absolute dating

• Absolute dating gives the date of geological materials as an age in years before present
• Isotopes


  The same element with the same number of protons but a different number of neutrons
• Half-lives


  The time it takes for half of the original radioactive parent atoms to decay to stable daughter atoms
• Calculating decay time


  If the ratio of parent to daughter atoms can be accurately measured and the decay rate is known, then how long decay has been proceeding can be calculated
• Most reliable dates are obtained when the rocks being dated are between 1 and 3 half-lives old
• There is a limit of 10 half-lives because there will only be a very small number of parent isotopes left
• Radioactive isotopes are found in

  • Small quantities in some rock forming minerals
• Crystals suitable for radiometric dating are found in

  • Igneous and metamorphic rocks
• Formation of daughter atoms

  When crystals form from radioactive isotopes within, they begin to decay to produce daughter atoms
• Blocking temperature


  About 300°C when the crystal lattice is cool and rigid enough to stop daughter atoms from escaping
• Radiometric dating ages are minimum ages because some daughter atoms have decayed
• Reliable rocks for radiometric dating

  • Lavas due to fast cooling, small intrusions for faster cooling time, large intrusions for a minimum age for country rocks
• Metamorphism resets the radiometric clock
• Using K-Ar to radiometrically date minerals in a sedimentary rock would determine the age of the clasts and not the whole rock, also weathering may have caused a loss of argon
• Potassium-argon dating is the most widely used method of radiometric dating, potassium is abundant in lots of minerals and is used in igneous, metamorphic, and sedimentary rocks
• Potassium-argon dating process

  K produces two isotopes Ar40 and Ca40 but Ca40 cannot be used for dating as it is similar to regular Ca. 11% of K decays to stable Ar40 with a half-life of 1.26x10^9 years. Below the blocking temperature, this argon is trapped within crystals
• Measuring age using K40 and Ar40

  To get the age, K40 is measured separately in the sample. In order to determine the Ar40 content of a rock, it must be melted and then the composition of the released gas measured via a mass spec
• Dates are normally given with a +/- figure to give a range that you are 95% confident the data falls within
• Sources of error in radiometric dating

  • Probable loss of some daughter atoms before blocking temperature reached
  • Uncertainty/reliability of length of half-life
  • Contamination/addition of daughter atoms from another material
  • Laboratory error when dealing with small samples, measuring very small quantities of parent or daughter atoms