Absolute Age Dating

Geological Time Absolute time – putting dates on geological events using radiometric dating and other techniques.

Radiometric Dating Absolute dating using radioactive decay data

Alpha Emission Beta Emission Electron Capture

Fig Decay of 238 U General Decay Rules Atomic mass number No change Decrease Atomic number Decrease Decrease Increase Increase General Decay Rules Atomic mass number No change Decrease Atomic number Decrease Decrease Increase Increase Alpha emission Beta emission

Decay sequence for Uranium-238 Half life Half life – the time it takes for half of the original element to decay to the new element.

Half Life = 10,000 yrs Daughterium Decays to Daughterium Parentium Starting with 1,000,0000 nuclei of Parentium Parentium Hypothetical Element Parentium

TimeParentiumDaughterium 1,000, , ,000+10, ,000750, ,000

TimeParentiumDaughterium 125,000825,000 62,500937, ,000 31,250968, , ,000

Elapsed Time ParentiumDaughterium 0 10,000 20,000 30,000 40,000 50,000 1,000, , , ,000 62,500 31, , , , , ,750

,000, , ,000 0 Time (thousands of years) Nuclei Parentium Daughterium 500,000

40 K 238 U 235 U 87 Rb 40 Ar 206 Pb 207 Pb 87 Sr 1,250,000,000 4,500,000, ,000,000 49,000,000, ,000 - formation of Earth 10,000,000 - formation of Earth ParentDaughter Half-life (yrs) Useful Range (yrs) Geologically Useful Radioactive Isotopes Geologist collects samples of rock Geochemist finds radiometric age Geologist interprets the geologic history of area during the time that the rock was being formed

40 K 238 U 235 U 87 Rb 40 Ar 206 Pb 207 Pb 87 Sr 1,250,000,000 4,500,000, ,000,000 49,000,000, ,000 - formation of Earth 10,000,000 - formation of Earth ParentDaughter Half-life (yrs) Useful Range (yrs) Geologically Useful Radioactive Isotopes 40 Ar 40 K An analysis of a feldspar crystal in a rock reveals that it contains 75% 40 Ar and 25% 40 K. How many half-lives have passed since the formation of the feldspar? Assuming the feldspar and rock formed at the same time, how old is the rock?

238 U 235 U 206 Pb 207 Pb 4,500,000, ,000,000 10,000,000 - formation of Earth ParentDaughter Half-life (yrs) Useful Range (yrs) An analysis of a zircon crystal in a rock reveals that it contains: 238 U 206 Pb 50.0% 238 U and 50.0% 206 Pb 235 U 207 Pb 1.6% 235 U and 98.4% 207 Pb 238 U How many half-lives of 238 U have passed since the formation of the zircon? 235 U How many half-lives of 235 U have passed since the formation of the zircon? What is the age of the zircon?