Radiometric Dating

• • average rate of disintegration is fixed and doesn't vary with any typical changes in chemical/physical conditions, so radioactive decay offers a dependable means of keeping time
  • daughter atoms formed from old disintegrated parent elements; if daughter atoms identified/counted and rate of decay known you can work back to when there was only parent atoms
• Half-life - the time taken for one-half of the original number of radioactive atoms to decay.
• Radiometric methods may be incorrect:

  • some daughter elements may have been removed by percolating groundwater + geological events may have reset the clock by allowing earlier-formed daughter elements to escape giving a falsely young age for the rock (eg. Argon gas may diffuse out)
• The amounts of parent and daughter atoms present in a sample of rock are a measure of the time interval between now and the time the rock crystallised.

  • Absolute Dating - gives the date of geological materials/events as an age in years before present; known as radiometric dating as methods based on decay of radioactive isotopes.
• Principles of Radiometric Dating:

  • unstable radioactive isotopes decay at a constant rate to produce stable daughter atoms
  • after one half-life, half the original atoms are unstable parents and the other half are stable daughter atoms
  • if ratio of parent/daughter atoms can be measured and half life is known, then how long decay has been happening/age of rock can be calculated
• • Isotopes - atoms with the same number of protons but with a different number of neutrons.

  Radioactive isotopes; found in small quantities in some rock forming minerals + crystals in igneous/metamorphic rocks most suitable (eg. Micas, Horneblende, Feldspars, Uraninite, and Glauconite).
• Reliability of Radiometric Dating:

  • Lavas - reliable as fast cooling except if deeply weathered (causes loss of daughter atoms).
  • Small intrusions - reliable die to relatively fast cooling + give minimum age for country rocks.
  • Large intrusions - less reliable as take millions of years to cool (often in many stages) + doesn't give precise time ranges.
  • Metamorphism - resets radiometric clock + may hide polyphase metamorphism (several events).
• • one half life has elasped when 50% of parent atoms are left and two half lives have passed when 25% of parent atoms are left; relative amounts of daughter/parent atoms are measured with a mass spectrometer
• 40K - 40 Ar (Potassium - Argon):

  • half-life 1277Ma (1277x10$^6$) + most widely used method of radiometric dating + potassium component in many common rock-forming minerals
  • 11% of 40K decays into 40Ar (gas not formed any other way so useful) + useful for dating original cooling/uplift of volcanic/granitic rocks and dates of deposition of glauconite bearing sediments ('greensands')
• Limitations/Problems of Potassium-Argon decay:

  • 89% of 40K decays to form 40Ca (no use for dating as same as calcium found in many rock-forming minerals)
  • only dates rocks older than 100,000 years + Argon is a gas so easily lost through diffusion under metamorphism (so K/Ar decay may give date of metamorphism not age of rock)
• 147Sm - 143Nd (Samarium - Neodymium):

  • half-life of 106 billion years; both are rare earth elements (lanthanides) with similiar chemical properties (so loss by diffusion is reduced)
  • low concentration in surface waters indicate changes during low-temperature weathering are unlikely + Sm/Nd content of constituent minerals haven't been modified since rock crystallised
  • uses; dating 2.7 billion year old Gabbro (plagioclase/pyroxenes) as have different concentrations of Nd/Sm
• Limitations/Problems of Samarian-Neodymium dating:

  • only suited for rocks hundreds/billions of years old + difficult to seperate these very similiar elements (makes fractionation during crystallisation extremely limited) so high precision is needed to accurately measure