle Beta Particle Radioactive Atom Gamma Ray (X Ray)

Average Annual Dose Cosmic 28 mrem Terrestrial 28 mrem Internal 40 mrem Medical X-Rays 40 mrem Nuclear Medicine 14 mrem Consumer Products 10 mrem Other 3 mrem Radon 200 mrem Natural Sources Man-Made Sources

Comparison of Radiation Dose Doses shown in bar graph are in units of mrem

Half-Life New1 Half- Life 2 Half- Lives 3 Half- Lives 4 Half- Lives Activity The time required for the amount of radioactive material to decrease by one-half

Half-lives ParentDaughter Half life Potassium 40 Argon by Rubidium 87 Strontium by Uranium 235 Lead by Carbon 14 Nitrogen 14 6,000 years Sodium 22 Aluminum hours

Half life problem If you begin with 80 grams of 14 C after a time, 20 grams are left. How old is the sample? If you begin with 80 grams of 14 C after a time, 20 grams are left. How old is the sample?

14 Carbon Only accurate for ages less than 100,000 years Only accurate for ages less than 100,000 years Parent gets too small to accurately measure Parent gets too small to accurately measure

Mass spectrometer measures D/P ratio

Preparing a rock for mass spectrometer

Using daughter/parent ratio 1) D/P ratio 1) D/P ratio 2) figure out number of half-lives 2) figure out number of half-lives –Use graph 3) multiply number of half-lives by the time of one half-live 3) multiply number of half-lives by the time of one half-live Example: 240 g 14 N ; 8 g 14 C Example: 240 g 14 N ; 8 g 14 C

Another half-life problem 1,000 grams of radioactive element is in a rock when it is formed. The element’s half-life is 2 million years. After a time, 125 grams of the original element remain. How old is the rock? 1,000 grams of radioactive element is in a rock when it is formed. The element’s half-life is 2 million years. After a time, 125 grams of the original element remain. How old is the rock?